Implications of LHCb measurements and future prospects
Abstract
During 2011 the LHCb experiment at CERN collected 1.0 fb^{−1} of \(\sqrt{s} = 7\mbox{~TeV}\)pp collisions. Due to the large heavy quark production cross-sections, these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a dedicated experiment in the forward region at a hadron collider. This document discusses the implications of these first measurements on classes of extensions to the Standard Model, bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS. The physics potential of an upgrade to the LHCb detector, which would allow an order of magnitude more data to be collected, is emphasised.
1 Introduction
During 2011 the LHCb experiment [1] at CERN collected 1.0 fb^{−1} of \(\sqrt{s} = 7 ~\mathrm{TeV} \)pp collisions. Due to the large production cross-section, Open image in new window in the LHCb acceptance [2], with the comparable number for charm production about 20 times larger [3, 4], these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a flavour physics experiment in the forward region at a hadron collider.
The physics objectives of the first phase of LHCb were set out prior to the commencement of data taking in the “roadmap document” [5]. They centred on six main areas, in all of which LHCb has by now published its first results: (i) the tree-level determination of γ [6, 7], (ii) charmless two-body B decays [8, 9], (iii) the measurement of mixing-induced CP violation in \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) [10], (iv) analysis of the decay \(B ^{0}_{ s } \rightarrow \mu ^{+} \mu ^{-} \) [11, 12, 13, 14], (v) analysis of the decay B^{0}→K^{∗0}μ^{+}μ^{−} [15], (vi) analysis of \(B ^{0}_{ s } \rightarrow \phi\gamma\) and other radiative B decays [16, 17].^{1} In addition, the search for CP violation in the charm sector was established as a priority, and interesting results in this area have also been published [18, 19].
The results demonstrate the capability of LHCb to test the Standard Model (SM) and, potentially, to reveal new physics (NP) effects in the flavour sector. This approach to search for NP is complementary to that used by the ATLAS and CMS experiments. While the high-\(p_{\rm T}\) experiments search for on-shell production of new particles, LHCb can look for their effects in processes that are precisely predicted in the SM. In particular, the SM has a highly distinctive flavour structure, with no tree-level flavour-changing neutral currents, and quark mixing described by the Cabibbo–Kobayashi–Maskawa (CKM) matrix [20, 21] which has a single source of CP violation. This structure is not necessarily replicated in extended models. Historically, new particles have first been seen through their virtual effects since this approach allows one to probe mass scales beyond the energy frontier. For example, the observation of CP violation in the kaon system [22] was, in hindsight, the discovery of the third family of quarks, well before the observations of the bottom and top quarks. Crucially, measurements of both high-\(p_{\rm T}\) and flavour observables are necessary in order to decipher the nature of NP.
The early data also illustrated the potential for LHCb to expand its physics programme beyond these “core” measurements. In particular, the development of trigger algorithms that select events inclusively based on properties of b-hadron decays [23, 24] facilitates a much broader output than previously foreseen. On the other hand, limitations imposed by the hardware trigger lead to a maximum instantaneous luminosity at which data can most effectively be collected (higher luminosity requires tighter trigger thresholds, so that there is no gain in yields, at least for channels that do not involve muons). To overcome this limitation, an upgrade of the LHCb experiment has been proposed to be installed during the long shutdown of the LHC planned for 2018. The upgraded detector will be read out at the maximum LHC bunch-crossing frequency of 40 MHz so that the trigger can be fully implemented in software. With such a flexible trigger strategy, the upgraded LHCb experiment can be considered as a general purpose detector in the forward region.
The Letter of Intent for the LHCb upgrade [25], containing a detailed physics case, was submitted to the LHCC in March 2011 and was subsequently endorsed. Indeed, the LHCC viewed the physics case as “compelling”. Nevertheless, the LHCb Collaboration continues to consider further possibilities to enhance the physics reach. Moreover, given the strong motivation to exploit fully the flavour physics potential of the LHC, it is timely to update the estimated sensitivities for various key observables based on the latest available data. These studies are described in this paper, and summarised in the framework technical design report for the LHCb upgrade [26], submitted to the LHCC in June 2012 and endorsed in September 2012.
In the remainder of this introduction, a brief summary of the current LHCb detector is given, together with the common assumptions made to estimate the sensitivity achievable by the upgraded experiment. Thereafter, the sections of the paper discuss rare charm and beauty decays in Sect. 2, CP violation in the B system in Sect. 3 and mixing and CP violation in the charm sector in Sect. 4. There are several other important topics, not covered in any of these sections, that can be studied at LHCb and its upgrade, and these are discussed in Sect. 5. A summary is given in Sect. 6.
1.1 Current LHCb detector and performance
The LHCb detector [1] is a single-arm forward spectrometer covering the pseudorapidity range 2<η<5, designed for the study of particles containing b or c quarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system has a momentum resolution Δp/p that varies from 0.4 % at 5 GeV/c to 0.6 % at 100 GeV/c, and an impact parameter resolution of 20 μm for tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers. The trigger consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage which applies a full event reconstruction.
During 2011, the LHCb experiment collected 1.0 fb^{−1} of integrated luminosity during the LHC pp run at a centre-of-mass energy \(\sqrt {s} = 7 ~\mathrm{TeV} \). The majority of the data was recorded at an instantaneous luminosity of \(\mathcal{L}_{\rm inst} = 3.5 \times10^{32}~\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\), nearly a factor of two above the LHCb design value, and with a pile-up rate (average number of visible interactions per crossing) of μ∼1.5 (four times the nominal value, but below the rates of up to μ∼2.5 seen in 2010). A luminosity levelling procedure, where the beams are displaced at the LHCb interaction region, allows LHCb to maintain an approximately constant luminosity throughout each LHC fill. This procedure permitted reliable operation of the experiment and a stable trigger configuration throughout 2011. The hardware stage of the trigger produced output at around 800 kHz, close to the nominal 1 MHz, while the output of the software stage was around 3 kHz, above the nominal 2 kHz, divided roughly equally between channels with muons, b decays to hadrons and charm decays. During data taking, the magnet polarity was flipped at a frequency of about one cycle per month in order to collect equal sized data samples of both polarities for periods of stable running conditions. Thanks to the excellent performance of the LHCb detector, the overall data taking efficiency exceeded 90 %.
1.2 Assumptions for LHCb upgrade performance
LHC collisions will be at \(\sqrt{s} = 14 ~\mathrm{TeV} \), with heavy flavour production cross-sections scaling linearly with \(\sqrt{s}\);
the instantaneous luminosity^{2} in LHCb will be \(\mathcal{L}_{\rm inst} = 10^{33}~\mathrm{cm} ^{-2}\,\mathrm{s}^{-1}\): this will be achieved with 25 ns bunch crossings (compared to 50 ns in 2011) and μ=2;
LHCb will change the polarity of its dipole magnet with similar frequency as in 2011/12 data taking, to approximately equalise the amount of data taken with each polarity for better control of certain potential systematic biases;
the integrated luminosity will be \(\mathcal{L}_{\rm int} = 5~\mbox{fb}^{-1}\) per year, and the experiment will run for 10 years to give a total sample of 50 fb^{−1}.
2 Rare decays
2.1 Introduction
flavour-changing neutral current (FCNC) processes that are mediated by electroweak box and penguin type diagrams in the SM;
more exotic decays, including searches for lepton flavour or number violating decays of B or D mesons and for light scalar particles.
For the second class of decay, there is either no SM contribution or the SM contribution is vanishingly small and any signal would indicate evidence for physics beyond the SM. Grouped in this class of decay are searches for GeV scale new particles that might be directly produced in B or D meson decays. This includes searches for light scalar particles and for B meson decays to pairs of same-charge leptons that can arise, for example, in models containing Majorana neutrinos [27, 28, 29].
The focus of this section is on rare decays involving leptons or photons in the final states. There are also several interesting rare decays involving hadronic final states that can be pursued at LHCb, such as B^{+}→K^{−}π^{+}π^{+}, B^{+}→K^{+}K^{+}π^{−} [30, 31], \(B ^{0}_{ s } \rightarrow \phi \pi ^{0} \) and \(B ^{0}_{ s } \rightarrow \phi\rho^{0}\) [32]; however, these are not discussed in this document.
Section 2.2 introduces the theoretical framework (the operator product expansion) that is used when discussing rare electroweak penguin processes. The observables and experimental constraints coming from rare semileptonic, radiative and leptonic B decays are then discussed in Sects. 2.3, 2.4 and 2.5 respectively. The implications of these experimental constraints for NP contributions are discussed in Sects. 2.6 and 2.7. Possibilities with rare charm decays are then discussed in Sect. 2.8, and the potential of LHCb to search for rare kaon decays, lepton number and flavour violating decays, and for new light scalar particles is summarised in Sects. 2.9, 2.10 and 2.11 respectively.
2.2 Model-independent analysis of new physics contributions to leptonic, semileptonic and radiative decays
In the SM, models with minimal flavour violation (MFV) [35, 36] and models with a flavour symmetry relating the first two generations [37], the Wilson coefficients appearing in Eq. (1) are equal for q=d or s and the ratio of amplitudes for b→d relative to b→s transitions is suppressed by |V_{td}/V_{ts}|. Due to this suppression, at the current level of experimental precision, constraints on decays with a b→d transition are much weaker than those on decays with a b→s transition for constraining \(C_{i}^{(\prime)}\). In the future, precise measurements of b→d transitions will allow powerful tests to be made of this universality which could be violated by NP.
The dependence on the Wilson coefficients, and the set of operators that can contribute, is different for different rare B decays. In order to put the strongest constraints on the Wilson coefficients and to determine the room left for NP, it is therefore desirable to perform a combined analysis of all the available data on rare leptonic, semileptonic and radiative B decays. A number of such analyses have recently been carried out for subsets of the Wilson coefficients [38, 39, 40, 41, 42, 43].
The theoretically cleanest branching ratios probing the b→s transition are the inclusive decays B→X_{s}γ and B→X_{s}ℓ^{+}ℓ^{−}. In the former case, both the experimental measurement of the branching ratio and the SM expectation have uncertainties of about 7 % [44, 45]. In the latter case, semi-inclusive measurements at the B factories still have errors at the 30 % level [44]. At hadron colliders, the most promising modes to constrain NP are exclusive decays. In spite of the larger theory uncertainties on the branching fractions as compared to inclusive decays, the attainable experimental precision can lead to stringent constraints on the Wilson coefficients. Moreover, beyond simple branching fraction measurements, exclusive decays offer powerful probes of \(C_{7}^{(\prime)}\), \(C_{9}^{(\prime)}\) and \(C_{10}^{(\prime)}\) through angular and CP-violating observables. The exclusive decays most sensitive to NP in b→s transitions are B→K^{∗}γ, \(B ^{0}_{ s } \rightarrow \mu^{+}\mu^{-}\), B→Kμ^{+}μ^{−} and B→K^{∗}μ^{+}μ^{−}. These decays are discussed in more detail below.
2.3 Rare semileptonic B decays
The richest set of observables sensitive to NP are accessible through rare semileptonic decays of B mesons to a vector or pseudoscalar meson and a pair of leptons. In particular the angular distribution of B→K^{∗}μ^{+}μ^{−} decays, discussed in Sect. 2.3.2, provides strong constraints on \(C_{7}^{(\prime)}\), \(C_{9}^{(\prime)}\) and \(C_{10}^{(\prime)}\).
2.3.1 Theoretical treatment of rare semileptonic B→Mℓ^{+}ℓ^{−} decays
The theoretical treatment of exclusive rare semileptonic decays of the type B→Mℓ^{+}ℓ^{−} is possible in two kinematic regimes for the meson M: large recoil (corresponding to low dilepton invariant mass squared, q^{2}) and small recoil (high q^{2}). Calculations are difficult outside these regimes, in particular in the q^{2} region close to the narrow \(c \overline { c } \) resonances (the J/ψ and ψ(2S) states).
Within the QCDF/SCET approach, a general, quantitative method to estimate the important Λ_{QCD}/m_{b} corrections to the heavy quark limit is missing. In semileptonic decays, a simple dimensional estimate of 10 % is often used, largely from matching of the soft form factors to the full-QCD form factors (see also Ref. [56]).
The high q^{2} (low hadronic recoil) region, corresponds to dilepton invariant masses above the two narrow resonances of J/ψ and ψ(2S), with q^{2}≳(14–15) GeV^{2}. In this region, broad \(c\overline{c}\)-resonances are treated using a local operator product expansion [57, 58]. The operator product expansion (OPE) predicts small sub-leading corrections which are suppressed by either (Λ_{QCD}/m_{b})^{2} [58] or α_{S}Λ_{QCD}/m_{b} [57] (depending on whether full QCD or subsequent matching on heavy quark effective theory in combination with form factor symmetries [59] is adopted). The sub-leading corrections to the amplitude have been estimated to be below 2 % [58] and those due to form factor relations are suppressed numerically by \(C_{7} / C_{9} \sim{ \mathcal{O} } (0.1)\). Moreover, duality violating effects have been estimated within a model of resonances and found to be at the level of 2 % of the rate, if sufficiently large bins in q^{2} are chosen [58]. Consequently, like the low q^{2} region, this region is theoretically well under control.
At high q^{2} the heavy-to-light form factors are known only as extrapolations from light cone sum rules (LCSR) calculations at low q^{2}. Results based on lattice calculations are being derived [60], and may play an important role in the near future in reducing the form factor uncertainties.
2.3.2 Angular distribution of B^{0}→K^{∗0}μ^{+}μ^{−} and \(B ^{0}_{ s } \rightarrow \phi \mu ^{+} \mu ^{-} \) decays
When combining B and \(\overline{ B }{} \) decays, it is possible to form both CP-averaged and CP-asymmetric quantities: \(S_{i} =(J_{i} + \bar {J_{i}})/[d(\varGamma+ \bar{\varGamma})/d q^{2} ]\) and \(A_{i} = (J_{i} - \bar {J_{i}})/[d(\varGamma+ \bar{\varGamma})/d q^{2} ]\), from the J_{i} [53, 54, 62, 63, 64, 65, 66]. The terms J_{5,6,8,9} in the angular distribution are CP-odd and, consequently, the associated CP-asymmetry, A_{5,6,8,9} can be extracted from an untagged analysis (making it possible for example to measure A_{5,6,8,9} in \(B ^{0}_{ s } \rightarrow \phi \mu ^{+} \mu ^{-} \) decays). Moreover, the terms J_{7,8,9} are T-odd and avoid the usual suppression of the corresponding CP-asymmetries by small strong phases [64]. The decay B^{0}→K^{∗0}μ^{+}μ^{−}, where the K^{∗0} decays to K^{+}π^{−}, is self-tagging (the flavour of the initial B meson is determined from the decay products) and it is therefore possible to measure both the A_{i} and S_{i} for the twelve angular terms.
In addition, a measurement of the T-odd CP asymmetries, A_{7}, A_{8} and A_{9}, which are zero in the SM and are not suppressed by small strong phases in the presence of NP, would be useful to constrain non-standard CP violation. This is particularly true since the direct CP asymmetry in the inclusive B→X_{s}γ decay is plagued by sizeable long-distance contributions and is therefore not very useful as a constraint on NP [67].
2.3.3 Strategies for analysis of B^{0}→K^{∗0}ℓ^{+}ℓ^{−} decays
In 1.0 fb^{−1} of integrated luminosity, LHCb has collected the world’s largest samples of B^{0}→K^{∗0}μ^{+}μ^{−} (with K^{∗0}→K^{+}π^{−}) and \(B ^{0}_{ s } \rightarrow \phi \mu ^{+} \mu ^{-} \) decays, with around 900 and 80 signal candidates respectively reported in preliminary analyses [68, 69]. These candidates are however sub-divided into six q^{2} bins, following the binning scheme used in previous experiments [70]. With the present statistics, the most populated q^{2} bin contains ∼300B^{0}→K^{∗0}μ^{+}μ^{−} candidates which is not sufficient to perform a full angular analysis. The analyses are instead simplified by integrating over two of the three angles or by applying a folding technique to the ϕ angle, ϕ→ϕ+π for ϕ<0, to cancel terms in the angular distribution.
2.3.4 Theoretically clean observables in B^{0}→K^{∗0}ℓ^{+}ℓ^{−} decays
By the time that 5 fb^{−1} of integrated luminosity is available at LHCb, it will be possible to exploit the complete NP sensitivity of the B→K^{∗}ℓ^{+}ℓ^{−} both in the low- and high-q^{2} regions, by performing a full angular analysis. The increasing size of the experimental samples makes it important to design optimised observables (by using specifically chosen combinations of the J_{i}) to reduce theoretical uncertainties. In the low q^{2} region, the linear dependence of the amplitudes on the soft form factors allows for a complete cancellation of the hadronic uncertainties due to the form factors at leading order. This consequently increases the sensitivity to the structure of NP models [53, 54].
In the low q^{2} region, the so-called transversity observables \(A_{T}^{(i)}\), i=2,3,4,5 are an example set of observables that are constructed such that the soft form factor dependence cancels out at leading order. They represent the complete set of angular observables and are chosen to be highly sensitive to new right-handed currents via \(C_{7}^{\prime}\) [53, 54]. A second, complete, set of optimised angular observables was constructed (also in the cases of non-vanishing lepton masses and in the presence of scalar operators) in Ref. [55]. Recently the effect of binning in q^{2} on these observables has been considered [72]. In these sets of observables, the unknown \(\varLambda_{\rm QCD}/m_{b}\) corrections are estimated to be of order 10 % on the level of the spin amplitudes and represent the dominant source of theory uncertainty.
In general, the angular observables are shown to offer high sensitivity to NP in the Wilson coefficients of the operators O_{7}, O_{9}, and O_{10} and of the chirally flipped operators [53, 54, 62, 64]. In particular, the observables S_{3}, A_{9} and the CP-asymmetries A_{7} and A_{8} vanish at leading order in Λ_{QCD}/m_{b} and α_{S} in the SM operator basis [64]. Importantly, this suppression is absent in extensions with non-vanishing chirality-flipped \({C}^{\prime}_{7,9,10}\), giving rise to contributions proportional to \(\operatorname{Re} (C_{i} {C_{j}^{*}}^{\prime})\) or \(\operatorname{Im} (C_{i} {C_{j}^{*}}^{\prime})\) and making these terms ideal probes of right-handed currents [53, 54, 62, 64]. CP asymmetries are small in the SM, because the only CP-violating phase affecting the decay is doubly Cabibbo-suppressed, but can be significantly enhanced by NP phases in C_{9,10} and \(C^{\prime}_{9,10}\), which at present are poorly constrained. In a full angular analysis it can also be shown that CP-conserving observables provide indirect constraints on CP-violating NP contributions [54].
only on short-distance quantities (e.g. \(H_{T}^{(2,3)}\));
only on long-distance quantities (\(F_{\rm L}\) and low q^{2} optimised observables \(A_{T}^{(2,3)}\)).
In the SM operator basis it is interesting to note that \(A_{T}^{(2,3)}\), which are highly sensitive to short distance contributions (from \(C_{7}^{\prime}\)) at low q^{2}, instead become sensitive to long-distance quantities (the ratio of form factors) at high q^{2}. The extraction of form factor ratios is already possible with current data on S_{3} (\(A_{T}^{(2)}\)) and \(F_{\rm L}\) and leads to a consistent picture between LCSR calculations, lattice calculations and experimental data [41, 74]. In the presence of chirality-flipped Wilson coefficients, these observables are no longer short-distance free, but are probes of right-handed currents [42]. At high q^{2}, the OPE framework predicts \(H_{T}^{(2)} = H_{T}^{(3)}\) and J_{7}=J_{8}=J_{9}=0. Any deviation from these relationships, would indicate a problem with the OPE and the theoretical predictions in the high q^{2} region.
2.3.5 B^{+}→K^{+}μ^{+}μ^{−} and B^{+}→K^{+}e^{+}e^{−}
The branching fractions of B^{0(+)}→K^{0(+)}μ^{+}μ^{−} have been measured by BaBar, Belle and CDF [70, 75, 76]. In 1.0 fb^{−1} LHCb observes 1250 B^{+}→K^{+}μ^{+}μ^{−} decays [77], and in the future will dominate measurements of these processes.
Since the B→K transition does not receive contributions from an axial vector current, the primed Wilson coefficients enter the B^{0(+)}→K^{0(+)}μ^{+}μ^{−} observables always in conjunction with their unprimed counterparts as \((C_{i}+C_{i}^{\prime})\). This is in contrast to the B→K^{∗}μ^{+}μ^{−} decay and therefore provides complementary constraints on the Wilson coefficients and their chirality-flipped counterparts.
In the SM, the forward–backward asymmetry of the dilepton system is expected to be zero. Any non-zero forward–backward asymmetry would point to a contribution from new particles that extend the SM operator basis. Allowing for generic (pseudo-)scalar and tensor couplings, there is sizeable room for NP contributions in the range \(|A_{\rm FB}| \lesssim 15~\%\). The flat term, \(F_{\rm H}/2\), that appears with \(A_{\rm FB}\) in the angular distribution, is non-zero, but small (for ℓ=e,μ) in the SM. This term can also see large enhancements in models with (pseudo-)scalar and tensor couplings of up to \(F_{\rm H} \sim0.5\). Recent SM predictions at low- and high-q^{2} can be seen in Refs. [40, 56, 78, 79]. The current experimental limits on \(\mathcal {B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) now disfavour large C_{S} and C_{P}, and if NP is present only in tensor operators then NP contributions are expected to be in the range \(|A_{\rm FB}| \lesssim 5~\% \) and \(F_{\rm H} \lesssim 0.2\).
It is also interesting to note that at high q^{2} the differential decay rates and CP asymmetries of B^{0(+)}→K^{0(+)}ℓ^{+}ℓ^{−} and B^{0(+)}→K^{∗0(+)}ℓ^{+}ℓ^{−} (ℓ=e,μ) are correlated [40] and exhibit the same short-distance dependence (in the SM operator basis). Any deviation would point to a problem for the OPE used in the high q^{2} region.
2.3.6 Rare semileptonic b→dℓ^{+}ℓ^{−} decays
The b→d transitions can show potentially larger CP- and isospin-violating effects than their b→s counterparts due to the different CKM hierarchy [51]. These studies would need the large statistics provided by the future LHCb upgrade. A 50 fb^{−1} data sample will also enable a precision measurement of the ratio of the branching fractions of B^{+} meson decays to π^{+}μ^{+}μ^{−} and K^{+}μ^{+}μ^{−}. This ratio would enable a useful comparison of |V_{td}/V_{ts}| to be made using penguin processes (with form factors from lattice QCD) and box processes (using Δm_{s}/Δm_{d} and bag-parameters from lattice QCD) and provide a powerful test of MFV.
2.3.7 Isospin asymmetry of B^{0(+)}→K^{0(+)}μ^{+}μ^{−} and B^{0(+)}→K^{∗0(+)}μ^{+}μ^{−} decays
2.4 Radiative B decays
While the theoretical prediction of the branching ratio of the B→K^{∗}γ decay is problematic due to large form factor uncertainties, the mixing-induced asymmetry^{13}\(S_{K^{*}\gamma}\) provides an important constraint due to its sensitivity to the chirality-flipped magnetic Wilson coefficient \(C_{7}^{\prime}\). At leading order it vanishes for \(C_{7}^{\prime} \rightarrow 0\), so the SM prediction is tiny and experimental evidence for a large \(S_{K^{*}\gamma }\) would be a clear indication of NP effects through right-handed currents [89, 90]. Unfortunately it is experimentally very challenging to measure \(S_{K^{*}\gamma}\) in a hadronic environment, requiring both flavour tagging and the ability to reconstruct the K^{∗0} in the decay mode K^{∗0}→K^{0}π^{0}. However, the channel \(B ^{0}_{ s } \rightarrow \phi\gamma\), which is much more attractive experimentally, offers the same physics opportunities, with additional sensitivity due to the non-negligible width difference in the \(B ^{0}_{ s } \) system. Moreover, LHCb can study several other interesting radiative b-hadron decays.
2.4.1 Experimental status and outlook for rare radiative decays
In 1.0 fb^{−1} of integrated luminosity LHCb observes 5300 B^{0}→K^{∗0}γ and \(690\ B ^{0}_{ s } \rightarrow \phi\gamma \) [17] candidates. These are the largest samples of rare radiative B^{0} and \(B ^{0}_{ s } \) decays collected by a single experiment. The large sample of B^{0}→K^{∗0}γ decays has enabled LHCb to make the world’s most precise measurement of the direct CP-asymmetry \(\mathcal{A}_{ \mathit{CP} } ( K ^{*} \gamma) = 0.8 \pm1.7 \pm0.9~\%\), compatible with zero as expected in the SM [17].
With larger data samples, it will be possible to add additional constraints on the \(C_{7}\mbox{--}C_{7}^{\prime}\) plane through measurements of b→sγ processes. These include results from time-dependent analysis of \(B ^{0}_{ s } \rightarrow \phi\gamma \) [91], as described in detail in the LHCb roadmap document [5]. Furthermore, the large \({ \varLambda }^{0}_{ b } \) production cross-section will allow for measurements of the photon polarisation through the decays \({ \varLambda }^{0}_{ b } \rightarrow { \varLambda }^{(*)}\gamma\) [92, 93]. In fact, the study of \({ \varLambda }^{0}_{ b } \rightarrow { \varLambda }\) transitions is quite attractive from the theoretical point of view, since the hadronic uncertainties are under good control [94, 95, 96]. However, because the \({ \varLambda }^{0}_{ b } \) has \(J^{P} = \frac{1}{2}^{+}\) and can be polarised at production, it will be important to measure first the \({ \varLambda }^{0}_{ b } \) polarisation.
B→VPγ decays with a photon, a vector and a pseudoscalar particle in the final state can also provide sensitivity to \(C_{7}^{\prime}\) [97, 98, 99, 100]. The decays B→ϕKγ and B^{+}→K_{1}(1270)^{+}γ have been previously observed at the B factories [101, 102] and large samples will be available for the first time at LHCb.
2.5 Leptonic B decays
2.5.1 \(B ^{0}_{ s } \rightarrow \mu ^{+} \mu ^{-} \) and B^{0}→μ^{+}μ^{−}
Within the SM, C_{S} and C_{P} are negligibly small and the dominant contribution of C_{10} is helicity suppressed. The coefficients C_{i} are the same for \(B ^{0}_{ s } \) and B^{0} in any scenario (SM or NP) that obeys MFV. The large suppression of \(\mathcal{B}( B ^{0} \rightarrow \mu^{+}\mu^{-} ) \) with respect to \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) in MFV scenarios means that \(B ^{0}_{ s } \rightarrow \mu ^{+} \mu ^{-} \) is often of more interest than B^{0}→μ^{+}μ^{−} for NP searches. The ratio \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) / \mathcal{B}( B ^{0} \rightarrow \mu^{+}\mu^{-} ) \) is however a very useful probe of MFV.
The SM branching fraction depends on the exact values of the input parameters: \(f_{B_{q}}\), \(\tau_{B_{q}}\) and \(|V_{tb}V_{tq}^{*}|^{2}\). The \(B ^{0}_{ s } \) decay constant, \(f_{B_{s}}\), constitutes the main source of uncertainty on \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \). There has been significant progress in theoretical calculations of this quantity in recent years. As of the year 2009 there were two unquenched lattice QCD calculations of \(f_{B_{s}}\), by the HPQCD [107] and FNAL/MILC [108] Collaborations, which, when averaged, gave the value \(f_{B_{s}}=238.8\pm9.5 ~\mathrm{MeV} \) [109]. The FNAL/MILC calculation was updated in 2010 [110], and again in 2011 to give \(f_{B_{s}}=242\pm9.5 ~\mathrm{MeV} \) [111, 112]. Also in 2011, the ETM Collaboration reported a value of \(f_{B_{s}}=232\pm 10 ~\mathrm{MeV} \) [113]. The HPQCD Collaboration presented in 2011 a result, \(f_{B_{s}}=227\pm 10 ~\mathrm{MeV} \) [114], which has recently been improved upon with an independent calculation that gives \(f_{B_{s}}=225\pm4 ~\mathrm{MeV} \) [115].
NP models, especially those with an extended Higgs sector, can significantly enhance the \(B^{0}_{(s)} \rightarrow \mu^{+}\mu ^{-} \) branching fraction even in the presence of other existing constraints. In particular, it has been emphasised in many works [121, 122, 123, 124, 125, 126, 127, 128] that the decay \(B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} \) is very sensitive to the presence of SUSY particles. At large tanβ—where tanβ is the ratio of vacuum expectation values of the Higgs doublets^{14}—the SUSY contribution to this process is dominated by the exchange of neutral Higgs bosons, and both C_{S} and C_{P} can receive large contributions from scalar exchange.
Other NP models such as composite models (e.g. Littlest Higgs model with T-parity or Topcolour-assisted Technicolor), models with extra dimensions (e.g. Randall–Sundrum models) or models with fourth generation fermions can modify \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) [116, 131, 132, 133, 134, 135]. The NP contributions from these models usually arise via \((C_{10}\mbox{--}C^{\prime}_{10})\), and they are therefore correlated with the constraints from other b→sℓ^{+}ℓ^{−} processes, e.g. with \({ \mathcal{B} } ( B ^{+} \rightarrow K ^{+} \mu ^{+} \mu ^{-} )\) which depends on \((C_{10}+C^{\prime}_{10})\). The term \((C_{P} \mbox{--}C^{\prime}_{P})\) in the branching fraction adds coherently with the SM contribution from \((C_{10} \mbox{--}C^{\prime}_{10})\), and therefore can also destructively interfere. In such cases, if \((C_{S} \mbox{--}C^{\prime}_{S})\) remains small, \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) could be smaller than the SM prediction. A measurement of \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) well below the SM prediction would be a clear indication of NP and would be symptomatic of a model with a large non-degeneracy in the scalar sector (where \(C_{P}^{(\prime)}\) is enhanced but \(C_{S}^{(\prime)}\) is not). If only C_{10} is modified, these constraints currently require the branching ratio to be above 1.1×10^{−10} [42]. In the presence of NP effects in both C_{10} and \(C_{10}^{\prime}\), even stronger suppression is possible in principle.
With 50 fb^{−1} of integrated luminosity, taken with an upgraded LHCb experiment, a precision better than 10 % can be achieved in \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu ^{-} ) \), and ∼35 % on the ratio \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) / \mathcal{B}( B ^{0} \rightarrow \mu^{+}\mu^{-} ) \). The dominant systematic uncertainty is likely to come from knowledge of the ratio of fragmentation fractions, f_{d}/f_{s}, which is currently known to a precision of 8 % from two independent determinations.^{17} One method [140]^{18} is based on hadronic B decays [142, 143], and relies on knowledge of the B_{(s)}→D_{(s)} form factors from lattice QCD calculations [144]. The other [145] uses semileptonic decays, exploiting the expected equality of the semileptonic widths [146, 147]. However, the two methods have a common, and dominant, uncertainty which originates from the measurement of \({ \mathcal{B} } ( D ^{+}_{ s } \rightarrow K ^{+} K ^{-} \pi ^{+} )\), which in the PDG is given to 4.9 % (coming from a single measurement from CLEO [148]). A new preliminary result from Belle has recently been presented [149]—inclusion of this measurement in the world average will improve the uncertainty on \({ \mathcal{B} } ( D ^{+}_{ s } \rightarrow K ^{+} K ^{+} \pi ^{+} )\) to ∼3.5 %. With the samples available with the LHCb upgrade, it will be possible to go beyond branching fraction measurements and study the effective lifetime of \(B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} \), that provides additional sensitivity to NP [136].
In Sect. 2.7, the NP implications of the current measurements of \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} ) \) and the interplay with other observables, including results from direct searches, are discussed for a selection of specific NP models. In general, the strong experimental constraints on \(\mathcal{B}( B^{0}_{ s } \rightarrow \mu^{+}\mu ^{-} ) \) [13, 130, 150, 151] largely preclude any visible effects from scalar or pseudoscalar operators in other b→sℓ^{+}ℓ^{−} decays.^{19}
2.5.2 \(B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} \)
The leptonic decay \(B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} \) provides interesting information on the interaction of the third generation quarks and leptons. In many NP models, contributions to third generation quarks/leptons can be dramatically enhanced with respect to the first and second generation. This is true in, for example, scalar and pseudoscalar interactions in supersymmetric scenarios, for large values of tanβ. Interestingly, there is also an interplay between b→sτ^{+}τ^{−} processes and the lifetime difference \(\varGamma_{12}^{s}\) in \(B ^{0}_{ s } \) mixing (see Sect. 3). The correlation of both processes has been discussed model-independently [152, 153] and in specific scenarios, such as leptoquarks [154, 155] or Z′ models [156, 157, 158]. There are presently no experimental limits on \(B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} \), however the interplay with \(\varGamma_{12}^{s}\), and the latest LHCb-measurement of Γ_{d}/Γ_{s} would imply a limit of \({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} ) < 3~\%\) at 90 % C.L. Any improvement on this limit, which might be in reach with the existing LHCb data set, would yield strong constraints on models that couple strongly to third generation leptons. A large enhancement in b→sτ^{+}τ^{−} could help to understand the anomaly observed by the D0 experiment in their measurement of the inclusive dimuon asymmetry [159] and could also reduce the tension that exists with other mixing observables [152, 153].
The study of \(B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} \) at LHCb presents significant challenges. The τ leptons must be reconstructed in decays that involve at least one missing neutrino. Although it has been demonstrated that the decay Z→τ^{+}τ^{−} can be separated from background at LHCb, using both leptonic and hadronic decay modes [160], at lower energies the backgrounds from semileptonic heavy flavour decays cause the use of the leptonic decay modes to be disfavoured. However, in the case that “three-prong” τ decays are used, the vertices can be reconstructed from the three hadron tracks. The analysis can then benefit from the excellent vertexing capability of LHCb, and, due to the finite lifetime of the τ lepton, there are in principle sufficient kinematic constraints to reconstruct the decay. Work is in progress to understand how effectively the different potential background sources can be suppressed, and hence how sensitive LHCb can be in this channel.
2.6 Model-independent constraints
At 95 % C.L., all Wilson coefficients are compatible with their SM values.
For the coefficients present in the SM, i.e. C_{7}, C_{9} and C_{10}, the constraints on the imaginary part are looser than on the real part.
For the Wilson coefficients \(C_{10}^{(\prime)}\), the constraint on \({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow \mu ^{+} \mu ^{-} )\) is starting to become competitive with the constraints from the angular analysis of B→K^{(∗)}μ^{+}μ^{−}.
The constraints on \(C_{9}^{\prime}\) and \(C_{10}^{\prime}\) from B→Kμ^{+}μ^{−} and B→K^{∗}μ^{+}μ^{−} are complementary and lead to a more constrained region, and better agreement with the SM, than with B→K^{∗}μ^{+}μ^{−} alone.
A second allowed region in the C_{7}–\(C_{7}^{\prime}\) plane characterised by large positive contributions to both coefficients, which was found previously to be allowed e.g. in Refs. [38, 39], is now disfavoured at 95 % C.L. by the new B→K^{∗}μ^{+}μ^{−} data, in particular the measurements of the forward–backward asymmetry from LHCb.
Significant improvements of these constraints—or first hints for physics beyond the SM—can be obtained in the future by both improved measurements of the observables discussed above and by improvements on the theoretical side. From the theory side, there is scope for improving the estimates of the hadronic form factors from lattice calculations, which will reduce the dominant source of uncertainty on the exclusive decays. On the experimental side there are a large number of theoretically clean observables that can be extracted with a full angular analysis of B^{0}→K^{∗0}μ^{+}μ^{−}, as discussed in Sect. 2.3.2.
2.7 Interplay with direct searches and model-dependent constraints
The search for SUSY is the main focus of NP searches in ATLAS and CMS. Although the results so far have not revealed a positive signal, they have put strong constraints on constrained SUSY scenarios. The understanding of the parameters of SUSY models also depends on other measurements, such as the anomalous dipole moment of the muon, limits from direct dark matter searches, measurements of the dark matter relic density and various B physics observables. As discussed in Sect. 2.5, the rare decay channels studied in LHCb, such as \(B^{0}_{(s)} \rightarrow \mu^{+}\mu^{-} \), provide stringent tests of SUSY. In addition, the decays B→K^{(∗)}μ^{+}μ^{−} provide many complementary observables which are sensitive to different sectors of the theory. In this section, the implications of the current LHCb measurements in different SUSY models are explained, both in constrained scenarios and in a more general case.
First consider the constrained minimal supersymmetric standard model (CMSSM) and a model with non-universal Higgs masses (NUHM1). The CMSSM is characterised by the set of parameters {m_{0},m_{1/2},A_{0},tanβ,sgn(μ)} and invokes unification boundary conditions at a very high scale \(m_{\rm GUT}\) where the universal mass parameters are specified. The NUHM1 relaxes the universality condition for the Higgs bosons which are decoupled from the other scalars, adding then one extra parameter compared to the CMSSM.
The interplay with Higgs boson searches can also be very illuminating as any viable model point has to be in agreement with all the direct and indirect limits. As an example, if a scalar Higgs boson is confirmed at ∼125 GeV,^{22} the MSSM scenarios in which the excess would correspond to the heaviest CP-even Higgs (as opposed to the lightest Higgs) are ruled out by the \(B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} \) limit, since they would lead to a too light pseudoscalar Higgs.
It is clear that with more precise measurements a large part of the supersymmetric parameter space could be disfavoured. In particular the large tanβ region is strongly affected by \(B^{0}_{ s } \rightarrow \mu^{+}\mu^{-} \) as can be seen in Fig. 5. Also, a measurement of \({ \mathcal{B} } ( B^{0}_{ s } \rightarrow \mu^{+}\mu ^{-} )\) lower than the SM prediction would rule out a large variety of supersymmetric models. In addition, B→K^{∗}μ^{+}μ^{−} observables play a complementary role especially for smaller tanβ values. With reduced theoretical and experimental errors, the exclusion bounds in Figs. 6 and 7 for example would shrink leading to important consequences for SUSY parameters.
2.8 Rare charm decays
So far the focus of this chapter has been on rare B decays, but the charm sector also provides excellent probes for NP in the form of very rare decays. Unlike the B decays described in the previous sections, the smallness of the d, s and b quark masses makes the Glashow–Iliopoulos–Maiani (GIM) cancellation in loop processes very effective. Branching ratios governed by FCNC are hence not expected to exceed \(\mathcal{O}(10^{-10})\) in the SM. These processes can then receive contributions from NP scenarios which can be several orders of magnitude larger than the SM expectation.
2.8.1 Search for D^{0}→μ^{+}μ^{−}
The branching fraction of the D^{0}→μ^{+}μ^{−} decay is dominated in the SM by the long distance contributions due to the two photon intermediate state, D^{0}→γγ. The experimental upper limit on the two photon mode can be combined with theoretical predictions to constrain \(\mathcal{B}(D^{0}\rightarrow \mu^{+}\mu^{-})\) in the framework of the SM: \(\mathcal{B}(D^{0}\rightarrow \mu^{+}\mu^{-})<6\times10^{-11} \) at 90 % C.L. [176]. Particular NP models where this decay is enhanced include supersymmetric models with R-parity violation (RPV), which provides tree-level contributions that would enhance the branching fraction. In such models, the branching fraction would be related to the D^{0}–\(\overline{ D }{} ^{0} \) mixing parameters. Once the experimental constraints on the mixing parameters are taken into account, the corresponding tree-level couplings can still give rise to \(\mathcal{B}(D^{0}\rightarrow\mu^{+}\mu^{-})\) of up to \(\mathcal {O}(10^{-9})\) [177].
2.8.2 Search for \(D^{+}_{(s)}\rightarrow h^{+} \mu^{+}\mu^{-}\) and D^{0}→hh′μ^{+}μ^{−}
The \(D^{+}_{(s)}\rightarrow h^{+} \mu^{+}\mu^{-}\) decay rate is dominated by long distance contributions from tree-level \(D^{+}_{(s)}\rightarrow h^{+} V\) decays, where V is a light resonance (V=ϕ,ρ,ω). The long-distance contributions have an effective branching fraction (with V→μ^{+}μ^{−}) above 10^{−6} in the SM. Large deviations in the total decay rate due to NP are therefore unlikely. However, the regions of the dimuon mass spectrum far from these resonances are interesting probes. Here, the SM contribution stems only from FCNC processes, that should yield no partial branching ratio above 10^{−11} [179]. NP contributions could enhance the branching fraction away from the resonances by several orders of magnitude: e.g. in the RPV model mentioned above, or in models involving a fourth quark generation [179, 180].
The LHCb experiment is well-suited to search for \(D^{+}_{(s)}\rightarrow h^{\mp} \mu^{+}\mu^{\pm}\) decays. The long distance contributions can be used to normalise the decays searched for at high and low dimuon mass: their decay rate will be measured relative to that of \(D^{+}_{(s)}\rightarrow\pi^{+} \phi(\mu ^{+}\mu^{-})\). These resonant decays have a clean experimental signature and their final state only differs from the signal in the kinematic distributions, which helps to reduce the systematic uncertainties. The sensitivity of the LHCb experiment can be estimated by comparing the yields of \(D^{+}_{(s)}\rightarrow\pi^{+} \phi(\mu^{+}\mu^{-})\) decays observed in LHCb with those obtained by the D0 experiment, which established the best limit on these modes so far [181]. With an integrated luminosity corresponding to 1.0 fb^{−1}, upper limits on the D^{+} (\(D ^{+}_{ s } \)) modes are expected close to 10^{−8} (10^{−7}) at 90 % C.L.
In analogy to the B sector, there is a wealth of observables potentially available in four-body rare decays of D mesons. In the decays D^{0}→hh′μ^{+}μ^{−} (with h^{(′)}=K or π), forward–backward asymmetries or asymmetries based on T-odd quantities could reveal NP effects [179, 182, 183]. Clearly the first challenge is to observe the decays which, depending on their branching fractions, may be possible with the 2011 data set. However, the 50 fb^{−1} collected by the upgraded LHCb detector will be necessary to exploit the full set of observables in these modes.
2.9 Rare kaon decays
The cross-section for \(K ^{0}_{\mathrm{S}} \) production at the LHC is such that \({\sim}10^{12} K ^{0}_{\mathrm{S}} \rightarrow \pi ^{+}\pi^{-}\) would be reconstructed and selected in LHCb with a fully efficient trigger. This provides a good opportunity to search for rare \(K ^{0}_{\mathrm{S}} \) decays in channels with high trigger efficiency, in particular \(K ^{0}_{\mathrm{S}} \rightarrow \mu ^{+}\mu^{-} \).
2.10 Lepton flavour and lepton number violation
The experimental observation of neutrino oscillations provided the first signature of lepton flavour violation (LFV). The consequent addition of mass terms for the neutrinos in the SM implies LFV also in the charged sector, but with branching fractions smaller than 10^{−40}. NP could significantly enhance the rates but, despite steadily improving experimental sensitivity, charged lepton flavour violating (cLFV) processes like μ^{−}→e^{−}γ, μ–N→e–N, μ^{−}→e^{+}e^{−}e^{−}, τ^{−}→ℓ^{−}γ and τ^{−}→ℓ^{+}ℓ^{−}ℓ^{−} (with ℓ^{−}=e^{−},μ^{−}) have not been observed. Numerous theories beyond the SM predict larger LFV effects in τ^{−} decays than μ^{−} decays, with branching fractions within experimental reach [188]. An observation of cLFV would thus be a clear sign for NP, while lowering the experimental upper limit will help to further constrain theories [189].
Another approach to search for NP is via lepton number violation (LNV). Decays with LNV are sensitive to Majorana neutrino masses—their discovery would answer the long-standing question of whether neutrinos are Dirac or Majorana particles. The strongest constraints on minimal models that introduce neutrino masses come from neutrinoless double beta decay processes, but searches in heavy flavour decays provide competitive and complementary limits in models with extended neutrino sectors.
In this section, LFV and LNV decays of τ leptons and B mesons with only charged tracks in the final state are discussed.
2.10.1 Lepton flavour violation
The neutrinoless decay τ^{−}→μ^{+}μ^{−}μ^{−} is a particularly sensitive mode in which to search for LFV at LHCb as the inclusive τ^{−} production cross-section at the LHC is large (∼80 μb, coming mainly from \(D ^{+}_{ s } \) decays^{23}) and muon final states provide clean signatures in the detector. This decay is experimentally favoured with respect to the decays τ^{−}→μ^{−}γ and τ^{−}→e^{+}e^{−}e^{−} due to the considerably better particle identification of the muons and better possibilities for background discrimination. LHCb has reported preliminary results from a search for the decay τ^{−}→μ^{+}μ^{−}μ^{−} using 1.0 fb^{−1} of data [191]. The upper limit on the branching fraction was found to be \({ \mathcal{B} }(\tau^{-} \rightarrow \mu^{+}\mu^{-}\mu^{-})<7.8~(6.3) \times10^{-8}\) at 95 % (90 %) C.L, to be compared with the current best experimental upper limit from Belle: \({ \mathcal{B} }(\tau^{-} \rightarrow \mu^{+}\mu^{-}\mu^{-})<2.1 \times10^{-8}\) at 90 % C.L. As the data sample increases this limit is expected to scale as the square root of the available statistics, with possible further reduction depending on improvements in the analysis. The large integrated luminosity that will be collected by the upgraded experiment will provide sensitivity corresponding to an upper limit of a few times 10^{−9}. Searches will also be conducted in modes such as \(\tau^{-} \rightarrow \bar{p} \mu^{+} \mu^{-}\) or τ^{−}→ϕμ^{−}, where the existing limits are much weaker, and low background contamination is expected in the data sample.^{24}
The pseudoscalar meson decays probe transitions of the type q→q′ℓℓ′ and hence are particularly sensitive to leptoquark-models and thus provide complementarity to leptonic decay LFV processes [193, 194]. For the LHCb experiment, both decays from D and B mesons are accessible. Sensitivity studies for the decays \(B^{0}_{(s)} \rightarrow e^{-} \mu^{+}\) and D^{0}→e^{−}μ^{+} are ongoing. Present estimates indicate that LHCb will be able to match the sensitivity of the existing limits from the B factories and CDF in the near future.
2.10.2 Lepton number violation
In lepton number violating B and D meson decays a search can be made for Majorana neutrinos with a mass of \(\mathcal{O}(1 ~\mathrm {GeV} )\). These indirect searches are performed by analysing the production of same sign charged leptons in D or B decays such as \(D ^{+}_{ s } \rightarrow \pi ^{-} \mu ^{+} \mu ^{+} \) or B^{+}→π^{−}μ^{+}μ^{+} [28, 195]. These same sign dileptonic decays can only occur via exchange of heavy Majorana neutrinos. Resonant production may be possible if the heavy neutrino is kinematically accessible, which could put the rates of these decays within reach of the future LHCb luminosity. Non-observation of these LNV processes, together with low energy neutrino data, would lead to better constraints for neutrino masses and mixing parameters in models with extended neutrino sectors.
Using 0.4 fb^{−1} of integrated luminosity from LHCb, limits have been set on the branching fraction of \(B^{+} \rightarrow D_{(s)}^{-} \mu ^{+} \mu ^{+} \) decays at the level of a few times 10^{−7} and on B^{+}→π^{−}μ^{+}μ^{+} at the level of 1×10^{−8} [196, 197]. These branching fraction limits imply a limit on, for example, the coupling |V_{μ4}| between ν_{μ} and a Majorana neutrino with a mass in the range 1<m_{N}<4 GeV/c^{2} of |V_{μ4}|^{2}<5×10^{−5}.
2.11 Search for NP in other rare decays
Many extensions of the SM predict weakly interacting particles with masses from a few MeV to a few GeV [198, 199, 200, 201, 202] and there are some experimental hints for these particles from astrophysical and collider experiments [203, 204]. For example, the HyperCP Collaboration has reported an excess of Σ^{+}→pμ^{+}μ^{−} events with dimuon invariant masses around 214 MeV/c^{2} [205]. These decays are consistent with the decay Σ^{+}→pX with the subsequent decay X→μ^{+}μ^{−}. Phenomenologically, X can be interpreted as a pseudoscalar or axial-vector particle with lifetimes for the pseudoscalar case estimated to be about 10^{−14} s [206, 207, 208]. Such a particle could, for example, be interpreted as a pseudoscalar sgoldstino [207] or a light pseudoscalar Higgs boson [209].
The LHCb experiment has recorded the world’s largest data sample of B and D mesons which provides a unique opportunity to search for these light particles. Preliminary results from a search for decays of \(B^{0}_{(s)} \rightarrow \mu^{+} \mu^{-} \mu^{+} \mu^{-} \) have been reported [210]. Such decays could be mediated by sgoldstino pair production [211]. No excess has been found and limits of 1.3 and 0.5×10^{−8} at 95 % C.L. have been set for the \(B ^{0}_{ s } \) and B^{0} modes respectively. The analysis can naturally be extended to D^{0}→μ^{+}μ^{−}μ^{+}μ^{−} decays, as well as \(B^{0}_{(s)} \rightarrow V^{0} \mu^{+} \mu^{-}\) (V^{0}=K^{(∗)0},ρ^{0},ϕ), where the dimuon mass spectrum can be searched for any resonant structure. Such an analysis has been performed by the Belle Collaboration [212]. With the larger data sample and flexible trigger of the LHCb upgrade, it will be possible to exploit several new approaches to search for exotic particles produced in decays of heavy flavoured hadrons (see, e.g. Ref. [213]).
3 CP violation in the B system
3.1 Introduction
CP violation, i.e. violation of the combined symmetry of charge conjugation and parity, is one of three necessary conditions to generate a baryon asymmetry in the Universe [214]. Understanding the origin and mechanism of CP violation is a key question in physics. In the SM, CP violation is fully described by the CKM mechanism [20, 21]. While this paradigm has been successful in explaining the current experimental data, it is known to generate insufficient CP violation to explain the observed baryon asymmetry of the Universe. Therefore, additional sources of CP violation are required. Many extensions of the SM naturally contain new sources of CP violation.
The b hadron systems provide excellent laboratories to search for new sources of CP violation, since new particles beyond the SM may enter loop-mediated processes such as b→q FCNC transitions with q=s or d, leading to discrepancies between measurements of CP asymmetries and their SM expectations. Two types of b→q FCNC transitions are of special interest: neutral B meson mixing (ΔB=2) processes, and loop-mediated B decay (ΔB=1) processes.
The LHCb experiment exploits the large number of b hadrons, including the particularly interesting \(B ^{0}_{ s } \) mesons, produced in proton–proton collisions at the LHC to search for CP-violating NP effects. Section 3.2 provides a review of the status and prospects in the area of searches for NP in \(B^{0}_{(s)}\) mixing, in particular through measurements of the mixing phases ϕ_{d(s)} and the semileptonic asymmetries \(a_{\rm sl}^{d(s)}\). The LHCb efforts to search for NP in hadronic b→s penguin decays, such as \(B ^{0}_{ s } \rightarrow \phi\phi\), are discussed in Sect. 3.3. Section 3.4 describes the LHCb programme to measure the angle γ of the CKM unitarity triangle (UT) in decay processes described only by tree amplitudes, such as B^{±}→DK^{±}, B^{0}→DK^{∗0} and \(B ^{0}_{ s } \rightarrow D ^{\mp}_{ s } K ^{\pm} \). These measurements allow precise tests of the SM description of quark-mixing via global fits to the parameters of the CKM matrix, as well as direct comparisons with alternative determinations of γ in decay processes involving loop diagrams, such as \(B ^{0}_{ s } \rightarrow K ^{+} K ^{-} \). At the end of each section, a brief summary of the most promising measurements with the upgraded LHCb detector and their expected/projected sensitivities is provided.
3.2 \(B^{0}_{(s)}\) mixing measurements
3.2.1 \(B^{0}_{(s)}\)–\(\overline{B}^{0}_{(s)}\) mixing observables
- the mass difference between the heavy and light mass eigenstateswhere \(\phi_{12}^{q} = \arg(-M^{q}_{12}/\varGamma^{q}_{12})\) is convention-independent;$$ \Delta m_q \equiv M^q_{\rm H}- M^q_{\rm L} \approx2\bigl|M^q_{12}\bigr| \biggl( 1-\frac{|\varGamma^q_{12}|^2}{8|M^q_{12}|^2}\sin^2\phi_{12}^q \biggr); $$(25)
- the decay width difference between the light and heavy mass eigenstates
- the flavour-specific asymmetry^{26}
The ϕ_{s} notation has been used in the LHCb measurements of the CP-violating phase in \(B ^{0}_{ s } \) mixing, using J/ψϕ [10, 139] and J/ψf_{0}(980) [218, 219] final states. By using the same notation for different decays, an assumption that \(\arg(\bar{A}_{f}/A_{f})\) is common for different final states is being made. This corresponds to an assumption that the penguin contributions to these decays are negligible. Although this is reasonable with the current precision, as the measurements improve it will be necessary to remove such assumptions–several methods to test the contributions of penguin amplitudes are discussed below. These include measuring ϕ_{q} with different decay processes governed by different quark-level transitions. Previous experiments have used the notation \(2\beta^{\rm eff}\) in particular for measurements based on \(b \rightarrow q\bar{q}s\) (q=u,d,s) transitions; for symmetry the notation \(2\beta^{\rm eff}_{s}\) is used in corresponding cases in the \(B ^{0}_{ s } \) system, although the cancellation of the mixing and decay phases in \(B ^{0}_{ s } \) decays governed by \(b \rightarrow q\bar {q}s\) amplitudes is expected to lead to a vanishing CP violation effect (within small theoretical uncertainties).
In the SM, the mixing observables can be predicted using CKM parameters from a global fit to other observables and hadronic parameters (decay constants and bag parameters) from lattice QCD calculation. These predictions can be compared to their direct measurements to test the SM and search for NP in neutral B mixing.
3.2.2 Current experimental status and outlook
Status of B mixing measurements and corresponding SM predictions. New results presented at ICHEP 2012 and later are not included. The inclusive same-sign dimuon asymmetry \(A^{b}_{\rm SL}\) is defined below and in Ref. [159]
Observable | Measurement | Source | SM prediction | References |
---|---|---|---|---|
\(B ^{0}_{ s } \) system | ||||
Δm_{s} (ps^{−1}) | 17.719±0.043 | HFAG 2012 [44] | 17.3±2.6 | |
17.725±0.041±0.026 | LHCb (0.34 fb^{−1}) [226] | |||
ΔΓ_{s} (ps^{−1}) | 0.105±0.015 | HFAG 2012 [44] | 0.087±0.021 | |
0.116±0.018±0.006 | LHCb (1.0 fb^{−1}) [139] | |||
ϕ_{s} (rad) | \(-0.044 \,^{+0.090}_{-0.085}\) | HFAG 2012 [44] | −0.036±0.002 | |
−0.002±0.083±0.027 | LHCb (1.0 fb^{−1}) [139] | |||
\(a^{s}_{\rm sl}\) (10^{−4}) | \(-17\pm91\,^{+14}_{-15}\) | D0 (no \(A^{b}_{\rm SL}\)) [227] | \(0.29\,^{+0.09}_{-0.08}\) | |
−105±64 | HFAG 2012 (including \(A^{b}_{\rm SL}\) ) [44] | |||
Admixture of B^{0} and \(B ^{0}_{ s } \) systems | ||||
\(A^{b}_{\rm SL}\) (10^{−4}) | −78.7±17.1±9.3 | D0 [159] | −2.0±0.3 | |
B^{0} system | ||||
Δm_{d} (ps^{−1}) | 0.507±0.004 | HFAG 2012 [44] | 0.543±0.091 | |
ΔΓ_{d}/Γ_{d} | 0.015±0.018 | HFAG 2012 [44] | 0.0042±0.0008 | |
sin2β | 0.679±0.020 | HFAG 2012 [44] | \(0.832\,^{+0.013}_{-0.033} \) | |
\(a^{d}_{\rm sl}\) (10^{−4}) | −5±56 | HFAG 2012 [44] | \(-6.5\,^{+1.9}_{-1.7}\) |
The HFAG average of the \(B ^{0}_{ s } \) mass difference Δm_{s} in Table 1 is based on measurements performed at CDF [228] and LHCb [226, 229]. It is dominated by the preliminary LHCb result obtained using 0.34 fb^{−1} of data [226], which is also given in Table 1. These are all consistent with the SM prediction. Improving the precision of the SM prediction is desirable to further constrain NP in \(M^{s}_{12}\), and requires improving the accuracy of lattice QCD evaluations of the decay constant and bag parameter (see Ref. [216] and references therein).
The LHCb \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) and \(B ^{0}_{ s } \rightarrow { J / \psi } \pi ^{+} \pi^{-}\) analyses discussed above only used opposite side flavour tagging [239, 240]. Future updates of these analyses will gain in sensitivity by also using the same side kaon tagging information, which so far has been used in a preliminary determination of Δm_{s} [226, 241]. Currently, the systematic uncertainty on ϕ_{s} is dominated by imperfect knowledge of the background, angular acceptance effects and by neglecting potential contributions of direct CP violation. All of these uncertainties are expected to be reduced with more detailed understanding and some improvements in the analysis. Therefore it is expected that the determination of ϕ_{s} will remain limited by statistical uncertainties, even with the data samples available after the upgrade of the LHCb detector. In addition to \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) and \(B ^{0}_{ s } \rightarrow { J / \psi } \pi ^{+} \pi^{-}\), other \(b \rightarrow c \overline{ c } s\) decay modes of \(B ^{0}_{ s } \) mesons, such as J/ψη, J/ψη′ [242] and \(D _{s}^{+} D _{s}^{-}\) [243] will be investigated. These decays have been measured at LHCb [244, 245].
The SM prediction ϕ_{s}=−0.036±0.002 rad could receive a small correction from doubly CKM-suppressed penguin contributions in the decay. The value of this correction is not precisely known, and may depend on the decay mode. Moreover, NP in the \(b \rightarrow c \overline{ c } s\) decay may also affect the results. Although such effects are already constrained by results from B^{+} and B^{0} decays, NP in the decay amplitudes can lead to polarisation-dependent mixing-induced CP asymmetries and triple product asymmetries in \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) [246]. Such effects will be searched for in future analyses.
The flavour-specific asymmetries provide important complementary constraints on ΔB=2 processes. The D0 collaboration has performed a direct measurement of \(a^{s}_{\rm sl}\) in semileptonic \(B ^{0}_{ s } \) decays [227], which is only weakly constraining.^{31} However, a measurement of the inclusive same-sign dimuon asymmetry provides better precision, and shows evidence of a large deviation from its SM prediction [159]. The inclusive measurement is sensitive to a linear combination of the flavour-specific asymmetries, \(A_{\rm SL}^{b} = C_{d} a_{\rm sl}^{d} + C_{s} a_{\rm sl}^{s}\), where C_{q} depend on the production fractions and mixing probabilities, and are determined to be C_{d}=0.594±0.022, C_{s}=0.406±0.022 [159].^{32} As discussed in Sect. 3.2.3, the D0 \(A_{\rm SL}^{b}\) result is in tension with other ΔB=2 observables. Improved measurements of \(a^{s}_{\rm sl}\) and \(a^{d}_{\rm sl}\) from LHCb are needed to solve this puzzle.
LHCb has already presented first results on Δm_{d} [229, 253] and sin2β [254]. The Δm_{d} result is the world’s most precise single measurement of this quantity, while the sensitivity on sin2β will be competitive with the B factory results using the data sample that will be collected by the end of 2012. LHCb can also search for enhancements in the value of ΔΓ_{d} above the tiny value expected in the SM, e.g. by comparing the effective lifetimes of \(B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} \) and B^{0}→J/ψK^{∗0} [255]. Significantly improving the precisions of the B^{0} mixing observables is an important goal of the LHCb upgrade, as will be discussed in Sect. 3.2.6.
The SM predictions of b-hadron lifetimes and ΔΓ_{q} are all obtained within the framework of the heavy quark expansion. LHCb is actively working on measurements of b-hadron lifetimes and lifetime ratios, which will be used to test these predictions. The knowledge obtained from this work will allow to improve the SM predictions of ΔΓ_{q} for the purpose of searching for NP. Furthermore, a more precise measurement of the ratio of \(B ^{0}_{ s } \) to B^{0} lifetimes could either support or strongly constrain the existence of NP in \(\varGamma^{s}_{12}\) [152, 153, 216, 220, 256].
3.2.3 Model independent constraints on new physics in B mixing
NP contributions to the absorptive part \(\varGamma_{12}^{q}\) of B mixing can enter through ΔB=1 decays b→qX with light degrees of freedom X of total mass below m_{B}. In some particular models such contributions can arise [154, 260] and interfere constructively or destructively with the SM contribution. The recent measurements of ΔΓ_{q} and of \(A_{\rm SL}^{b}\) revived interest in this possibility. Model-independent analyses have confirmed that the \(A_{\rm SL}^{b}\) measurement cannot be accommodated within the SM [261, 262]. A model-independent fit assuming NP in both \(M_{12}^{q}\) and \(\varGamma _{12}^{q}\) has been considered in the framework of an extended CKM fit [256]. In this case, the experimental data can be accommodated, and the \(B ^{0}_{ s } \) system remains rather SM-like, but large NP contributions in the B^{0} system are required.
Model-independent analyses based on Eq. (34) are restricted to a particular set of observables, mainly those with ΔB=2, since correlations with ΔB=1 measurements are difficult to quantify. Either additional assumptions on the nature of X in b→qX or explicit NP models will permit better exploitation of the wealth of future experimental information. In fact, such analyses have found it difficult to accommodate the hypothesis of large NP in \(\varGamma^{q}_{12}\) with current ΔB=1 measurements, therefore NP in \(\varGamma^{q}_{12}\) seems unlikely to provide a full explanation of the measured value of \(A_{\rm SL}^{b}\). In the case of \(X = f\bar{f}\), the ΔB=1 operators \(b \rightarrow (d,s) f\bar{f}\) (f=q or ℓ) are strongly constrained [152], with the exception of \(b \rightarrow s c \overline{ c } \) and b→sτ^{+}τ^{−}. Currently, only a weak upper bound on \({ \mathcal {B} } ( B ^{+} \rightarrow K ^{+} \tau ^{+} \tau ^{-} ) \lesssim 3.3 \times 10^{-3}\) at 90 % C.L. [263] exists whereas other decays \(B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} \), B→X_{s}τ^{+}τ^{−} might be indirectly constrained with additional assumptions (see also the discussion in Sect. 2.5.2). As an example, the improved LHCb measurement of \(\tau_{ B ^{0}_{ s } }/\tau _{ B ^{0} }\) allowed the derivation of a stronger bound on \({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow \tau ^{+} \tau ^{-} )\). Still, a model-independent analysis of the complete set of b→sτ^{+}τ^{−} operators does not allow for deviations larger than 35 % from the SM in \(\varGamma_{12}^{s}\) [153], which is much too small to resolve the tension with \(A_{\rm SL}^{b}\). For b→dτ^{+}τ^{−} operators there exists a stronger constraint \({ \mathcal{B} } ( B ^{0} \rightarrow \tau ^{+} \tau ^{-} ) \lesssim 4 \times 10^{-3}\) and even smaller NP effects are expected in \(\varGamma_{12}^{d}\). Other proposed solutions such as the existence of new light spin-0 [264] or spin-1 [265] X states could be seriously challenged by improved measurements of quantities, such as ratios of lifetimes, which are theoretically under good control [220].
In summary, NP contributions to |Δ_{q}| are already quite constrained due to Δm_{q} measurements and theoretical progress is required in order to advance. Although the phases \(\phi_{q}^{\Delta}\) are constrained by the recent LHCb measurement of ϕ_{s}, and B factory measurements of ϕ_{d}, there is a mild tension with the SM in model-independent fits of ΔB=2 measurements [153, 256, 261, 262], especially when allowing for NP in \(\varGamma_{12}^{q}\). On the other hand, NP effects in \(\varGamma_{12}^{q}\) are expected to be limited when constraints from ΔB=1 observables are taken into account. Independent improved measurements of \(a_{\rm sl}^{q}\) are needed in order to resolve the nature of the current discrepancies between the ΔB=2 observables with their SM expectations and other observables entering global CKM fits. Further, improved measurements of Γ_{q} and ΔΓ_{q}, as well as of control channels, are needed to constrain NP in \(\varGamma^{q}_{12}\).
3.2.4 CKM unitarity fits in SM and beyond
This section presents the results of the unitarity triangle (UT) analysis performed by two groups: UTfit [266] and CKMfitter [252].^{35} The main aim of the UT analysis is the determination of the values of the CKM parameters, by comparing experimental measurements and theoretical predictions for several observables. The popular Wolfenstein parametrisation allows for a transparent expansion of the CKM matrix in terms of the sine of the small Cabibbo angle, λ, with the other three parameters being A, \(\bar {\rho}\) and \(\bar{\eta}\). Assuming the validity of the SM, one can perform a fit to the available measurements. LHCb results already make important contributions to the constraints on γ and Δm_{s}. With more statistics, LHCb results are expected to impact on other CKM fit inputs, including α and sin2β. It is important to note the crucial role of lattice QCD calculations as input to the CKM fits. For example, the parameters \(f_{B_{s}}\sqrt{B_{B_{s}}}\) and ξ enter the constraints on Δm_{s} and Δm_{d}/Δm_{s}. At the end of 2011, the precision of the calculations was at the level of 5.4 % and 2.6 %, respectively [109]. The necessary further progress to obtain the full benefit of the LHCb measurements appears to be in hand exploiting algorithmic advances as well as ever increasing computing power for the lattice calculations.
Predictions for some parameters of the SM fit and their measurements as combined by the UTfit and CKMfitter groups. Note that the two groups use different input values for some parameters. The lines marked with (*) are not used in the full fit. Details of the pull calculation can be found in Refs. [259, 269]. New results presented at ICHEP2012 and later are not included in these analyses
Parameter | UTfit | CKMfitter | ||||
---|---|---|---|---|---|---|
Prediction | Measurement | Pull | Prediction | Measurement | Pull | |
α(^{∘}) | 87.5±3.8 | 91.4±6.1 | +0.5σ | \(95.9\,^{+2.2}_{-5.6}\) | \(88.7\,^{+2.2}_{-5.9}\) | −1.0σ |
sin2β | 0.809±0.046 | 0.667±0.024 | −2.7σ | \(0.820\,^{+0.024}_{-0.028}\) | 0.679±0.020 | −2.6σ |
γ(^{∘}) | 67.8±3.2 | 75.5±10.5 | +0.7σ | \(67.2\,^{+4.4}_{-4.6}\) | \(66\,^{+12}_{-12}\) | −0.1σ |
V_{ub}(10^{−3}) | 3.62±0.14 | 3.82±0.56 | +0.3σ | \(3.55\,^{+0.15}_{-0.14}\) | 3.92±0.09±0.45 | 0.0σ |
V_{cb}(10^{−3}) | 42.26±0.89 | 41±1 | −0.9σ | \(41.3\,^{+0.28}_{-0.11}\) | 40.89±0.38±0.59 | 0.0σ |
ε_{k}(10^{−3}) | 1.96±0.20 | 2.229±0.010 | +1.3σ | \(2.02\,^{+0.53}_{-0.52}\) | 2.229±0.010 | 0.0σ |
\(\Delta m_{s}\ ( {\rm ps}^{-1} )\) | 18.0±1.3 | 17.69±0.08 | −0.2σ | \(17.0\,^{+2.1}_{-1.5}\) | 17.731±0.045 | 0.0σ |
\({ \mathcal{B} } ( B \rightarrow \tau\nu)(10^{-4})\) | 0.821±0.0077 | 1.67±0.34 | +2.5σ | \(0.733\,^{+0.121}_{-0.073}\) | 1.68±0.31 | +2.8σ |
\(\beta_{s}\ \rm rad \) (*) | 0.01876±0.0008 | \(0.01822\,^{+0.00082}_{-0.00080}\) | ||||
\({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow \mu\mu)(10^{-9})\) (*) | 3.47±0.27 | \(3.64\,^{+0.21}_{-0.32}\) |
In order to estimate the origin of the tensions, the UTfit and CKMfitter groups have performed analyses including model-independent NP contributions to neutral meson mixing processes (see Refs. [256, 270] for details). The NP effects are introduced through the real valued C and ϕ parameters (\(A_{\rm NP} = C e^{i\phi} A_{\rm SM}\)) in case of UTfit and the complex valued Δ parameter (\(A_{\rm NP} = \Delta A_{\rm SM}\)) for CKMfitter. The parameters are added separately for the \(B ^{0}_{ s } \) and B^{0} sectors. In the absence of NP, the expected values are C=1, ϕ=0^{∘}, and Δ=1. For the B^{0} sector the fits return C=0.94±0.14 and ϕ=(−3.6±3.7)^{∘}, and \(\Delta= (0.823\,^{+0.143}_{-0.095})+i(-0.199\,^{+0.062}_{-0.048})\). The results for both groups show some disagreement with the SM, driven by tensions in the input parameters mentioned above. In the \(B ^{0}_{ s } \) sector, on the other hand, the situation is much closer to the SM than before the LHCb measurements were available: C=1.02±0.10 and ϕ=(−1.1±2.2)^{∘}, and \(\Delta= (0.92\,^{+0.13}_{-0.08})+i(0.00 \pm0.10)\).
The results of the studies by both groups point to the absence of big NP effects in ΔB=2 processes. Nevertheless there is still significant room for NP in mixing in both B^{0} and \(B ^{0}_{ s } \) systems. More precise results, in particular from LHCb, can enable more careful studies. Besides providing null tests of the SM hypothesis, improved ϕ_{s} and \(a_{\rm sl}^{s}\) measurements are crucial to quantity effects of NP in mixing. In addition a precise γ determination is essential, not only for a SM global consistency test, but also to fix the apex of the UT in the extended fits.
3.2.5 Penguin pollution in \(b \rightarrow c \overline { c } s \) decays
In addition to the very clear experimental signature, precise determination of the B^{0} and \(B ^{0}_{ s } \) mixing phases is possible due to the fact that in the “golden modes”, \(B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} \) and \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\), explicit calculation of the relevant matrix elements can be avoided, once subleading doubly Cabibbo-suppressed and loop-suppressed terms are assumed to vanish [271]. Estimates yield corrections of the order O(10^{−3}) only [272, 273, 274]; it is however notoriously difficult to actually calculate the relevant matrix elements, and non-perturbative enhancements cannot be excluded. Given the future experimental precision for these and related modes, a critical reconsideration of this assumption is mandatory.
The main problem lies in the fact that once the assumption of negligible penguin contributions is dropped, the evaluation of hadronic matrix elements again becomes necessary, which still does not seem feasible to an acceptable precision for the decays in question. To avoid explicit calculation, symmetry relations can be used, exploiting either flavour SU(3) or U-spin symmetry [275, 276, 277, 278, 279, 280, 281]. Without taking into account any QCD evaluation and only using control channels to estimate the size of the penguin amplitude, the analyses in Refs. [278, 281] still allow a phase shift of up to a few degrees for ϕ_{d}, which would correspond to a very large non-perturbative enhancement of the penguin size. In Ref. [278] a negative sign is preferred which (slightly) reduces the tension in the unitarity triangle fit shown in Fig. 12. The reason for the large allowed range of the shift of ϕ_{d} is due to the limited precision to which the corresponding control channels B^{0}→J/ψπ^{0} and \(B ^{0}_{ s } \rightarrow { J / \psi } K ^{0} \), which are Cabibbo-suppressed compared to the golden modes, are known. For ϕ_{s}, an analogous analysis [277] cannot yet constrain the penguin contribution, due to the lack of a B→J/ψV control channel data for \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\). However, in principle the effects in the B→J/ψV modes are expected to be of the same order of magnitude as in the B→J/ψP modes. The control channel \(B ^{0}_{ s } \rightarrow { J / \psi } K^{*0}\) has already been observed at CDF [282] and LHCb [283], and work is ongoing to measure its decay rate, polarisations and direct CP asymmetries. This will enable the first direct constraint on the shift of ϕ_{s} due to penguin contributions in the decay \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\).^{36} For \(B ^{0}_{ s } \rightarrow { J / \psi } f_{0}(980)\) there is an additional complication due to the unknown hadronic structure of the f_{0}(980) [235].
In addition to insufficient data, there are, at present, theoretical aspects limiting the precision of this method at present, the most important of which is the violation of SU(3) symmetry. Regarding the B^{0} mixing phase, a full SU(3) analysis can be performed [285] (instead of using only one control channel) to be able to model-independently include SU(3) breaking. The inclusion of SU(3)-breaking contributions is important: their neglect can lead to an overestimation of the subleading effects. Including recent data for two of the relevant modes [286, 287], the analysis shows that the data are at the moment actually compatible with vanishing penguin contributions, with SU(3)-breaking contributions of the order 20 %. Including the penguin contributions, an upper limit on the shift of the mixing-induced CP asymmetry ΔS=sinϕ_{d}−sin2β is derived: |ΔS|≲0.01, with a negative sign for ΔS slightly preferred.^{37} This is the most stringent limit available, despite the more general treatment of SU(3) breaking. In this analysis still some (conservatively chosen) theoretical inputs are needed to exclude fine-tuned solutions: SU(3)-breaking effects have been restricted to at most 40 % for a few parameters which are not well determined by the fit and also have only small influence on the CP violation observables, and the penguin matrix elements are constrained to be at most 50 % of the leading contributions. Importantly, these theory inputs can be replaced by experimental measurements, namely of the CP asymmetries in the decay \(B ^{0}_{ s } \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} \), the decay rate of which has already been measured at LHCb [287] after its observation at CDF [282]. Furthermore, data from all the corresponding modes (i.e. B_{d,u,s}→J/ψP, with light pseudoscalar meson P=π,η^{(′)} or K) can be used to determine the shift more precisely, i.e. the related uncertainty is not irreducible, but can be reduced with coming data.
Turning to the second golden mode, \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\), in general, the absolute shift is not expected to be larger than in the B^{0} case. At the moment the data are not yet available to make a comparable analysis. While the penguin decay mode \(B ^{0}_{ s } \rightarrow \phi\phi\) is not related by symmetry with \(B ^{0}_{ s } \rightarrow { J / \psi } \phi \), comparing their decay rates indicates that the penguin contributions are small, and there are no huge enhancements to be expected for the penguin matrix elements in question.
Nonetheless, a quantitative analysis will ultimately be warranted here as well. In principle, these methods can be adapted to extract the \(B ^{0}_{ s } \) mixing phase including penguin contributions and model-independent SU(3) breaking, thereby improving the method proposed in Ref. [277]. The corresponding partners of the golden mode \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) are all the decays B_{u,d,s}→J/ψV, with the light vector mesons V=K^{∗}, ρ, ϕ or ω. However, the complete analysis requires results on the polarisation fractions and CP asymmetries for each of these final states, and for some of them the experimental signature is quite challenging. In addition, the ϕ meson is a superposition of octet and singlet, therefore the “control channels” involving K^{∗} and ρ are not as simply related as in the case with a pseudoscalar meson, but require the usage of nonet symmetry, whose precision has to be investigated in turn.
Nevertheless, significant progress can be expected. Several B→J/ψV modes, including \(B^{0}_{(s)} \rightarrow { J / \psi } K^{*0}\) [283], are being studied at LHCb. While measurements of the modes involving b→d transitions are expected to exhibit rather large uncertainties at first, the advantage of the proposed method is the long “lever arm” due to the relative enhancement ∼1/λ^{2} in the control channels, so that even moderate precision will be very helpful.
3.2.6 Future prospects with LHCb upgrade
LHCb measurements of ϕ_{s}. The quoted uncertainties are statistical and systematic, respectively
Final state | Current value (rad) with 1.0 fb^{−1} | Projected uncertainty (50 fb^{−1}) |
---|---|---|
J/ψϕ | −0.001±0.101±0.027 | 0.008 |
J/ψπ^{+}π^{−} | \(-0.019\,^{+0.173}_{-0.174}\,^{+0.004}_{-0.003} \) | 0.014 |
Both | −0.002±0.083±0.027 | 0.007 |
As discussed in Sect. 3.2.5, contributions from doubly CKM-suppressed SM penguin diagrams could have a non-negligible effect on the mixing-induced CP asymmetry and bias the extracted value of ϕ_{s}. Naive estimates of the bias are of the order O(10^{−3}) only [272, 273, 274], but this must be examined with experimental data using flavour symmetries to exploit control channels. LHCb can perform an SU(3) analysis using measurements of the decays rates and CP asymmetries in \(B ^{0}_{ s } \rightarrow { J / \psi } K ^{*0} \), B^{0}→J/ψρ^{0} and B^{0}→J/ψϕ as control channels for \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\). The necessary high precision can only be reached using the large data sample that will be collected with the upgraded LHCb detector. The 50 fb^{−1} data sample will also allow to measure ϕ_{s} in the penguin-free (\(b \rightarrow c \bar{u} s / u \bar{c} s\)) \(B ^{0}_{ s } \rightarrow D \phi \) decay [288, 289].
Another important goal is a more precise determination of sin2β in the B^{0} system, motivated by the tension between the direct and indirect determinations of sin2β seen by both UTfit and CKMfitter groups, as shown in Table 2. With the upgraded detector, using the \(B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} \) final state alone, a statistical precision of ±0.006 is expected, to be compared to the current error from the B factories of ±0.023 [190]. Given experience with the current detector it seems feasible to control the systematic uncertainties to a similar level. Such precision, together with better control of the penguin pollution, will allow us to pin down any NP effects in B^{0} mixing. In addition, the penguin-free (\(b \rightarrow c \bar{u} d / u \bar{c} d\)) B^{0}→Dρ^{0} channel can be used to get another handle on sin2β [290, 291].
The importance of improved measurements of ΔΓ_{q} has been emphasised in Sects. 3.2.1–3.2.3. LHCb has made a preliminary measurement of ΔΓ_{s} in \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\) using a 1.0 fb^{−1} data sample [139]. The effective lifetime of \(B ^{0}_{ s } \rightarrow { J / \psi } f_{0}(980)\) [292] has also been measured [236]. Based on this, the statistical precision on ΔΓ_{s} with 50 fb^{−1} is projected to be ∼0.003 ps^{−1}. It is hoped that the systematic uncertainty can be controlled to the same level.
A measurement of ΔΓ_{d} is of interest as any result larger than the tiny value expected in the SM would clearly signal NP [154, 255, 293]. To determine this quantity, LHCb will compare the effective lifetimes of the two decay modes \(B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} \) with B^{0}→J/ψK^{∗0}. The estimated precision for 1.0 fb^{−1} is ∼0.02 ps^{−1}. With the upgraded detector and 50 fb^{−1} a statistical precision of ∼0.002 ps^{−1} on ΔΓ_{d} can be achieved. The systematic uncertainty is under study.
3.3 CP violation measurements with hadronic b→s penguins
3.3.1 Probes for new physics in penguin-only \(b \rightarrow s q \bar{q}\) decays
Direct CP asymmetries. In the SM \(b \rightarrow s q \overline{ q } \) decays are dominated by the penguin diagram with an internal top quark. As a consequence, the direct CP asymmetry is expected to be small. If there is a NP amplitude with comparable size interfering with the SM amplitude, and it has different strong and weak phases than the SM amplitude, a much larger direct CP asymmetry can arise.
- Polarisation and triple product asymmetries. For B decays into two vector mesons V_{1} and V_{2}, followed by vector to two pseudoscalar decays \(V_{1} \rightarrow P_{1} P^{\prime}_{1}\) and \(V_{2} \rightarrow P_{2} P^{\prime}_{2}\), there are three transversity states, labelled “longitudinal” (0), “perpendicular” (⊥) and “parallel” (∥). Measurements of the fractions of the total decay rate in each of these states, which correspond to determinations of the polarisation in the final state, provide useful information about the chiral structure of the electroweak currents, as well about non-perturbative effects such as rescattering and penguin annihilation. In the SM, the decay to each transversity state is dominated by a single amplitude with magnitude |A_{j}|, weak phase Φ_{j} and strong phase δ_{j}. The CP-violating observables \(\operatorname{Im} (A_{\perp}A^{*}_{j}-{\bar{A}}_{\perp }{\bar{A}}^{*}_{j})\) are then The values of these observables are tiny since in the SM the weak phases are the same to a very good approximation, but \(\operatorname{Im} (A_{\perp }A^{*}_{j}-{\bar{A}}_{\perp}{\bar{A}}^{*}_{j})\) can significantly differ from zero if there is a sizeable CP-violating NP contribution in the loop.These observables can be extracted from the differential distributions in terms of the angles θ_{1}, θ_{2} and ϕ, where θ_{1} (θ_{2}) is the polar angle of P_{1} (P_{2}) in the rest frame of V_{1} (V_{2}) with respect to the opposite of the direction of motion of the B meson, and ϕ is the angle between the decay planes of \(V_{1} \rightarrow P_{1}P^{\prime}_{1}\) and \(V_{2} \rightarrow P_{2} P^{\prime}_{2}\) in the rest frame of the B meson. The two observables can also be related to two triple product asymmetries for CP-averaged decays^{39} which are equal to asymmetries between the number of events with positive and negative values of U=sin2ϕ and \(V={\rm sign}(\cos \theta_{1} \cos\theta_{2})\sin\phi\): A review of this subject can be found in Ref. [298] and references therein.
Mixing-induced CP asymmetries. Mixing-induced CP asymmetries in \(b \rightarrow s q \overline{ q } \) decays of neutral B to CP eigenstates are precisely predicted. Due to the fact that the penguin diagram with an internal top quark is expected to dominate, the values of \(2\beta^{\rm eff}\) determined using \(B ^{0} \rightarrow \phi K ^{0}_{\mathrm{S}} \), \(B ^{0} \rightarrow \eta^{\prime} K ^{0}_{\mathrm{S}} \), \(B ^{0} \rightarrow f_{0}(980) K ^{0}_{\mathrm{S}} \), etc., are all expected to give ≈2β (see, e.g. Refs. [299, 300] and the discussion in Ref. [44]). Similarly, the values of \(2\beta_{s}^{\rm eff}\) determined from \(B ^{0}_{ s } \rightarrow \phi\phi\), \(B ^{0}_{ s } \rightarrow K^{*0} {\overline{K}}^{*0}\), etc., are expected to vanish due to cancellation of weak phases between mixing (top box) and decay (top penguin) amplitudes. Higher order corrections from subleading diagrams are expected to be small compared to the precision that can be achieved in the near-term, but further theoretical studies will be needed as the upgrade era approaches. NP with a flavour structure different from the SM will alter these CP asymmetries through the decay amplitudes, even if there is no NP in B mixing. A number of quasi-two-body or three-body decay modes can be studied.
Correlations between direct and mixing-induced asymmetries. Penguin-only decay modes are particularly interesting as the difference between formal “tree” and “penguin” contributions boils down to a difference in the quark-flavour running in the loop of the penguins. This difference, dominated by short distances, can be assessed accurately using QCD factorisation, and it can be used to correlate the branching ratio and the CP asymmetries of penguin-mediated modes. As discussed in Refs. [138, 301, 302], these observables can be correlated not only within the SM, but can also be used to extract the \(B ^{0}_{ s } \) mixing phase even in the presence of NP affecting only this phase.
3.3.2 Current status and outlook of LHCb measurements
3.3.3 Future prospects with LHCb upgrade
There are several more NP probes in \(b \rightarrow s q \overline{ q } \) decays that can be exploited at LHCb and its upgrade, such as mixing-induced CP asymmetries and triple product asymmetries in both \(B ^{0}_{ s } \rightarrow \phi\phi\) and \(B ^{0}_{ s } \rightarrow K ^{*0} \overline { K }{} ^{*0} \) decays. The statistical precision of \(\phi_{s}^{\rm eff}\) with each channel is estimated to be 0.3–0.4 rad for 1.0 fb^{−1}. The projected precision for 50 fb^{−1} is about 0.03 rad each. This can be compared with the uncertainties of their SM predictions of about 0.02 rad. It is also possible to perform a combined analysis of \(B ^{0}_{ s } \rightarrow K ^{*0} \overline{ K }{} ^{*0} \) and its U-spin related channel \(B ^{0} \rightarrow K ^{*0} \overline { K }{} ^{*0} \), which will put strong constraint on the subleading penguin diagrams in \(B ^{0}_{ s } \rightarrow K ^{*0} \overline { K }{} ^{*0} \), thus further reducing the theoretical uncertainty in the measurement of \(\phi^{\rm eff}_{s}\) [307, 308]. The statistical precision of A_{U} and A_{V} is estimated to be about 0.004, compared with an upper bound of 0.02 on their possible sizes in the SM [298].
Current and projected precisions of the key observables in \(b \rightarrow s q \overline{ q } \) decays
Observable | Current | LHCb upgrade (50 fb^{−1}) | Theory uncertainty |
---|---|---|---|
\(A_{U,V}( B ^{0}_{ s } \rightarrow \phi\phi)\) | 0.04 (LHCb 1.0 fb^{−1}) | 0.004 | 0.02 [309] |
\(\phi^{\rm eff}_{s}( B ^{0}_{ s } \rightarrow \phi\phi)\) | – | 0.03 | 0.02 [306] |
\(\phi^{\rm eff}_{s}( B ^{0}_{ s } \rightarrow K^{*0} \overline{K}^{*0})\) | – | 0.03 | 0.02 [306] |
\(\sin2\beta^{\rm eff}( B ^{0} \rightarrow \phi K ^{0}_{\mathrm{S}} )\) | 0.12 (B factories) | 0.06 | 0.02 [179] |
3.4 Measurements of the CKM angle γ
3.4.1 Measurements of γ using tree-mediated decays
The CKM angle γ, defined as the phase \(\gamma= \arg [ -V_{ud}V_{ub}^{*}/\allowbreak (V_{cd}V_{cb}^{*}) ]\), is one of the angles of the unitarity triangle formed from the hermitian product of the first (d) and third (b) columns of the CKM matrix V. It is one of the least well known parameters of the quark mixing matrix. However, since it can be determined entirely through decays of the type B→DK^{40} that involve only tree amplitudes—an unusual, even unique, property amongst all CP violation parameters—it provides a benchmark measurement. The determination from tree level decays has essentially negligible theoretical uncertainty, at the level of \(\delta\gamma/\gamma= \mathcal{O}(10^{-6})\), as will be shown in the next section. This makes γ a very appealing “standard candle” of the CKM sector. It serves as a reference point for comparison with γ values measured from loop decays (see Sect. 3.4.4).
Moreover, the determination of γ is crucial to improve the precision of the global CKM fits, and resulting limits on (or evidence for) NP contributions (see Sect. 3.2.4). In particular, the measurement of Δm_{d} and the oscillation phase sin2β in B^{0}–\(\overline{ B }{} ^{0} \) mixing can be converted to a measurement of γ (in the SM). This can be compared to the reference value from B→DK—their consistency verifies that the Kobayashi–Maskawa mechanism of CP violation is the dominant source in quark flavour-changing processes. Existing measurements provide tests at the level of \({\mathcal{O}}(10~\% )\), but improving the precision to search for smaller effects of NP is well motivated.
Several established methods to measure γ in tree decays exploit the B^{−}→D^{(∗)}K^{(∗)−} decays. They are based on the interference between the b→u and b→c tree amplitudes, which arises when the neutral D meson is reconstructed in a final state accessible to both D^{0} and \(\overline{ D }{} ^{0} \) decays. The interference between the amplitudes results in observables that depend on their relative weak phase γ. Besides γ they also depend on hadronic parameters, namely the ratio of magnitudes of amplitudes r_{B}≡|A(b→u)/A(b→c)| and the relative strong phase δ_{B} between the two amplitudes. These hadronic parameters depend on the B decay under investigation. They can not be precisely calculated from theory (see, however, Ref. [310]), but can be extracted directly from data by simultaneously reconstructing several different D final states.
singly Cabibbo-suppressed (SCS) decays (the GLS method [316]).
The best sensitivity to γ obviously comes from combining the results of all different analyses. This not only improves the precision on γ, but provides additional constraints on the hadronic parameters. It also allows one to overcome the fact that CP-odd final states such as \(K ^{0}_{\mathrm{S}} \pi ^{0} \) are not easily accessible in LHCb’s hadronic environment.
Besides the established methods based on direct CP violation in B→DK decays, it is also possible to measure γ using time-dependent analyses of neutral B^{0} and \(B ^{0}_{ s } \) tree decays [328, 329, 330]. The method still relies on the interference of b→u and b→c amplitudes, but interference is achieved through B^{0} (\(B ^{0}_{ s } \)) mixing. Thus one measures the sum of γ and the mixing phase, namely γ+2β and γ−2β_{s} in the B^{0} and \(B ^{0}_{ s } \) systems, respectively. Since both sin2β and β_{s} are becoming increasingly well measured, these measurements provide sensitivity to γ.
The LHCb experiment has the necessary decay time resolution, tagging power and access to large enough signal yields to perform this time-dependent CP measurement.^{42} The signal yields can be seen from the measurement of \({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow D ^{\mp }_{ s } K ^{\pm} )\) [140] (see Sect. 3.4.3 below). The identification of the initial flavour of the signal \(B ^{0}_{ s } \) candidate can be done combining both the responses of opposite-side and same-side kaon tagging algorithms, as is planned for other measurements of mixing-induced CP-violation in \(B ^{0}_{ s } \) decays, and has already been implemented in the preliminary analysis of \(B ^{0}_{ s } \rightarrow D^{-}_{s} \pi^{+}\) decays [226].
3.4.2 Theoretical cleanliness of γ from B→DK decays
Existing measurements place strong constraints on tree-level NP effects, yet the possibility of discoveries in this sector in the near term is not ruled out. In the far future, with much larger statistics, the measurement of γ is well suited to search for high scale NP since it is theoretically very clean. For example, NP with contributions of different chirality could give different shifts in γ, so the above test is meaningful.
A useful question to ask is, what is the energy scale that could be probed in principle? To answer this, the irreducible theoretical uncertainty in the determination of γ must be estimated. There are several sources that can induce a bias in the determination of γ from B→DK decays. However, most of these can be avoided, either (i) with more statistics (for example, the Dalitz plot model uncertainty where a switch to a model-independent method is possible), or (ii) by modifying the equations used to determine γ (an example is to correct for effects of D^{0}–\(\overline{ D }{} ^{0} \) mixing [337, 338]). The remaining, irreducible, theory uncertainties are then from the electroweak corrections.
The size of this effect is estimated by integrating over both t and b at the same time. The electroweak corrections in the effective theory are then described by a local operator whose matrix elements are easier to estimate. Although the Wilson coefficient of the operator contains large logarithms, log(m_{b}/m_{W}), for \({\mathcal{O}}(1)\) estimates, the precision obtained without resummation is sufficient. If one resums log(m_{b}/m_{W}) then nonlocal contributions are also generated. As a rough estimate only the local contributions need be kept. The irreducible theory error on γ is conservatively estimated to be \(\delta\gamma/\gamma<{\mathcal{O}}(10^{-6})\) (most likely it is even \(\delta\gamma/\gamma\lesssim{\mathcal{O}}(10^{-7})\)).
Ultimate NP scales that can be probed using different observables listed in the first column. They are given by saturating the theoretical errors given respectively by (1) δγ/γ=10^{−6}, (2) optimistically assuming no error on f_{B}, so that the ultimate theoretical error is only from electroweak corrections, (3) using SM predictions in Ref. [31], (4) optimistically assuming perturbative error estimates δβ/β 0.1 % [339], and (5) from bounds for \(\operatorname{Re} C_{1} ( \operatorname{Im} C_{1})\) from UTfitter [270]
Probe | \(\varLambda_{\rm NP}\) for (N)MFV NP | \(\varLambda_{\rm NP}\) for gen. FV NP |
---|---|---|
γ from B→DK^{(1)} | \(\varLambda\sim{\mathcal{O}}(10^{2} ~\mathrm{TeV} )\) | \(\varLambda\sim{\mathcal{O}}(10^{3} ~\mathrm{TeV} )\) |
B→τν^{(2)} | \(\varLambda\sim{\mathcal{O}}(1 ~\mathrm{TeV} )\) | \(\varLambda\sim{\mathcal{O}}(30 ~\mathrm{TeV} )\) |
\(b \rightarrow ss\bar{d}^{(3)}\) | \(\varLambda\sim{\mathcal{O}}(1 ~\mathrm{TeV} )\) | \(\varLambda\sim{\mathcal{O}}(10^{3} ~\mathrm{TeV} )\) |
β from \(B \rightarrow { J / \psi } K {}^{0}_{\mathrm{S}} {}^{(4)}\) | \(\varLambda\sim{\mathcal{O}}(50 ~\mathrm{TeV} )\) | \(\varLambda\sim{\mathcal{O}}(200 ~\mathrm{TeV} )\) |
\(K\mbox{--}\overline{K}\) mixing^{(5)} | Λ>0.4 TeV (6 TeV) | Λ>10^{3(4)} TeV |
Since an experimental precision of δγ/γ∼10^{−6} is not achievable in the near future, the NP scale reach must be adjusted for more realistic data sets. This is easily done, since the scale \(\varLambda_{\rm NP}\) probed goes as the fourth root of the yield. With the LHCb upgrade, an uncertainty of <1^{∘} on γ can be achieved (see Sect. 3.4.6), so that NP scales approaching \(\varLambda_{\rm NP}\sim5 (50) ~\mathrm {TeV} \) can be probed for MFV (general FV) NP.
3.4.3 Current LHCb experimental situation
First results from LHCb in this area include a measurement using B^{−}→DK^{−} with the GLW and ADS final states [6].^{43} A measurement of the branching ratio of \(B ^{0}_{ s } \rightarrow D^{\mp}_{s} K ^{\pm} \) has also been performed [140]. Several other analyses, including studies of GGSZ-type final states, are in progress.^{44}
These measurements all share common selection strategies. They benefit greatly from boosted decision tree algorithms, which combine up to 20 kinematic variables to effectively suppress combinatorial backgrounds. Charmless backgrounds are suppressed by exploiting the large forward boost of the \(D_{(s)}^{+}\) meson through a cut on its flight distance.
The analysis of the \(B ^{0}_{ s } \rightarrow D^{\mp}_{s} K ^{\pm} \) decay mode [140] is based on a sample corresponding to an integrated luminosity of 0.37 fb^{−1}, collected in 2011 at a centre-of-mass energy of \(\sqrt{s} = 7 ~\mathrm{TeV} \). This decay mode has been observed by the CDF [347] and Belle [348] Collaborations, who measured its branching fraction with an uncertainty around 23 % [190]. In addition to \(B^{0}_{s} \rightarrow D^{\mp}_{s} K^{\pm} \), the channels B^{0}→D^{−}π^{+} and \(B^{0}_{s} \rightarrow D^{-}_{s} \pi^{+} \) are analysed. They are characterised by a similar topology and therefore are good control and normalisation channels. Particle identification criteria are used to separate the CF decays from the suppressed modes, and to suppress misidentified backgrounds.
3.4.4 Measurements of γ using loop-mediated two-body B decays
CP violation in \(B^{0}_{(s)}\) decays plays a fundamental role in testing the consistency of the CKM paradigm in the SM and in probing virtual effects of heavy new particles.
With the advent of the B factories, the Gronau–London (GL) [350] isospin analysis of B→ππ decays has been a precious source of information on the phase of the CKM matrix. Although the method allows a full determination of the weak phase and of the relevant hadronic parameters, it suffers from discrete ambiguities that limit its constraining power. It is however possible to reduce the impact of discrete ambiguities by adding information on hadronic parameters [351, 352]. In particular, as noted in Refs. [353, 354, 355], the hadronic parameters entering the B^{0}→π^{+}π^{−} and the \(B ^{0}_{ s } \rightarrow K^{+}K^{-}\) decays are connected by U-spin, so that experimental knowledge of \(B ^{0}_{ s } \rightarrow K^{+} K^{-}\) can improve the extraction of the CKM phase with the GL analysis. Indeed, in Ref. [352], the measurement of \({ \mathcal{B} } ( B ^{0}_{ s } \rightarrow K^{+} K^{-})\) was used to obtain an upper bound on one of the hadronic parameters.
Experimental data on B→ππ and \(B ^{0}_{ s } \rightarrow K ^{+} K ^{-} \) decays. The correlation column refers to that between S_{f} and C_{f} measurements. Except for the preliminary results in Ref. [356], all other measurements have been averaged by HFAG [44]. The CP asymmetry of B^{+}→π^{+}π^{0} has been reported for completeness, although it has not been used in the analysis. New results on time-dependent CP violation in B^{0}→π^{+}π^{−} reported by Belle at CKM2012 [358] are not included
Channel | \({ \mathcal{B} } \times10^{6}\) | S_{f} (%) | C_{f} (%) | Corr. | Ref. |
---|---|---|---|---|---|
B^{0}→π^{+}π^{−} | 5.11±0.22 | −65±7 | −38±6 | −0.08 | |
B^{0}→π^{+}π^{−} | – | −56±17±3 | −11±21±3 | 0.34 | [356] |
B^{0}→π^{0}π^{0} | 1.91±0.23 | – | −43±24 | – | |
B^{+}→π^{+}π^{0} | 5.48±0.35 | – | −2.6±3.9 | – | |
\(B ^{0}_{ s } \rightarrow K^{+} K^{-}\) | 25.4±3.7 | 17±18±5 | −2±18±4 | 0.1 |
The LHCb preliminary results on direct and mixing-induced CP violation parameters in B^{0}→π^{+}π^{−} and \(B ^{0}_{ s } \rightarrow K^{+}K^{-}\) decays [356] are shown in Table 6. The measurements of \(C_{ \pi ^{+} \pi ^{-} }\) and \(S_{ \pi ^{+} \pi ^{-} }\) are compatible with those from the B factories, whereas \(C_{ K ^{+} K ^{-} }\) and \(S_{ K ^{+} K ^{-} }\) are measured for the first time and are consistent with zero within the current uncertainties.
Beyond the SM, NP can affect both the \(B^{0}_{(s)}\)–\(\overline{B}^{0}_{(s)}\) amplitudes and the b→d(s) penguin amplitudes. Taking the phase of the mixing amplitudes from other measurements, for example from \(b \rightarrow c \bar{c} s\) decays, one can obtain a constraint on NP in b→s (or b→d) penguins. Alternatively, assuming no NP in the penguin amplitudes, one can obtain a constraint on NP in mixing. The analysis discussed here is based on a simplified framework [357], using as input values sin2β=0.679±0.024 [44] and 2β_{s}=(0±5)^{∘} [139] obtained from \(b \rightarrow c \bar{c} s\) decays. The optimal strategy will be to include the combined GL and Fleischer analysis in a global fit of the CKM matrix plus possible NP contributions.
In Fig. 20 the PDF for γ obtained with the Fleischer method for two different values of the U-spin breaking parameter κ=0.1,0.5 is shown. The method is very precise for small amounts of U-spin breaking (κ=0.1), but becomes clearly worse for κ=0.5. Thus, a determination of γ from the Fleischer method alone is subject to uncertainty on the size of U-spin breaking.
Finally, \(B ^{0}_{ s } \rightarrow KK\) decays can also be used to extract 2β_{s} in the SM. The optimal choice in this respect is represented by \(B ^{0}_{ s } \rightarrow K^{(*)0} \overline{K}^{(*)0}\) (with \(B^{0} \rightarrow K^{(*)0} \overline{K}^{(*)0}\) as U-spin related control channels to constrain subleading contributions), since in this channel there is no tree contribution proportional to e^{iγ} [307, 308]. However, the combined analysis described above, in the framework of a global SM fit, can serve for the same purpose. To illustrate this point, the GL+F analysis is performed, taking as input the SM fit result γ=(69.7±3.1)^{∘} [270] and not using the measurement of 2β_{s} from \(b \rightarrow c \bar{c} s\) decays. In this way, 2β_{s}=(3±14)^{∘} is obtained for κ=0.5. The analysis can also be performed without using the measurement of γ, in this case the result is 2β_{s}=(6±14)^{∘}. With improved experimental accuracy, this determination could become competitive with that from \(b \rightarrow c \bar{c} s\) decays. Once results of time-dependent analyses of the \(B^{0}_{(s)} \rightarrow K^{(*)0} \overline{K}^{(*)0}\) channels are available these may also provide useful constraints.^{48}
To conclude, the usual GL analysis to extract α from B^{0}→ππ can be supplemented with the inclusion of the \(B ^{0}_{ s } \rightarrow K^{+} K^{-}\) modes, in the framework of a global CKM fit. The method optimises the constraining power of these decays and allows the derivation of constraints on NP contributions to penguin amplitudes or on the \(B ^{0}_{ s } \) mixing phase and illustrates these capabilities with a simplified analysis, neglecting correlations with other SM observables.
3.4.5 Studies of CP violation in multibody charmless b hadron decays
Multibody charmless b hadron decays can be used for a variety of studies of CP violation, including searches for NP and determination of the angle γ. Due to the resonant structure in multibody decays, these can offer additional possibilities to search for both the existence and features of NP. Model-independent analyses [374, 375] can be performed to first establish the presence of a CP violation effect, and then to identify the regions of the phase space in which it is most pronounced.^{49} To further establish whether any observed CP violation can be accommodated within the Standard Model, amplitude analyses can be used to quantify the effects associated with resonant contributions to the decay. A number of methods have been proposed to determine γ from such processes [378, 379, 380, 381, 382, 383, 384, 385, 386, 387], in general requiring input not only from charged B decays, but also from B^{0} and \(B ^{0}_{ s } \) decays (to states such as \(K ^{0}_{\mathrm{S}} h^{+}h^{\prime-}\) and π^{0}h^{+}h^{′−}).^{50} The potential for LHCb to study multibody charmless \({ \varLambda }^{0}_{ b } \) decays adds further possibilities for novel studies of CP violation effects.
3.4.6 Prospects of future LHCb measurements
- 1.
B^{+}→DK^{+}π^{−}π^{+} where, similarly to the B→DK mode, the neutral D can be reconstructed either in the two-body (ADS and GLW-like measurement) or multibody (GGSZ-like measurement) final state. The observation of the CF mode in LHCb data [389] indicates a yield only twice lower than that for the B→DK mode, which makes it competitive for the measurement of γ.^{51} However, two unknown factors affect the expected γ sensitivity. First, since this is a multibody decay, the overlap between the interfering amplitudes is in general less than 100 %; this is accounted for by a coherence factor between zero and unity which enters the interference term in Eqs. (54), (55), (59), (60) as an unknown parameter. Second, the value of r_{B} can be different from that in B→DK and is as yet unmeasured, although it is expected [318] that it can be larger in this decay than in B→DK.
- 2.
B^{0}→DK^{+}π^{−}. Although the rate of these decays is smaller that of B^{+}→DK^{+}, both interfering amplitudes are colour-suppressed, therefore the expected value of r_{B} is larger, r_{B}≃0.3. As a result, the sensitivity to γ should be similar to that in the B→DK modes.^{52} Depending on the content of B^{0}→D^{0}K^{+}π^{−} and \(B^{0} \rightarrow \overline {D}{}^{0}K^{+}\pi^{-}\) amplitudes, the optimal strategy may involve Dalitz plot analysis of the B^{0} decay [390, 391]. In this case, control of amplitude model uncertainty will become essential for a precision measurement; it can be eliminated by studying the decays B^{0}→DK^{+}π^{−} with \(D \rightarrow K ^{0}_{\mathrm{S}} \pi^{+}\pi^{-}\) [392].
- 3.
\(B ^{0}_{ s } \rightarrow D\phi\). This mode is not self-tagging, but sensitivity to γ can be obtained from untagged time-integrated measurements using several different neutral D decay modes [393, 394]. The first evidence for the three-body decay \(B ^{0}_{ s } \rightarrow \overline{ D }{} ^{0} K ^{+} K ^{-} \) has just been reported by LHCb [395], and investigation of its resonant structure is in progress.
- 4.
\(B _{ c } ^{+} \rightarrow DD_{s}^{+}\). \(B _{ c } ^{+} \) production in pp collisions is significantly suppressed, however, in this mode the magnitude of CP violation is expected to be \(\mathcal{O}(100~\%)\): the two interfering amplitudes are of the same magnitude because the \(b \rightarrow u\bar{c}s\) amplitude is colour allowed, while the \(b \rightarrow c\bar{u}s\) amplitude is colour suppressed [396, 397, 398, 399].
- 5.
\({ \varLambda }^{0}_{ b } \rightarrow D{ \varLambda }\) and \({ \varLambda }^{0}_{ b } \rightarrow DpK^{-}\). Measurement of γ from analysis of the \({ \varLambda }^{0}_{ b } \rightarrow D{ \varLambda }\) decay mode was proposed in Ref. [400]. This method allows one to measure γ in a model-independent way by comparing the S- and P-wave amplitudes. However, this mode is problematic to reconstruct at LHCb because of the poorly defined \({ \varLambda }^{0}_{ b } \) vertex (both particles from its decay are long-lived) and low efficiency of Λ reconstruction. Alternatively, one can consider a similar measurement with the decay \({ \varLambda }^{0}_{ b } \rightarrow DpK^{-}\). A preliminary observation of this mode in early LHCb data has been reported [401].
Estimated precision of γ measurements with 50 fb^{−1} for various charmed B decay modes
Decay mode | γ sensitivity |
---|---|
B→DK with D→hh′, D→Kπππ | 1.3^{∘} |
B→DK with \(D \rightarrow K ^{0}_{\mathrm{S}} \pi\pi\) | 1.9^{∘} |
B→DK with D→4π | 1.7^{∘} |
B^{0}→DKπ with D→hh′, \(D \rightarrow K ^{0}_{\mathrm{S}} \pi\pi\) | 1.5^{∘} |
B→DKππ with D→hh′ | ∼3^{∘} |
Time-dependent \(B ^{0}_{ s } \rightarrow D ^{\mp}_{ s } K ^{\pm} \) | 2.0^{∘} |
Combined | ∼0.9^{∘} |
Measurement of γ and 2β_{s} by means of the CP-violating observables from loop-mediated decays B^{0}→π^{+}π^{−} and \(B ^{0}_{ s } \rightarrow K^{+} K^{-}\) was discussed in Sect. 3.4.4. Extrapolating the current sensitivity on C and S to the upgrade scenario, when 50 fb^{−1} of integrated luminosity will be collected, LHCb will be able to reach a statistical sensitivity \(\sigma_{\rm stat}(C) \approx\sigma_{\rm stat}(S) \simeq0.008\) in both B^{0}→π^{+}π^{−} and \(B ^{0}_{ s } \rightarrow K^{+} K^{-}\). This corresponds to a precision on γ of 1.4^{∘}, and on 2β_{s} of 0.01 rad, assuming perfect U-spin symmetry.
4 Mixing and CP violation in the charm sector
4.1 Introduction
The study of D mesons offers a unique opportunity to access up-type quarks in flavour-changing neutral current (FCNC) processes. It probes scenarios where up-type quarks play a special role, such as supersymmetric models with alignment [402, 403]. It offers complementary constraints on possible NP contributions to those arising from the measurements of FCNC processes of down-type quarks (B or K mesons).
The neutral D system is the latest and last system of neutral mesons where mixing between particles and anti-particles has been established. The mixing rate is consistent with, but at the upper end of, SM expectations [404] and constrains many NP models [405]. More precise D^{0}–\(\overline{ D }{} ^{0} \) mixing measurements will provide even stronger constraints. However, the focus has been shifting to CP violation observables, which provide cleaner tests of the SM [406, 407, 408]. First evidence for direct CP violation in the charm sector has been reported by the LHCb Collaboration in the study of the difference of the time-integrated asymmetries of D^{0}→K^{+}K^{−} and D^{0}→π^{+}π^{−} decay rates through the parameter \(\Delta \mathcal{A}_{ \mathit{CP} } \) [18]. No evidence of indirect CP violation has yet been found. As discussed in detail below, these results on CP violation in the charm sector appear marginally compatible with the SM but contributions from NP are not excluded.
The mass eigenstates of neutral D mesons, |D_{1,2}〉, with masses m_{1,2} and widths Γ_{1,2} can be written as linear combinations of the flavour eigenstates \(|D_{1,2}\rangle=p| D ^{0} \rangle\pm{}q| \overline { D }{} ^{0} \rangle\), with complex coefficients p and q which satisfy |p|^{2}+|q|^{2}=1. The average mass and width are defined as m≡(m_{1}+m_{2})/2 and Γ≡(Γ_{1}+Γ_{2})/2. The D mixing parameters are defined using the mass and width difference as x_{D}≡(m_{2}−m_{1})/Γ and y_{D}≡(Γ_{2}−Γ_{1})/2Γ. The phase convention of p and q is chosen such that \(\mathit{CP} | D ^{0} \rangle =-| \overline{ D }{}^{0} \rangle\). First evidence for mixing of neutral D^{0} mesons was discovered in 2007 by Belle and BaBar [409, 410] and is now well established [44]: the no-mixing hypothesis is excluded at more than 10σ for the world average (\(x_{D}=0.63\,^{+0.19}_{-0.20}~\%\), y_{D}=0.75±0.12 %).^{53}
It is convenient to group hadronic charm decays into three categories. The CF decays, such as D^{0}→K^{−}π^{+}, are mediated by tree amplitudes, and therefore no direct CP violation effects are expected. The same is true for DCS decays, such as D^{0}→K^{+}π^{−}, even though these are much more rare. The SCS decays, on the other hand, can also have contributions from penguin amplitudes, and therefore direct CP violation is possible, even though the penguin contributions are expected to be small. Within this classification, it should be noted that some decays to final states containing \(K ^{0}_{\mathrm{S}} \) mesons, e.g. \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} \rho^{0}\), have both CF and DCS contributions which can interfere [412]. Within the SM, however, direct CP violation effects are still expected to be negligible in these decays.
LHCb is ideally placed to carry out a wide physics programme in the charm sector, thanks to the high production rate of open charm: with a cross-section of 6.10±0.93 mb [3, 4], one tenth of LHC interactions produce charm hadrons. Its ring-imaging Cherenkov detectors provide excellent separation between pions, kaons and protons in the momentum range between 2 and 100 GeV/c, and additional detectors also provide clean identification of muons and electrons. This allows high purity samples to be obtained both for hadronic and muonic decays. The large boost of the D hadrons produced at LHCb is beneficial for time-dependent studies. LHCb has the potential to improve the precision on all the key observables in the charm sector in the next years.
In the remainder of this section the key observables in the charm sector are described, and the current status and near term prospects of the measurements at LHCb are reviewed. A discussion of the implications of the first LHCb charm physics results follows, motivating improved measurements and studies of additional channels. The potential of the LHCb upgrade to make the precise measurements needed to challenge the theory is then described.
4.1.1 Key observables
An alternative way to search for CP violation in charm mixing is with a time-dependent Dalitz plot analysis of D^{0} and \(\overline{ D }{} ^{0} \) decays to \(K ^{0}_{\mathrm{S}} \pi ^{+} \pi ^{-} \) or \(K ^{0}_{\mathrm{S}} K ^{+} K ^{-} \). Such analyses have been carried out at the B factories [416, 417]. Also in these cases no CP violation was observed.
The current most accurate measurements of \(\Delta \mathcal{A}_{ \mathit{CP} } \) are from the LHCb and CDF Collaborations and are (−0.82±0.21±0.11) % [18] and (−0.62±0.21±0.10) % [422], respectively.^{56} These results show first evidence of CP violation in the charm sector: the world average is consistent with no CP violation at only 0.006 % C.L. [44].
4.1.2 Status and near-term future of LHCb measurements
LHCb has a broad programme of charm physics, including searches for rare charm decays (see Sect. 2), spectroscopy and measurements of production cross-sections and asymmetries (see Sect. 5). In this section only studies of mixing and CP violation are discussed. For reviews of the formalism, the reader is referred to Refs. [413, 424, 425] and the references therein, and for an overview of NP implications to Ref. [418].
Mixing and indirect CP violation occur only in neutral mesons. These are probed in a number of different decay modes, predominantly—but not exclusively—time-dependent ratio measurements. In most cases, the same analysis yields measurements of both mixing and CP violation parameters, so these are considered together. By contrast, direct CP violation may occur in decays of both neutral and charged hadrons, and the primary sensitivity to it comes from time-integrated measurements—though it may affect certain time-dependent asymmetries as well, as discussed in Sect. 4.7.1.
Measurements of the ratios of the effective D^{0} lifetimes in decays to quasi-flavour-specific states (e.g. D^{0}→K^{−}π^{+}) and CP eigenstates f_{CP} (e.g. D^{0}→K^{−}K^{+}). These yield y_{CP}. Comparing the lifetime of D^{0}→f_{CP} and \(\overline { D }{} ^{0} \rightarrow f_{ \mathit{CP} }\) yields the CP violation parameter A_{Γ}.
- Measurements of the time-dependence of the ratio of wrong-sign to right-sign hadronic decays (e.g. D^{0}→K^{+}π^{−} vs. D^{0}→K^{−}π^{+}). The ratio depends on \(y_{D}^{\prime} t\) and \((x_{D}^{\prime2} + y_{D}^{\prime2}) t^{2}\) (see, e.g., Ref. [424]), where and δ is the mode-dependent strong phase between the CF and DCS amplitudes. Note that \((x_{D}^{\prime2} + y_{D}^{\prime2}) = x_{D}^{2} + y_{D}^{2} \equiv r_{M}\). The mixing parameters can be measured independently for D^{0} and \(\overline{ D }{} ^{0} \) to constrain indirect CP violation, and the overall asymmetry in wrong-sign decay rates for D^{0} and \(\overline{ D }{} ^{0} \) gives the direct CP violation parameter A_{d}.
Time-dependent Dalitz plot fits to self-conjugate final states (e.g. \(D^{0} \rightarrow K ^{0}_{\mathrm{S}} \pi^{-} \pi^{+}\)). These combine features of the two methods above, along with simultaneous extraction of the strong phases relative to CP eigenstate final states. Consequently they yield measurements of x_{D} and y_{D} directly. Likewise, the indirect CP violation parameters |q/p| and ϕ may be extracted, along with the asymmetry in phase and magnitude of each contributing amplitude (in a model-dependent analysis).
Measurements of the ratio of time-integrated rates of wrong-sign to right-sign semileptonic decays (e.g. \(D ^{0} \rightarrow \overline{ D }{} ^{0} \rightarrow K^{+} l^{-} \bar{\nu}_{l}\) vs. D^{0}→K^{−}l^{+}ν_{l}). These yield r_{M} and A_{m}.
Projected statistical uncertainties with 1.0 and 2.5 fb^{−1} of LHCb data. Yields are extrapolated based on samples used in analyses of 2011 data; sensitivities are projected from these yields assuming \(1/\sqrt{N}\) scaling based on reported yields by LHCb, and using published input from BaBar, Belle, and CDF. The projected CP-violation sensitivities may vary depending on the true values of the mixing parameters
Sample | Observable | Sensitivity (1.0 fb^{−1}) | Sensitivity (2.5 fb^{−1}) |
---|---|---|---|
Tagged KK | y_{CP} | 5×10^{−4} | 4×10^{−4} |
Tagged ππ | y_{CP} | 10×10^{−4} | 7×10^{−4} |
Tagged KK | A_{Γ} | 5×10^{−4} | 4×10^{−4} |
Tagged ππ | A_{Γ} | 10×10^{−4} | 7×10^{−4} |
Tagged WS/RS Kπ | \(x_{D}^{\prime2}\) | 10×10^{−5} | 5×10^{−5} |
Tagged WS/RS Kπ | \(y_{D}^{\prime}\) | 20×10^{−4} | 10×10^{−4} |
Tagged \(K ^{0}_{\mathrm{S}} \pi\pi\) | x_{D} | 5×10^{−3} | 3×10^{−3} |
Tagged \(K ^{0}_{\mathrm{S}} \pi\pi\) | y_{D} | 3×10^{−3} | 2×10^{−3} |
Tagged \(K ^{0}_{\mathrm{S}} \pi\pi\) | |q/p| | 0.5 | 0.3 |
Tagged \(K ^{0}_{\mathrm{S}} \pi\pi\) | ϕ | 25^{∘} | 15^{∘} |
Measurement of differences in asymmetry between two related final states, such that systematic effects largely cancel—for example, \(\mathcal{A}_{ \mathit{CP} } (D^{0} \rightarrow K^{-}K^{+}) - \mathcal {A}_{ \mathit{CP} } (D^{0} \rightarrow \pi^{-}\pi^{+})\) [18]. This is simplest with two-body or quasi-two-body decays. This is discussed in more detail in Sect. 4.1.3.
Searching for asymmetries in the distributions of multi-body decays, such that differences in overall normalisation can be neglected and effects related to lab-frame kinematics are largely washed out—for example, in the Dalitz plot distribution of D^{+}→K^{−}K^{+}π^{+} [427].
In the longer term, the goal is to extract the CP asymmetries for D^{0}→K^{+}K^{−} and D^{0}→π^{+}π^{−} separately, along with those for other decay modes. To achieve this, it will be necessary to determine the production and detector efficiencies from data. Progress has been made in this area, notably in the \(D_{s}^{+}\) production asymmetry measurement [428], which involves determination of the pion reconstruction efficiency from D^{∗+}→D^{0}π^{+},D^{0}→K^{−}π^{−}π^{+}π^{+} decays in which one of the D^{0} daughter pions is not used in the reconstruction.^{58} The detector asymmetries need to be determined as functions of the relevant variables, and similarly, the production asymmetries can vary as functions of transverse momentum and pseudorapidity. Understanding these systematic effects with the level of precision and granularity needed for CP asymmetry measurements is difficult and it cannot be assumed that these challenges will be solved in a short time scale. Moreover, production asymmetries can be determined only with the assumption of vanishing CP asymmetry in a particular (usually CF) control mode. Therefore ultimately the resulting measurements of CP asymmetries for individual decay modes are essentially \(\Delta \mathcal{A}_{ \mathit{CP} } \) measurements relative to CF decays.
- D^{0}→K^{−}K^{+},π^{−}π^{+}:
Updates to the 0.6 fb^{−1}\(\Delta \mathcal{A}_{ \mathit{CP} } \) analysis [18] are in progress, using both prompt charm and charm from semileptonic B decays (see Sect. 4.1.3).
- \(D_{(s)}^{+} \rightarrow K ^{0}_{\mathrm{S}} h^{+}, \phi h^{+}\):
A \(\Delta \mathcal{A}_{ \mathit{CP} } \)-style analysis is possible by comparing asymmetries in a CF control mode (e.g. \(D^{+} \rightarrow K ^{0}_{\mathrm{S}} \pi^{+}\)) and the associated SCS mode (e.g. D^{+}→ϕπ^{+}), taking advantage of the inherent symmetry of the \(K ^{0}_{\mathrm{S}} \rightarrow \pi ^{-} \pi^{+}\) and ϕ→K^{−}K^{+} decays.^{59} The different kinematic distributions of the tracks (requiring binning or reweighting) and the CP asymmetry in the \(K ^{0}_{\mathrm{S}} \) decay need to be taken into account.
- D^{+}→π^{+}π^{−}π^{+},K^{+}K^{−}π^{+}:
A search for CP violation in D^{+}→K^{+}K^{−}π^{+} with the model-independent (so-called “Miranda”) technique [374] was published with the 2010 data sample [427], comprising 0.04 fb^{−1}. With such small data samples, detector effects are negligible. However, from studies of control modes such as \(D_{s}^{+} \rightarrow K^{-} K^{+} \pi ^{+}\) it is found that this is no longer the case with 1.0 fb^{−1} of data or more, so an update will require careful control of systematic effects. The π^{+}π^{−}π^{+} final state should be more tractable, since the π^{±} interaction asymmetry does not depend strongly on momentum.
- D^{0}→π^{−}π^{+}π^{−}π^{+},K^{−}K^{+}π^{−}π^{+}:
Previous publications have focused mainly on T-odd moments [430], but there is further information in the distribution of final-state particles. A Miranda-style binned analysis or a comparable unbinned method [375] can be used.^{60}
- Baryonic decays:
LHCb will collect large samples of charmed baryons, enabling novel searches for CP-violation effects [432]. Triggering presents a challenge, but trigger lines for several \({ \varLambda }^{+}_{ c } \) decay modes of the form Λh^{+} or ph^{−}h^{′+} are already incorporated, allowing large samples to be recorded. In addition to the considerations outlined above for D meson decays, the large proton-antiproton interaction asymmetry and the possibility of polarisation in the initial state must be taken into account.
4.1.3 Experimental aspects of \(\Delta \mathcal{A}_{ \mathit{CP} } \) and related measurements
the data can be partitioned into smaller kinematic regions such that within each region the raw asymmetries are constant and/or the K^{+}K^{−} and π^{+}π^{−} kinematic distributions are equal;
the data can be reweighted such that the K^{+}K^{−} and π^{+}π^{−} kinematic distributions are equalised.
There is another way in which the formalism could be broken: through the presence of peaking backgrounds which (a) fake the signal, (b) occur at different levels for the K^{+}K^{−} and π^{+}π^{−} final states, and (c) have a different raw asymmetry from the signal. The signal extraction procedure used in the published LHCb analysis is a fit to the mass difference from threshold \(\delta m \equiv m((h^{+}h^{-})_{ D ^{0} }\pi^{+}_{\mathrm{s}}) - m(h^{+}h^{-}) - m(\pi^{+})\). This is vulnerable to a class of background in which a real D^{∗+} decay occurs and the correct slow pion is found but the D^{0} decay is partly misreconstructed, e.g. D^{0}→K^{−}π^{+}π^{0} misidentified as D^{0}→K^{−}K^{+}. This typically creates a background which peaks in δm but is broadly distributed in m(h^{+}h^{−}). Only cases which lie within the narrow m(h^{+}h^{−}) signal window will survive. This is more common for the K^{+}K^{−} final state than for π^{+}π^{−}: the energy of a missing particle can be made up by misidentifying a pion as a kaon, but apart from D^{0}→π^{−}e^{+}ν_{e} there is little that can fake the kinematics of D^{0}→π^{+}π^{−}. In practise, the charged hadron identification at LHCb suppresses these background greatly, and their raw asymmetries are not expected to be very different from the signal. In the published LHCb analysis, the impact of these backgrounds on the asymmetry was estimated by measuring their size and asymmetry in the h^{+}h^{−} mass sidebands and computing the effect of such a background on the signal with a toy Monte Carlo study. The alternative approach would be to use a full 2D fit to m(h^{+}h^{−}) and δm, which would distinguish this class of peaking background from the signal by its m(h^{+}h^{−}) distribution.
Summary of absolute systematic uncertainties for \(\Delta \mathcal{A}_{ \mathit{CP} } \)
Source | Uncertainty |
---|---|
Fiducial requirement | 0.01 % |
Peaking background asymmetry | 0.04 % |
Fit procedure | 0.08 % |
Multiple candidates | 0.06 % |
Kinematic binning | 0.02 % |
Total | 0.11 % |
4.2 Theory status of mixing and indirect CP violation
4.2.1 Theoretical predictions for ΔΓ_{D}, Δm_{D} and indirect CP violation in the Standard Model
As discussed in Sect. 4.1, mixing of charmed mesons provides outstanding opportunities to search for physics beyond the SM. New flavour-violating interactions at some high-energy scale may, together with the SM interactions, mix the flavour eigenstates giving mixing parameters that differ from their SM expectations. It is known experimentally that D^{0}–\(\overline{ D }{} ^{0} \) mixing proceeds extremely slowly, which in the SM is usually attributed to the absence of super-heavy quarks.
A simple examination of Eq. (91) reveals that the local ΔC=2 interactions only affect x_{D}, thus one can conclude that it is more likely that x_{D} receives large NP contributions. Hence, it was believed that an experimental observation of x_{D}≫y_{D} would unambiguously reveal NP contributions to charm mixing. This simple signal for NP was found to not be realised in nature, but it is interesting that the reverse relation, x_{D}<y_{D} with y_{D} expected to be determined by the SM processes, might nevertheless significantly affect the sensitivity to NP of experimental analyses of D mixing [439]. Also, it is important to point out that, contrary to the calculations of the SM contribution to mixing, the contributions of NP models can be calculated relatively unambiguously [405, 440, 441].
There are currently two approaches, neither of which give very reliable results because m_{c} is in some sense intermediate between heavy and light. The “inclusive” approach is based on the OPE. In the \(m_{c} \gg\varLambda_{\rm QCD}\) limit, where \(\varLambda_{\rm QCD}\) is a scale characteristic of the strong interactions, Δm_{D} and ΔΓ_{D} can be expanded in terms of matrix elements of local operators [434, 435, 436, 437]. Such calculations typically yield x_{D},y_{D}<10^{−3}. The use of the OPE relies on local quark-hadron duality (see, for example, Ref. [443]), and on \(\varLambda_{\rm QCD}/E_{\rm released}\) (with \(E_{\rm released} \sim m_{c}\)) being small enough to allow a truncation of the series. Moreover, a careful reorganisation of the OPE series is needed, as terms with smaller powers of m_{s} are numerically more important despite being more suppressed by powers of 1/m_{c} [434, 435, 436, 437]. The numerically dominant contribution is composed of over twenty unknown matrix elements of dimension-12 operators, which are very hard to estimate. As a possible improvement of this approach, it would be important to perform lattice calculations of those matrix elements, as well as make perturbative QCD (pQCD) corrections to Wilson coefficients of those operators.
The “exclusive” approach sums over intermediate hadronic states, which may be modelled or fit to experimental data [444, 445, 446, 447, 448, 449]. Since there are cancellations between states within a given SU(3)_{f} multiplet, one needs to know the contribution of each state with high precision. However, the D meson is not light enough that its decays are dominated by a few final states. In the absence of sufficiently precise data on many decay rates and on strong phases, one is forced to use some assumptions. While most studies find x_{D},y_{D}<10^{−3}, Refs. [444, 445, 446, 447, 448, 449] obtain x_{D} and y_{D} at the 10^{−2} level by arguing that SU(3)_{f} violation is of order unity. Particular care should be taken if experimental data are used to estimate the mixing parameters, as the large cancellations expected in the calculation make the final result sensitive to uncertainties in the experimental inputs. It was shown that phase space effects alone provide enough SU(3)_{f} violation to induce x_{D},y_{D}∼10^{−2} [442]. Large effects in y_{D} appear for decays close to threshold, where an analytic expansion in SU(3)_{f} violation is no longer possible; a dispersion relation can then be used to show that x_{D} would receive contributions of similar order of magnitude. The dispersion calculation suffers from uncertainties associated with unknown (off-shell) q^{2}-dependences of non-leptonic transition amplitudes and thus cannot be regarded as a precision calculation, although it provides a realistic estimate of x_{D}. As a possible improvement of this approach, an estimate of SU(3)_{f} breaking in matrix elements should be performed. In addition, a calculation with V_{ub}≠0 should also be done, which is important to understand the size of CP violation in charm mixing.
Based on the above discussion, it can be seen that it is difficult to find a clear indication of physics beyond the SM in D^{0}–\(\overline { D }{} ^{0} \) mixing measurements alone. However, an observation of large CP violation in charm mixing would be a robust signal of NP.
CP violation in D decays and mixing can be searched for by a variety of methods. Most of the techniques that are sensitive to CP violation make use of the decay asymmetry \(\mathcal{A}_{ \mathit{CP} } (f)\) [418, 425]. For instance, time-dependent decay widths for D→Kπ are sensitive to CP violation in mixing. In particular, a combined analysis of D→Kπ and D→KK can yield interesting constraints on CP-violating parameters y_{CP} and A_{Γ}, as discussed in Sect. 4.1.1.
4.2.2 New physics in indirect CP violation
Indirect CP violation in charm mixing and decays is a unique probe for NP, since within the SM the relevant processes are described by the physics of the first two generations to an excellent approximation. Hence, observation of CP violation in D^{0}–\(\overline{ D }{} ^{0} \) mixing at a level higher than \(\mathcal{O}(10^{-3})\) (which is the SM contribution) would constitute an unambiguous signal of NP.
The contribution to x_{12} in the linear MFV case (\(r_{\rm GMFV}=0\)) is orders of magnitude below the current experimental sensitivity, assuming \(\mathcal{O}(1)\) proportionality coefficient in Eq. (99). Yet in the context of GMFV with two Higgs doublets and large tanβ, such that y_{b}∼1, observable signals can be obtained, as shown in Fig. 23 for \(r_{\rm GMFV}\) in the range [−3,3]. Note that strictly speaking \(r_{\rm GMFV}\) (and thus the resulting signal) is not bounded, but higher absolute values than those considered here are much less likely in realistic models. Indeed in the current example \(r_{\rm GMFV} \gtrsim2\) is excluded, as shown in the figure.
The available data on D^{0}–\(\overline{ D }{} ^{0} \) mixing can also be used to constrain the parameter space of specific theories, such as SUSY and warped extra dimensions (WED) [452]. This has been done e.g. in Refs. [440, 441] or Refs. [453, 454] where the interplay between the constraints from the K and D systems is presented. Here the influence of improving the current bounds is demonstrated.
To conclude, the experimental search for indirect CP violation in charm is one of the most promising channels for discovering NP or obtaining strong constraints. This is not negated by the large hadronic uncertainties in the D system, because of the very small SM short distance contribution to CP violation in D^{0}–\(\overline{ D }{} ^{0} \) mixing.
4.3 The status of calculations of \(\Delta \mathcal{A}_{ \mathit{CP} } \) in the Standard Model
The naive expectation for the SM penguin-to-tree ratio is based on estimates of the “short-distance” penguins with b-quarks in the loops. In fact, there is consensus that a SM explanation for \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) would have to proceed via dynamical enhancement of the long-distance “penguin contraction” contributions to the penguin amplitudes, i.e., penguins with s and d quarks inside the “loops”. Research addressing the direct CP asymmetry in the SM has largely fallen into one of two categories: (i) flavour SU(3)_{f} or U-spin fits to the D decay rates, to check that an enhanced penguin amplitude can be accommodated [457, 458, 459, 460, 461, 462, 463] (this, by itself, would not mean that \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) is due to SM dynamics); (ii) rough estimates of the magnitudes of certain contributions to the long-distance penguin contractions [461, 464, 465, 466], to check if, in fact, it is reasonable that SM dynamics could yield the enhanced penguin amplitudes returned by the SU(3)_{f} or U-spin fits.
The results obtained using the flavour symmetry decompositions can be summarised as follows. An SU(3)_{f} analysis of the D→PP decay amplitudes that incorporates CP violation effects was first carried out about 20 years ago [445, 457, 467]. Already in this study the possibility of large direct CP asymmetries was anticipated, e.g., as large as the percent level assuming that the penguins receive a large enhancement akin to the ΔI=1/2 rule in kaon decays. An updated analysis, working to first order in SU(3)_{f} breaking, has been presented [458], making use of branching ratio measurements for the D→Kπ,ππ and \(D ^{0} \rightarrow K ^{-} K ^{+} , \overline{ K }{} ^{0} \eta \) decay modes. The authors concluded that \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) can be easily reconciled with the measured branching ratios. This was also the conclusion of a study based on a diagrammatic SU(3)_{f} amplitude decomposition [459], which considered a larger set of D→PP decay modes. Again, this is only a statement about the possibility of accommodating the required amplitudes in the flavour decomposition, not about their realisation via long distance QCD dynamics. Both studies observe that a SM explanation of \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) could be combined with precise measurements of the individual asymmetries \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{0} \rightarrow K ^{-} K ^{+} )\) and \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{0} \rightarrow \pi ^{-} \pi ^{+} )\) to obtain predictions for \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{0} \rightarrow \pi ^{0} \pi ^{0} )\). The conclusion, based on current data, is that percent level asymmetries for the latter could be realised. Reference [459] also discusses implications for \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{+} \rightarrow K ^{+} \overline{ K }{} ^{0} )\).
Finally, the estimates for the long-distance penguin contractions [466, 468] are reviewed to see if the required enhancement can be realised. Reference [468] employs the one-gluon exchange approximation. The essential ingredients are: (i) 1/N_{c} counting; (ii) D branching ratio data which shows that certain formally 1/m_{c} power-suppressed amplitudes are of same order as their leading (1/m_{c})^{0} counterparts; (iii) translation of this breakdown of the 1/m_{c} expansion to the penguin contraction amplitudes, in the approximation of a hard gluon exchange; (iv) use of a partonic quantity as a rough estimator of the hadronic interactions, e.g., final state interactions, underlying the penguin contraction “loops”. This results in a rough estimate for \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) at the few per mille level. The authors of Ref. [468] thus conclude that a SM explanation is plausible, given that their estimate suffers from large uncertainties. In Ref. [466] the penguin contractions are estimated using isospin and information from ππ scattering and unitarity. A fit of the CP-conserving contributions from the CP-averaged branching ratios provides information on the isospin amplitudes and the underlying renormalisation group invariant amplitude contributions. Allowing for three coupled channel contributions to ππ,KK scattering the authors conclude that the observed asymmetries are marginally compatible with the SM.
To summarise, flavour SU(3) or U-spin fits to the D→PP data can accommodate the enhanced penguin amplitudes required to reproduce \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \). There is consensus that in this case \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{0} \rightarrow \pi ^{0} \pi ^{0} )\) could lie at the percent level, while \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{+} \rightarrow K ^{+} \overline{ K }{} ^{0} )\) could certainly lie at the few per mille level. Under the assumption of nominal SU(3)_{f} breaking in D→PP decays, the enhancement of the long-distance penguin contractions required to realise \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) is not surprising, given the large difference between the D^{0}→K^{−}K^{+} and D^{0}→π^{−}π^{+} decay rates. It would of course be of interest to extend the above CP violation studies to the SCS D→VP and D→VV decay modes. Finally, among the works which have attempted to estimate directly the magnitudes of the long distance penguin contractions, there is no consensus on whether they can be enhanced by an order of magnitude beyond the naive penguin amplitude estimates, as would be required in order to explain \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \). Ultimately this question will have to be answered directly via lattice studies.
In the following section, future prospects are discussed. In subsequent sections, several definitive CP-violating signals for NP in SCS D decays will be discussed.
4.4 \(\Delta \mathcal{A}_{ \mathit{CP} } \) in the light of physics beyond the Standard Model
4.4.1 General considerations
4.4.2 Universality of CP violation in flavour-changing decay processes
The viability of the remaining 4-quark operators in \(\mathcal{H}^{\text{eff-NP}}_{|\Delta C|=1}\) as explanations of the experimental \(\Delta \mathcal{A}_{ \mathit{CP} } \) value depends crucially on their flavour and chiral structure (a full list can be found in Ref. [469]). In particular, operators involving purely right-handed quarks are unconstrained in the effective theory analysis but may be subject to severe constraints from their UV sensitive contributions to D mixing observables. On the other hand, QED and QCD dipole operators are at present only weakly constrained by nuclear electric dipole moments (EDMs) and thus present the best candidates to address the \(\Delta \mathcal{A}_{ \mathit{CP} } \) puzzle [469].
Finally, note that it was shown that the impact of universality of CP within the alignment framework is to limit the amount of CP violation in D^{0}–\(\overline{ D }{} ^{0} \) mixing to below ∼20 %, which is interestingly near the current bound. The expected progress in this measurement with the LHCb detector is therefore going to start probing this framework.
4.4.3 Explanations of \(\Delta \mathcal{A}_{ \mathit{CP} } \) within NP models
Since the announcement of the LHCb result, several prospective explanations of \(\Delta \mathcal {A}_{ \mathit{CP} } \) within various NP frameworks have appeared. In the following the implications within some of the well-motivated NP models are discussed.
Finally, it is possible to relate \(\Delta \mathcal{A}_{ \mathit{CP} } \) to the anomalously large forward–backward asymmetry in the \(t\bar{t}\) system measured at the Tevatron [482] through a minimal model. Among the single-scalar-mediated mechanisms that can explain the top data, only the t-channel exchange of a colour-singlet weak doublet, with a very special flavour structure, is consistent with the total and differential \(t\bar{t}\) cross-section, flavour constraints and electroweak precision measurements [483]. The required flavour structure implies that the scalar unavoidably contributes at tree level to \(\Delta \mathcal{A}_{ \mathit{CP} } \) [484]. The relevant electroweak parameters are either directly measured, or fixed by the top-related data, implying that, for a plausible range of the hadronic parameters, the scalar-mediated contribution is of the right size.
4.4.4 Shedding light on direct CP violation via D→Vγ decays
- 1.
The first key observation to estimate DCPV asymmetries in radiative decay modes is the strong link between the ΔC=1 chromomagnetic operator (\(Q_{8} \sim\bar{u}_{L} \sigma_{\mu\nu} T^{a} g_{s} G_{a}^{\mu\nu} c_{R}\)) and the ΔC=1 electromagnetic-dipole operator (\(Q_{7} \sim\bar{u}_{L} \sigma_{\mu\nu} Q_{u} e F^{\mu\nu} c_{R} \)). In most explicit new-physics models the short-distance Wilson coefficients of these two operators (C_{7,8}) are expected to be similar. Moreover, even assuming that only a non-vanishing C_{8} is generated at some high scale, the mixing of the two operators from strong interactions implies C_{7,8} of comparable size at the charm scale. Thus if \(\Delta \mathcal{A}_{ \mathit{CP} } \) is dominated by NP contributions generated by Q_{8}, it can be inferred that \(|\operatorname{Im}[C^{\rm NP}_{7}(m_{c})] | \approx|\operatorname{Im}[C^{\rm NP}_{8}(m_{c})] | = (0.2 \mbox{--} 0.8) \times10^{-2} \).
- 2.The second important ingredient is the observation that in the Cabibbo-suppressed D→Vγ decays, where V is a light vector meson with \(u \overline{ u } \) valence quarks (V=ρ^{0},ω), Q_{7} has a sizeable hadronic matrix element. More explicitly, the short-distance contribution induced by Q_{7}, relative to the total (long-distance) amplitude, is substantially larger with respect to the corresponding relative weight of Q_{8} in D→P^{+}P^{−} decays. Estimating the SM long-distance contributions from data, and evaluating the short-distance CP-violating contributions under the hypothesis that \(\Delta \mathcal{A}_{ \mathit{CP} } \) is dominated by (dipole-type) NP, leads to the following estimate for the maximal direct CP asymmetries in the D→(ρ,ω)γ modes [485]:(112)In the first bin, close to the ϕ peak, the leading contribution is due to the ϕ-exchange amplitude. The contribution due to the nonresonant amplitudes becomes more significant further from the ϕ peak, where the CP asymmetry can become larger.$$ \everymath{\displaystyle} \begin{array}{@{}l} \bigl| a_{ \mathit{CP} }^{{ \rm dir }} \bigl( D \rightarrow K ^+ K ^- \gamma \bigr)\bigr|^{\rm max} \approx2~\% , \\\noalign{\vspace{6pt}} 2m_K < \sqrt{s} < 1.05 ~\mathrm{GeV} , \\\noalign{\vspace{6pt}} \bigl| a_{ \mathit{CP} }^{{ \rm dir }} \bigl( D \rightarrow K ^+ K ^- \gamma \bigr)\bigr|^{\rm max} \approx6~\% , \\\noalign{\vspace{6pt}} 1.05 ~\mathrm{GeV} < \sqrt{s} < 1.20 ~\mathrm{GeV} . \end{array} $$(113)
- 3.
In order to establish the significance of these results, two important issues have to be clarified: (1) the size of the CP asymmetries within the SM, (2) the role of the strong phases.
As far as the SM contribution is concerned, it can first be noticed that short-distance contributions generated by the operator Q_{7} are safely negligible. Using the result in Ref. [486], asymmetries are found to be below the 0.1 % level. The dominant SM contribution is expected from the leading non-leptonic four-quark operators, for which the general arguments discussed in Ref. [469] can be applied. The CP asymmetries can be decomposed as \(| a_{ \mathit{CP} }^{{{\rm SM}}} (f)| \approx2 \xi \operatorname{Im} (R_{f}^{{\rm SM}}) \approx0.13~\% \times\operatorname{Im} (R_{f}^{{\rm SM}})\), where ξ≡|V_{cb}V_{ub}/V_{cs}V_{us}|≈0.0007 and \(R_{f}^{{\rm SM}}\) is a ratio of suppressed over leading hadronic amplitudes, naturally expected to be smaller than one. This decomposition holds both for f=ππ,KK and for f=Vγ channels. The SM model explanations of Δa_{CP} require \(R_{ \pi \pi , K K }^{{\rm SM}}\sim3\). While this possibility cannot be excluded from first principles, a further enhancement of one order of magnitude in the D→Vγ mode is beyond any reasonable explanation in QCD. As a result, an observation of \(| a_{ \mathit{CP} }^{{ \rm dir }} ( D \rightarrow V \gamma )| \gtrsim 3~\%\) would be a clear signal of physics beyond the SM, and a clean indication of new CP-violating dynamics associated to dipole operators.
Having clarified that large values of \(| a_{ \mathit{CP} }^{{ \rm dir }} ( D \rightarrow V \gamma )|\) would be a clear footprint of non-standard dipole operators, it can be asked if potential tight limits on \(| a_{ \mathit{CP} }^{{ \rm dir }} ( D \rightarrow V \gamma )|\) could exclude this non-standard framework. Unfortunately, uncertainty on the strong phases does not allow this conclusion to be drawn. Indeed the maximal values for the DCPV asymmetries presented above are obtained in the limit of maximal constructive interference of the various strong phases involved. In principle, this problem could be overcome via time-dependent studies of \(D ( \overline { D }{} ) \rightarrow V \gamma \) decays or using photon polarisation, accessible via lepton pair conversion in D→V(γ^{∗}→ℓ^{+}ℓ^{−}); however, these types of measurements are certainly more challenging from the experimental point of view.
4.4.5 Testing for CP-violating new physics in the ΔI=3/2 amplitudes
It is possible, at least in principle, to distinguish between NP and the SM as the origin of \(\Delta \mathcal {A}_{ \mathit{CP} } \). If \(\Delta \mathcal{A}_{ \mathit{CP} } \) is due to a chromomagnetic operator, i.e. due to ΔI=1/2 contributions, one can measure CP violation in radiative D decays, as explained in the previous section. Examples of NP models that can be tested in this way are, e.g., flavour-violating supersymmetric squark-gluino loops that mediate the c→ug transition [418, 472, 473]. On the other hand, if \(\Delta \mathcal {A}_{ \mathit{CP} } \) is due to ΔI=3/2 NP one can use isospin symmetry to write sum rules for direct CP asymmetries in D decays [487]. If the sum rules are violated, then NP would be found. An example of a NP model that can be tested in this way is an addition of a single new scalar field with nontrivial flavour couplings [484].
The basic idea behind the ΔI=3/2 NP tests [487, 488] is that in the SM the CP violation in SCS D decays arises from penguin amplitudes which are ΔI=1/2 transitions. On the other hand, ΔI=3/2 amplitudes are CP-conserving in the SM. Moreover, there are no ΔI=5/2 terms in the SM short-distance effective Hamiltonian, and though such contributions can be generated by electromagnetic rescattering (as has been discussed in the context of B→ππ decays [489, 490]) they would also be CP conserving. Observing any CP violation effects in ΔI=3/2 amplitudes would therefore be a clear signal of NP.
In the derivation of the sum rules it is important to pay attention to the potentially important effects of isospin breaking. Isospin symmetry is broken at \({\mathcal{O}}(10^{-2})\), which is also the size of the interesting CP asymmetries. There are two qualitatively different sources of isospin breaking: due to electromagnetic interactions, u and d quark masses, which are all CP-conserving effects, and due to electroweak penguin operators that are a CP-violating source of isospin breaking. The CP-conserving isospin breaking is easy to cancel in the sum rules. As long as the CP-conserving amplitudes completely cancel in the sum rules, which is the case in Ref. [487], the isospin breaking will only enter suppressed by the small CP violation amplitude and is therefore negligible. The electroweak penguin operators, on the other hand, are suppressed by \(\alpha/ \alpha_{S} \sim {\mathcal{O}}(10^{-2})\) compared to the leading CP-violating but isospin conserving penguin contractions of the Q_{1,2} operators, and can thus also be safely neglected.
The above results apply also to D→ρρ decays, but for each polarisation amplitude separately. The corrections due to finite ρ width can be controlled experimentally in the same way as in B→ρρ decays [492]. As long as the polarisations of the ρ resonances are measured (or if the longitudinal decay modes dominate, as is the case in B→ρρ decays), the search for ΔI=3/2 NP could be easier experimentally in D→ρρ decays since there are more charged tracks in the final state. The most promising observable where polarisation measurement is not needed is \(\mathcal{A}_{ \mathit{CP} } ( D ^{+} \rightarrow \rho ^{+} \rho ^{0} )\), which if found nonzero (after the correction for the effect of finite ρ decay widths) would signal ΔI=3/2 NP.
4.5 Potential for lattice computations of direct CP violation and mixing in the D^{0}–\(\overline{ D }{} ^{0} \) system
In searches for NP using charmed mesons, it is obviously crucial to determine accurately the size of SM contributions. In the next few paragraphs the prospects for such a determination in the future using the methods of lattice QCD are discussed.
Lattice QCD provides a first-principles method for determining the strong-interaction contributions to weak decay and mixing processes. It has developed into a precision tool, allowing determinations of the light hadron spectrum, decay constants, and matrix elements such as B_{K} and B_{B} with percent-level accuracy. For reviews and collections of recent results, see Refs. [109, 494]. The results provide confirmation that QCD indeed describes the strong interactions in the non-perturbative regime, as well as providing predictions that play an important role in searching for new physics by looking for inconsistencies in unitarity triangle analyses.
Results with high precision are, however, only available for processes involving single hadrons and a single insertion of a weak operator. For the D^{0} system, the “high-precision” quantities are thus the matrix elements describing the short-distance parts of D^{0}–\(\overline{ D }{} ^{0} \) mixing and the matrix elements of four-fermion operators arising after integrating out NP. The methodology for such calculations is in place (and has been applied successfully to the K and B meson systems), and results are expected to be forthcoming in the next one to two years.
More challenging, and of course more interesting, are calculations of the decay amplitudes to ππ and \(K \overline { K }{} \). For kaon physics, this is the present frontier of lattice calculations. One must deal with two technical challenges: (i) the fact that one necessarily works in finite volume so the states are not asymptotic two-particle states and (ii) the need to calculate Wick contractions (such as the penguin-type contractions) which involve gluonic intermediate states in some channels. The former challenge has been solved in principle by the work of Lüscher [495, 496] and Lellouch and Lüscher [497] for the K→ππ case, while advances in lattice algorithms and computational power have allowed the numerical aspects of both challenges to be overcome. There are now well controlled results for the K→(ππ)_{I=2} amplitude [498] and preliminary results for the K→(ππ)_{I=0} amplitude [499]. It is likely that results to ∼10 % accuracy for all amplitudes will be available in a few years. Note that, once a lattice calculation is feasible, it will be of roughly equal difficulty to obtain results for the CP-conserving and CP-violating parts.
To extend these results to the charm case, one must face a further challenge. This is that, even when one has fixed the strong-interaction quantum numbers of a final state, say to I=S=0, the strong interactions necessarily bring in multiple final states when E=m_{D}. For example, ππ and \(K \overline { K }{} \) states mix with ηη, 4π, 6π, etc. The finite-volume states that are used by lattice QCD are inevitably mixtures of all these possibilities, and one must learn how, in principle and in practise, to disentangle these states so as to obtain the desired matrix element. Recently, in Ref. [500], a first step towards developing a complete method has been taken, in which the problem has been solved in principle for any number of two-particle channels, assuming that the scattering is dominantly S-wave. This is encouraging, and it may be that this method will allow semi-quantitative results for the amplitudes of interest to be obtained. Turning this method into practise is expected to take three to five years due to a number of numerical challenges (in particular the need to calculate several energy levels with good accuracy). It is also expected to be possible to generalise the methodology to include four particle states; several groups are actively working on the theoretical issues. It is unclear at this stage, however, what time scale one should assign to this endeavour.
Finally, the possibility of calculating long-distance contributions to D^{0}–\(\overline{ D }{} ^{0} \) mixing using lattice methods should be considered. Here the challenge is that there are two insertions of the weak Hamiltonian, with many allowed states propagating between them. Some progress has been made recently on the corresponding problem for kaons [501, 502] but the D^{0} system is much more challenging. The main problem is that, as for the decay amplitudes, there are many strong-interaction channels with E<m_{D}. Further theoretical work is needed to develop a practical method.
4.6 Interplay of \(\Delta \mathcal{A}_{ \mathit{CP} } \) with non-flavour observables
4.6.1 Direct CP violation in charm and hadronic electric dipole moments
Models in which the primary source of flavour violation is linked to the breaking of chiral symmetry (left–right flavour mixing) are natural candidates to explain direct CP violation in SCS D meson decays, via enhanced ΔC=1 chromomagnetic operators. Interestingly, the chromomagnetic operator generates contributions to D^{0}–\(\overline{ D }{} ^{0} \) mixing and ϵ′/ϵ that are always suppressed by at least the square of the charm Yukawa couplings, thus naturally explaining why they have remained undetected.
On the other hand, the dominant constraints are posed by the neutron and nuclear EDMs, which are expected to be close to their experimental bounds. This result is fairly robust because the Feynman diagram contributing to quark EDMs has essentially the same structure as that contributing to the chromomagnetic operator.
In the following the connection between \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) and hadronic EDMs in concrete NP scenarios is discussed, following the analyses of Refs. [472, 473].
Supersymmetry
The leading SUSY contribution to \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) stems from loops involving up-squarks and gluinos and off-diagonal terms in the squark squared-mass matrix in the left–right up sector, the so-called \((\delta^{u}_{12})_{LR}\) mass-insertion. As can be seen from Eqs. (108)–(109) and taking into account the large uncertainties involved in the evaluation of the matrix element, it can be concluded that a supersymmetric theory with left-right up-squark mixing can potentially explain the LHCb result.
Generalised trilinear terms
It is found that \(\theta^{q}_{ij}\) can all be of order unity not only in the up, but also in the down sector, thanks to the smallness of the down-type quark masses entering \((\delta^{d}_{ij})_{LR}\). The only experimental bounds in tension with this scenario are those on \(|\theta^{u,d}_{11}|\) coming from the neutron EDM.
Split families
Supersymmetric flavour models
In models where the flavour structure of the soft breaking terms is dictated by an approximate flavour symmetry, \((\delta^{u}_{LR})_{12}\) is generically flavour-suppressed by \((m_{c}\vert V_{us}\vert /\tilde{m})\), which is of order a few times 10^{−4}. There is however additional dependence on the ratio between flavour-diagonal parameters, \(A/\tilde{m}\), and on unknown coefficients of order one, that can provide enhancement by a small factor. In most such models, the selection rules that set the flavour structure of the soft breaking terms relate \((\delta^{u}_{LR})_{12}\) to \((\delta ^{d}_{LR})_{12}\) and to \((\delta^{u,d}_{LR})_{11}\), which are bounded from above by, respectively, ϵ′/ϵ and EDM constraints. Since both ϵ′/ϵ and EDMs suffer from hadronic uncertainties, small enhancements due to the flavour-diagonal supersymmetric parameters cannot be ruled out. It is thus possible to accommodate \(\Delta \mathcal{A}_{ \mathit{CP} } \sim 0.006\) in supersymmetric models that are non-minimally flavour violating, but—barring hadronic enhancements in charm decays—it takes a fortuitous accident to lift the supersymmetric contribution above the permille level [473].
New-physics scenarios with Z-mediated FCNC
New-physics scenarios with scalar-mediated FCNC
4.6.2 Interplay of collider physics and a new physics origin for \(\Delta \mathcal{A}_{ \mathit{CP} } \)
The first evidence for direct CP violation in SCS D decays may have interesting implications for NP searches around the TeV scale at the LHC. The NP contribution to \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) can be fully parametrised by a complete set of ΔC=1 effective operators at the charm scale. As shown by the authors of Ref. [469] only a few of these operators can accommodate the LHCb result without conflicting with present bounds from D^{0}–\(\overline{ D }{} ^{0} \) mixing and ϵ′/ϵ. In particular four-fermion operators of the form \(\mathcal{O}^{q}=(\bar{u}_{R}\gamma^{\mu}c_{R})(\bar{q}_{R}\gamma_{\mu}q_{R})\) with q=u,d,s are promising since they do not lead to flavour violation in the down-type quark sector. The corresponding Wilson coefficients are defined as \(1/\varLambda_{q}^{2}\). Assuming the SM expectation for \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) is largely subdominant, the LHCb measurement suggests a scale of Λ_{q}≃15 TeV [469].
There is an immediate interplay between charm decay and flavour (and CP) conserving observables at much higher energies provided \(\mathcal {O}^{q}\) arises from a heavy NP state exchanged in the s-channel. Under this mild assumption \(\mathcal{O}^{q}\) factorises as the product of two quark currents and the same NP induces D^{0}–\(\overline { D }{} ^{0} \) mixing and quark compositeness through the \((\bar{u}_{R} \gamma_{\mu}c_{R})^{2}\) and \((\bar{q}_{R} \gamma_{\mu}q_{R})^{2}\) operators, respectively. Denoting their respective Wilson coefficients by \(\varLambda_{\bar{u} c}\) and \(\varLambda_{\bar{q} q}\), the relation \(\varLambda_{q}=\sqrt{\varLambda_{\bar{u} c}\varLambda_{\bar{q} q}}\) is predicted. The D^{0}–\(\overline{ D }{} ^{0} \) mixing bound on NP implies \(\varLambda_{\bar{u} c}\gtrsim1200 ~\mathrm{TeV} \) [441]. Combining this stringent ΔC=2 bound with the ΔC=1 scale suggested by \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) thus generically requires \(\varLambda_{\bar{q} q}\lesssim200 ~\mathrm{GeV} \), which is a rather low compositeness scale for the light quark flavours.
Quark compositeness can be probed at the LHC through dijet searches. Actually for the up or the down quark the low scale suggested by \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) is already excluded by the Tevatron [503, 504]. On the other hand dijet searches are less sensitive to contact interactions involving only the strange quark since the latter, being a sea quark, has a suppressed parton distribution function in the proton. The authors of Ref. [505] showed that a first estimation at the partonic level of the extra dijet production from a \((\bar{s}_{R} \gamma_{\mu}s_{R})^{2}\) operator with a scale of \(\varLambda_{\bar{s} s}\sim200 ~\mathrm{GeV} \) is marginally consistent, given the \(\mathcal{O}(1)\) uncertainty of the problem, with the bounds from the ATLAS and CMS experiments [506, 507].
One concludes that an \(\mathcal{O}^{s}\) operator induced by a s-channel exchanged NP can accommodate the \(\Delta a_{ \mathit{CP} }^{{ \rm dir }} \) measurement without conflicting with ΔC=2, ϵ′/ϵ and dijet searches. Furthermore such a NP scenario makes several generic predictions both for charm and high-\(p_{\rm T}\) physics: (1) most of the CP asymmetry is predicted to be in the K^{+}K^{−} channel, (2) CP violation in D^{0}–\(\overline { D }{} ^{0} \) mixing should be observed in the near future, and (3) an excess of dijets at the LHC is expected at a level which should be visible in the 2012 data.
4.7 Future potential of LHCb measurements
4.7.1 Requirements on experimental precision
The ultimate goal of mixing and CP violation measurements in the charm sector is to reach the precision of the SM predictions (or better). In some cases this requires measurements in several decay modes in order to distinguish enhanced contributions of higher order SM diagrams from effects caused by new particles.
Indirect CP violation is constrained by the observable A_{Γ} (see Eq. (82)). The CP-violating parameters in this observable are multiplied by the mixing parameters x_{D} and y_{D}, respectively. Hence, the relative precision on the CP-violating parameters is limited by the relative precision of the mixing parameters. Therefore, aiming at a relative precision below 10 % and taking into account the current mixing parameter world averages, the target precision would be 2–3×10^{−4}. Indirect CP violation is expected in the SM at the order of 10^{−4}, and therefore the direct CP violation parameter contributing to A_{Γ} has to be measured to a precision of 10^{−3} in order to distinguish the two types of CP violation in A_{Γ}.
Direct CP violation is not expected to be as large as the current world average of \(\Delta \mathcal {A}_{ \mathit{CP} } \) in most other decay modes. However, a few large CP violation signatures are expected in various models, as discussed in the previous sections. Estimations based on flavour-SU(3) and U-spin symmetry lead to expectations of \(a_{ \mathit{CP} }^{{ \rm dir }} ( D ^{+} \rightarrow K ^{+} \overline { K }{} ^{0} )\gtrsim0.1~\% \) and \(a_{ \mathit{CP} }^{{ \rm dir }} (D^{0} \rightarrow K ^{0}_{\mathrm{S}} K ^{0}_{\mathrm{S}} ) \sim 0.6~\%\). Considerations assuming universality of ΔF=1 transitions lead to a limit of \(a_{ \mathit{CP} }^{{ \rm dir }} ( D \rightarrow \pi e ^{+} e ^{-} )\lesssim2~\%\). Enhanced electromagnetic dipole operators can lead to \(a_{ \mathit{CP} }^{{ \rm dir }} ( D \rightarrow V\gamma )\) of a few %, equivalent to the influence of chromomagnetic dipole operators on \(\Delta \mathcal{A}_{ \mathit{CP} } \). Additional information can be obtained from time-dependent studies of D→Vγ decays or from angular analyses of D→Vl^{+}l^{−} decays.
Analyses of ΔI=3/2 transitions involve asymmetry measurements of several related decay modes. Examples are the decays D→ππ, D→ρπ, D→ρρ, \(D \rightarrow \overline{ K } K \pi \), and \(D ^{+}_{ s } \rightarrow K ^{*} \pi \). The number of final state particles in these decays varies from two to six (counting the pions from \(K ^{0}_{\mathrm{S}} \) decays) and many of these modes contain neutral pions in their final state. The precision for modes involving neutral pions or photons will be limited by the ability of the calorimeter to identify these particles in the dense hadronic environment. An upgraded calorimeter with smaller Molière radius would greatly extend the physics reach in this area.^{68}
In general, a precision of 5×10^{−4} or better for asymmetry differences as well as individual asymmetries is needed for measurements of other SCS charm decays. While measurements of time-integrated raw asymmetries at this level should be well within reach, the challenge lies in the control of production and detection asymmetries in order to extract the physics asymmetries of individual decay modes. This can be achieved by assuming that there is no significant CP violation in CF decay modes.
4.7.2 Prospects of future LHCb measurements
Numbers of D^{0} and D^{∗+}→D^{0}π^{+} signal events observed in the 2011 data in a variety of channels and those projected for 50 fb^{−1}. These channels can be used for mixing studies, for indirect CP violation studies, and for direct CP violation studies. As discussed in the text, the numbers of events in any one channel can vary from one analysis to another, depending on the level of cleanliness required. Hence, all numbers should be understood to have an inherent variation of a factor of 2. To control systematic uncertainties with the very high level of precision that will be required by the upgrade, it may be necessary to sacrifice some of the statistics
Mode | 2011 yield (10^{3} events) | 50 fb^{−1} yield (10^{6} events) |
---|---|---|
Untagged D^{0}→K^{−}π^{+} | 230 000 | 40 000 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}π^{+} | 40 000 | 7 000 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{+}π^{−} | 130 | 20 |
D^{0}→K^{−}K^{+} | 25 000 | 4 600 |
D^{0}→π^{−}π^{+} | 6 500 | 1 200 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+} | 4 300 | 775 |
D^{∗+}→D^{0}π^{+}; D^{0}→π^{−}π^{+} | 1 100 | 200 |
D^{∗+}→D^{0}π^{+}; \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} \pi ^{-} \pi ^{+} \) | 300 | 180 |
D^{∗+}→D^{0}π^{+}; \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} K ^{-} K ^{+} \) | 45 | 30 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}π^{+}π^{−}π^{+} | 7 800 | 1 400 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+}π^{−}π^{+} | 120 | 20 |
D^{∗+}→D^{0}π^{+}; D^{0}→π^{−}π^{+}π^{−}π^{+} | 470 | 85 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}μ^{+}X | – | 4 000 |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{+}μ^{−}X | – | 0.1 |
Numbers of D^{+} and \(D ^{+}_{ s } \) signal events observed in the 2011 data in a variety of channels and those projected for 50 fb^{−1}. These channels can be used for direct CP violation studies. As discussed in the text, the numbers of events in any one channel can vary from one analysis to another, depending on the level of cleanliness required. To control systematic uncertainties with the very high level of precision that will be required by the upgrade, it may be necessary to sacrifice some of the statistics
Mode | 2011 yield (10^{3} events) | 50 fb^{−1} yield (10^{6} events) |
---|---|---|
D^{+}→K^{−}π^{+}π^{+} | 60 000 | 11 000 |
D^{+}→K^{+}π^{+}π^{−} | 200 | 40 |
D^{+}→K^{−}K^{+}π^{+} | 6 500 | 1 200 |
D^{+}→ϕπ^{+} | 2 800 | 500 |
D^{+}→π^{−}π^{+}π^{+} | 3 200 | 575 |
\(D ^{+} \rightarrow K ^{0}_{\mathrm{S}} \pi ^{+} \) | 1 500 | 1 000 |
\(D ^{+} \rightarrow K ^{0}_{\mathrm{S}} K ^{+} \) | 525 | 330 |
D^{+}→K^{−}K^{+}K^{+} | 60 | 10 |
\(D ^{+}_{ s } \rightarrow K ^{-} K ^{+} \pi ^{+} \) | 8 900 | 1 600 |
\(D ^{+}_{ s } \rightarrow \phi \pi ^{+} \), (ϕ→K^{−}K^{+}) | 5 350 | 1 000 |
\(D ^{+}_{ s } \rightarrow \pi ^{-} \pi ^{+} \pi ^{+} \) | 2 000 | 360 |
\(D ^{+}_{ s } \rightarrow K ^{-} \pi ^{+} \pi ^{+} \) | ||
\(D ^{+}_{ s } \rightarrow \pi ^{-} K ^{+} \pi ^{+} \) | 555 | 100 |
\(D ^{+}_{ s } \rightarrow K ^{-} K ^{+} K ^{+} \) | 50 | 10 |
\(D ^{+}_{ s } \rightarrow K ^{0}_{\mathrm{S}} K ^{+} \) | 410 | 260 |
\(D ^{+}_{ s } \rightarrow K ^{0}_{\mathrm{S}} \pi ^{+} \) | 33 | 20 |
Estimating the physics reach with the projected data sets requires a number of assumptions. The statistical precision generally improves as \(1 / \sqrt{N}\). Estimating the systematic error, and therefore ultimate physics reach, is more of an art. It is often the case that data can be used to control systematic uncertainties at the level of the statistical error, but the extent to which this will be possible cannot be reliably predicted. In some cases controlling systematic uncertainties will require sacrificing some of the statistics to work with cleaner signals or with signals which populate only parts of the detector where the performance is very well understood. Estimates of sensitivity to CP violation in mixing generally depend on the values of the mixing parameters—the larger the number of mixed events, the larger the effective statistics contributing to the corresponding CP violation measurement.
Estimated statistical uncertainties for mixing and CP violation measurements which can be made with the projected samples for 50 fb^{−1} described in Table 10
Sample | Parameter(s) | Precision |
---|---|---|
WS/RS Kπ | \(( x_{D}^{\prime2}, y_{D}^{\prime} ) \) | \(\mathcal{O} [ ( 10^{-5} , 10^{-4} ) ] \) |
WS/RS Kμν | r_{M} | \(\mathcal{O} ( 5\times10^{-7}) \) |
WS/RS Kμν | |p/q|_{D} | \(\mathcal{O} (1~\% ) \) |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+},π^{−}π^{+} | \(\Delta \mathcal{A}_{ \mathit{CP} } \) | 0.015 % |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+} | \(\mathcal{A}_{ \mathit{CP} } \) | 0.010 % |
D^{∗+}→D^{0}π^{+}; D^{0}→π^{−}π^{+} | \(\mathcal{A}_{ \mathit{CP} } \) | 0.015 % |
D^{∗+}→D^{0}π^{+}; \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} \pi ^{-} \pi ^{+} \) | (x_{D},y_{D}) | (0.015 %,0.010 %) |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+}, (π^{−}π^{+}) | y_{CP} | 0.004 % (0.008 %) |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+}, (π^{−}π^{+}) | A_{Γ} | 0.004 % (0.008 %) |
D^{∗+}→D^{0}π^{+}; D^{0}→K^{−}K^{+}π^{−}π^{+} | \(\mathcal{A}_{\rm T} \) | 2.5×10^{−4} |
The projected sensitivities for the two-body direct CP violation measurements are relatively solid: the 2011 \(\Delta \mathcal{A}_{ \mathit{CP} } \) measurements provide benchmark samples with full analysis cuts including fiducial cuts necessary to control systematic uncertainties for measuring \(\Delta \mathcal{A}_{ \mathit{CP} } \). The systematic errors for the separate \(\mathcal {A}_{ \mathit{CP} } ( K ^{-} K ^{+} )\) and \(\mathcal{A}_{ \mathit{CP} } ( \pi ^{-} \pi ^{+} )\) measurements will be more challenging and may require sacrificing statistical precision. The projections for measuring y_{CP} and A_{Γ} using K^{−}K^{+} and π^{−}π^{+} should also be robust as the same samples will be used for these analyses as for the \(\mathcal{A}_{ \mathit{CP} } \) measurements.
The projected precision for measuring (x_{D},y_{D}) from \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} \pi ^{-} \pi ^{+} \) comes from scaling the Belle [416] and BaBar [511] sensitivities. The statistical precisions could be even better as LHCb’s prompt sample will be enhanced at higher decay times where the mixing effects are larger. By contrast, D^{0} mesons from semileptonic B decays should be unbiased in this variable, providing a useful sample at lower decay times.
Estimated statistical uncertainties for CP violation measurements which can be made with the projected D^{+} samples for 50 fb^{−1} described in Table 11
Sample | Parameter(s) | Precision |
---|---|---|
\(D ^{+} \rightarrow K ^{0}_{\mathrm{S}} K ^{+} \) | Phase-space integrated CP violation | 10^{−4} |
D^{+}→K^{−}K^{+}π^{+} | Phase-space integrated CP violation | 5×10^{−5} |
D^{+}→π^{−}π^{+}π^{+} | Phase-space integrated CP violation | 8×10^{−5} |
D^{+}→K^{−}K^{+}π^{+} | CP violation in phases, amplitude model | (0.01–0.10)^{∘} |
D^{+}→K^{−}K^{+}π^{+} | CP violation in fraction differences, amplitude model | (0.01–0.10) % |
D^{+}→π^{−}π^{+}π^{+} | CP violation in phases, amplitude model | (0.01–0.10)^{∘} |
D^{+}→π^{−}π^{+}π^{+} | CP violation in fraction differences, amplitude model | (0.01–0.10) % |
D^{+}→K^{−}K^{+}π^{+} | CP violation in phases, model-independent | (0.01–0.10)^{∘} |
D^{+}→K^{−}K^{+}π^{+} | CP violation in fraction differences, model-independent | (0.01–0.10) % |
D^{+}→π^{−}π^{+}π^{+} | CP violation in phases, model-independent | (0.01–0.10)^{∘} |
D^{+}→π^{−}π^{+}π^{+} | CP violation in fraction differences, model-independent | (0.01–0.10) % |
4.8 Conclusion
LHCb has proven its capability of performing high-precision charm physics measurements. The experiment is ideally suited for CP violation searches and for measurements of decay-time-dependent processes such as mixing.
Finding evidence for a non-zero value of \(\Delta \mathcal{A}_{ \mathit{CP} } \) has raised the question of whether or not this may be interpreted as the first hint of physics beyond the SM at the LHC. Within the SM the central value can only be explained by significantly enhanced penguin amplitudes. This enhancement is conceivable when estimating flavour SU(3) or U-spin breaking effects from fits to D→PP data. However, attempts at estimating the long distance penguin contractions directly have not yielded conclusive results to explain the enhancement.
Lattice QCD has the potential of assessing the penguin enhancement directly. However, several challenges arise which make these calculations impossible at the moment. Following promising results on K→ππ decays, additional challenges arise in the charm sector as ππ and KK states mix with ηη, 4π, 6π and other states. Possible methods have been proposed and results may be expected in three to five years time.
General considerations on the possibility of interpreting \(\Delta \mathcal{A}_{ \mathit{CP} } \) in models beyond the SM have led to the conclusion that an enhanced chromomagnetic dipole operator is required. These operators can be accommodated in minimal supersymmetric models with non-zero left-right up-type squark mixing contributions or, similarly, in warped extra dimensional models. Tests of these interpretations beyond the SM are needed. One promising group of channels are radiative charm decays where the link between the chromomagnetic and the electromagnetic dipole operator leads to predictions of enhanced CP asymmetries of several percent. These can be measured to sufficient precision at the LHCb upgrade.
Another complementary test is to search for contributions beyond the SM in ΔI=3/2 amplitudes. This class of amplitudes leads to several isospin relations which can be tested in a range of decay modes, e.g. D→ππ, D→ρπ, \(D \rightarrow K \overline{ K }\), etc. Several of these measurements, such as the Dalitz plot analysis of the decay D^{0}→π^{+}π^{−}π^{0}, can be performed at LHCb.
Beyond charm physics, the chromomagnetic dipole operators would affect the neutron and nuclear EDMs, which are expected to be close to the current experimental bound. Similarly, rare FCNC top decays are expected to be enhanced, if kinematically allowed. Furthermore, quark compositeness can be related to the \(\Delta \mathcal{A}_{ \mathit{CP} } \) measurement and tested in dijet searches. Current results favour the NP contribution to be located in the D^{0}→K^{−}K^{+} decay as the strange quark compositeness scale is less well constrained. Measurements of the individual asymmetries of sufficient precision will be possible at the LHCb upgrade.
The charm mixing parameters have not yet been precisely calculated in the SM. An inclusive approach based on an operator product expansion relies on the expansion scale being small enough to allow convergence and furthermore involves the calculation of a large number of unknown matrix elements. An exclusive approach sums over intermediate hadronic states and requires very precise branching ratio determinations of these final states which are currently not available. Contrary to the SM, contributions beyond the SM can be calculated reliably. With the SM contribution to indirect CP violation being <10^{−4}, the LHCb upgrade is ideally suited to cover the parameter space available for enhanced asymmetries beyond the SM. Measurements in several complementary modes will permit the extraction of the underlying theory parameters with high precision.
The LHCb upgrade will allow to constrain CP asymmetries and mixing observables to a level of precision which, in most of the key modes, cannot be matched by any other experiment foreseen on a similar timescale. This level of precision should permit us not only to discover CP violation in charm decays but also to unambiguously understand its origin.
5 The LHCb upgrade as a general purpose detector in the forward region
The previous sections have focussed on flavour physics observables that are sensitive to physics beyond the SM. However, LHCb has excellent potential in a range of other important topics. As discussed in this section, the detector upgrade will further enhance the capability of LHCb in these areas, so that it can be considered as a general purpose detector in the forward region. LHCb may also be able to make a unique contribution to the field of heavy ion physics, by studying soft QCD and heavy flavour production in pA collisions. The first pA run of the LHC will clarify soon the potential of LHCb in this field.
5.1 Quarkonia and multi-parton scattering
The mechanism of heavy quarkonium production is a long-standing problem in QCD. An effective field theory, non-relativistic QCD (NRQCD), provides the foundation for much of the current theoretical work. According to NRQCD, the production of heavy quarkonium factorizes into two steps: a heavy quark–antiquark pair is first created perturbatively at short distances and subsequently evolves non-perturbatively into quarkonium at long distances. The NRQCD calculations depend on the colour-singlet (CS) and colour-octet (CO) matrix elements, which account for the probability of a heavy quark–antiquark pair in a particular colour state to evolve into heavy quarkonium. The CS model [512, 513], which provides a leading-order (LO) description of quarkonia production, was first used to describe experimental data. However, it underestimates the observed cross-section for single J/ψ production at high p_{T} at the Tevatron [514]. To resolve this discrepancy the CO mechanism was introduced [515]. The corresponding matrix elements were determined from the large-p_{T} data as the CO cross-section falls more slowly than that for CS. More recent higher-order calculations [516, 517, 518, 519] close the gap between the CS predictions and the experimental data [520] reducing the need for large CO contributions.
As the cross-sections for charmonium production at the LHC are large [521, 522, 523, 525], the question of multiple production of these states in a single proton–proton collision naturally arises. Studies of double hidden charm and hidden and associated open charm production have been proposed as probes of the quarkonium production mechanism [526]. In proton–proton collisions contributions from other mechanisms, such as double parton scattering (DPS) [527, 528, 529] or the intrinsic charm content of the proton [530], are possible. First studies of both processes have been carried out with the current LHCb data; more details can be found in Refs. [304, 531].
LO colour singlet calculations for the gg→J/ψJ/ψ process in perturbative QCD exist and give results consistent with the data [532, 533, 534]. In the LHCb fiducial region (2<y_{J/ψ}<4.5, \(p^{\mathrm {T}}_{ { J / \psi } }<10~\mathrm {GeV}/c\), where y_{J/ψ} and \(p^{\mathrm{T}}_{ { J / \psi } }\) represent the rapidity and transverse momentum of the J/ψ, respectively) these calculations predict the J/ψJ/ψ production cross-section to be 4.1±1.2 nb [534] in agreement with the measured value of 5.1±1.0 nb [531]. Similar calculations exist for the case of double ϒ(1S) production. For the case of J/ψ plus ϒ(1S) production no leading order diagrams contribute and hence the rate is expected to be suppressed in Single Parton Scattering (SPS). This leads to an “unnatural” ordering of the cross-section values: \(\sigma^{ { J / \psi } { J / \psi } }_{gg} > \sigma^{ \varUpsilon{(1S)} \varUpsilon{(1S)} }_{gg} > \sigma^{ \varUpsilon{(1S)} { J / \psi } }_{gg}\).
The DPS contributions to all these double onia production modes can be estimated, neglecting partonic correlations in the proton, as the product of the measured cross-sections of the sub-processes involved divided by an effective cross-section [527, 528, 529, 535]. The value of the latter is determined from multi-jet events at the Tevatron to be \(\sigma^{\mathrm {DPS}}_{\mathrm{eff}}= 14.5\pm1.7^{+1.7}_{-2.3}~\mathrm{mb}\) [536]. At \(\protect\sqrt{s} = 7 ~\mathrm{TeV} \) the contribution from this source to the total cross-section is similar in size to the LO contribution from SPS. For DPS the ordering of the cross-section values is: \(\sigma ^{ { J / \psi } { J / \psi } }_{\rm DPS} > \sigma^{ \varUpsilon{(1S)} { J / \psi } }_{\rm DPS} >\sigma ^{ \varUpsilon{(1S)} \varUpsilon {(1S)} }_{\rm DPS}\).
Expected cross-sections in the LHCb acceptance and yields for double quarkonia production with 50 fb^{−1} at \(\protect\sqrt{s} = 14 ~\mathrm{TeV} \)
Mode | σ_{gg} [nb] | Yield [SPS] | σ_{DPS} [nb] | Yield [DPS] |
---|---|---|---|---|
J/ψJ/ψ | 7.2 | 270 000 | 11 | 430 000 |
J/ψψ(2S) | 3.2 | 14 000 | 4.0 | 19 000 |
ψ(2S) ψ(2S) | 0.4 | 180 | 0.6 | 300 |
J/ψχ_{c0} | – | – | 4.3 | 200 |
J/ψχ_{c1} | – | – | 6.6 | 14 000 |
J/ψχ_{c2} | – | – | 8.6 | 11 000 |
J/ψϒ(1S) | 0.0036 | 360 | 0.27 | 20 000 |
J/ψϒ(2S) | 0.0011 | 90 | 0.07 | 5300 |
J/ψϒ(3S) | 0.0005 | 50 | 0.035 | 2000 |
ϒ(1S) ϒ(1S) | 0.014 | 1100 | 0.0027 | 200 |
As well as probing the production mechanism these studies are sensitive to a potential first observation of tetraquark states [534] and of χ_{b} and η_{b} states decaying in the double J/ψ mode. Based on the cross-sections and branching ratios given in Ref. [537], 500 (1500) fully reconstructed χ_{b0}(1P) (χ_{b2}(1P)) are expected with the upgraded detector and these decays will be visible at LHCb. In the case of the η_{b} state, several estimates exist, based on values of the branching ratio η_{b}→J/ψJ/ψ ranging from 10^{−6} to 10^{−8} [538], corresponding to yields of 0.02 to 5 events.
The upgraded detector is expected to have excellent hadron identification capabilities both offline and at the trigger level. As discussed in Ref. [539], this allows charmonium studies to be performed in hadronic decay modes. A particularly convenient mode is the \(p\overline{p}\) final state. This is accessible for the J/ψ, η_{c}, χ_{cJ}, h_{c} and ψ(2S) mesons. Extrapolating from studies with the current detector large inclusive samples of these decays will be collected. For example around 0.5 million \(\eta _{ c } \rightarrow p \overline {p}\) will be collected.
Hadronic decays of heavy bottomonium have received less attention in the literature [538]. The high mass implies a large phase space for many decay modes, but consequently the branching ratio for each individual mode is reduced. In Ref. [538] it is estimated that the \(\eta_{b} \rightarrow D^{*} \overline{D}\) branching fraction is 10^{−5} and the \(\eta_{b} \rightarrow D \overline{D} \pi\) rate may be a factor of ten higher. Though no specific studies have been performed, based on the studies of double open charm production given in Ref. [304] it is plausible that an η_{b} signal will be detected in this mode with the upgraded detector.
5.2 Exotic meson spectroscopy
The spectroscopy of bound states formed by heavy quark–antiquark pairs (c or b quarks), has been extensively studied from both theoretical and experimental points of view since the discovery of the J/ψ state in 1974 [540, 541] and the discovery of the ϒ(1S) state in 1977 [542]. Until recently, all experimentally observed charmonium (\(c\bar{c}\)) and bottomonium (\(b\bar{b}\)) states matched well with expectations.
However, in 2003, a new and unexpected charmonium state was observed by the Belle experiment [543] and then confirmed independently by the BaBar [544], CDF [545] and D0 [546] experiments. This new particle, referred to as the X(3872), was observed in B→X(3872)K decays, in the decay mode X(3872)→J/ψπ^{+}π^{−} and has a mass indistinguishable (within uncertainties) from the \(D^{*0} \overline{ D }{} ^{0} \) threshold [520]. Several of the X(3872) parameters are unknown (such as its spin) or have large uncertainties, but this state does not match any predicted charmonium state [520]. The discovery of the X(3872) has led to a resurgence of interest in exotic spectroscopy and subsequently many new states have been claimed. For example: the Y family, Y(4260),Y(4320) and Y(4660), of spin parity 1^{−}, or the puzzling charged Z family, Z(4050)^{+},Z(4250)^{+} and Z(4430)^{+}, so far observed only by the Belle experiment [547, 548, 549], and not confirmed by BaBar [550, 551]. The nature of these states has drawn much theoretical attention and many models have been proposed. One possible explanation is that they are bound molecular states of open charm mesons [552]. Another is that these are tetraquarks [553] states formed of four quarks (e.g. \(c,\bar{c}\), one light quark and one light anti-quark). Other interpretations have been postulated such as quark–gluon hybrid [553] or hadrocharmonium models [554], but experimental data are not yet able to conclude definitely. For reviews, see Refs. [520, 552, 554, 555, 556, 557, 558].
The bottomonium system should exhibit similar exotic states to the charmonium case. The Belle experiment recently reported the observation of exotic bottomonium charged particles Z_{b}(10610)^{+} and Z_{b}(10650)^{+} in the decays Z_{b}→ϒ(nS)π^{+} and Z_{b}→h_{b}(nP)π^{+} [559]. Evidence for a neutral isopartner has also been reported [560].^{70} These states appear similar to, but narrower than, the Z(4430)^{+} observed in the charmonium case. In addition, neutral states analogous to the X(3872) and the Y states are expected in the bottomonium system.
Studies of the X(3872) have already been performed with the current detector [562]. The 50 fb^{−1} of integrated luminosity collected with the upgraded detector will contain over one million X(3872)→J/ψππ candidates, by far the largest sample ever collected and allow study of this meson with high precision. A significant fraction of the X(3872) sample will originate from the decays of B mesons (the remainder being promptly produced) allowing the quantum numbers and other properties to be determined. With such a large sample the missing ^{3}D_{2} state of the charmonium system [563] will be also be observed and studied with high precision.
Another study being pursued with the current detector is to clarify the status of the Z(4430)^{+} state. If confirmed, the Z(4430)^{+} will be copiously produced at \(\sqrt {s}=14 ~\mathrm{TeV} \) and the larger data set will allow detailed study of its properties in different B decay modes, thus setting the basis for all future searches for exotic charged states.
Similar to the charmonium-like states, exotic bottomonium states will mainly be searched for in the ϒ(nS)π^{+}π^{−} channel, with ϒ(nS)→μ^{+}μ^{−}. The excellent resolution observed in the ϒ(nS) analysis [524] allows efficient separation of the three states, which is crucial in searching for exotic bottomonium states in these channels.
All these studies, and searches for other exotica such as pentaquarks will profit from the increased integrated luminosity.
5.3 Precision measurements of b- and c-hadron properties
A major focus of activity with the current LHCb detector is the study of the properties of beauty and charm hadrons. This is a wide ranging field including studies of properties such as mass and lifetime, observation of excited b hadrons and the measurements of branching ratios. These studies provide important input to pQCD models. Three topics are considered here: b decays to charmonia, \(B^{+}_{c}\), and b-baryon decays.
One important field being studied with the current detector is exclusive b decays to charmonia. Studies of these modes are important to improve understanding of the shape of the momentum spectrum of J/ψ produced in b hadron decays, as measured by the B factories [564, 565]. To explain the observed excess at low momentum, new contributions to the total b→J/ψX rate are needed. Several sources have been proposed in the literature: intrinsic charm [566], baryonium formation [567] and as yet unobserved exotic states [568]. One of the first proposed explanations for the excess was a contribution from an intrinsic charm component to the b-hadron wave-function [566]. This would lead to an enhancement of b-hadron decays to J/ψ in association with open charm. The B-factories have set limits on such decays at the level of 10^{−5} [190], which considerably restricts, but does not exclude, contributions from intrinsic charm models. The branching ratios of these decays have been estimated in pQCD [569]. In the case of B^{0}→J/ψD^{0} the branching ratio has been estimated to be 7×10^{−7}. If this value is correct, several hundred fully reconstructed events will be collected with the upgraded detector. Similar decay modes are possible for \(B ^{0}_{ s } \) and \(B _{ c } ^{+} \) mesons though no limits (or predictions) exist.
Another possibility to explain the shape of the J/ψ spectrum is contributions from exotic strange baryonia formed in decays such as \(B^{+} \rightarrow { J / \psi } \overline{\varLambda}{} ^{0} p\). This decay has been observed by BaBar [570], with a branching ratio of (1.18±0.31)×10^{−5}. The related decay \(B ^{0} \rightarrow { J / \psi } p \overline{p}\) is unobserved, with an upper limit on the branching ratio of 8.3×10^{−7} at 90 % confidence level [571]. At present, these decays are experimentally challenging due to the low Q-values involved. The larger data samples available at the time of the upgrade, together with improved proton identification at low momentum, may lead to their observation.
Compared to the case of B^{0} and B^{+}, the \(B ^{0}_{ s } \) sector is less well explored both experimentally and theoretically. Decays such as \(B ^{0}_{ s } \rightarrow { J / \psi } K^{*0} \overline {K}^{*0}\) and \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\rho\) should be observable with the present detector. With the upgraded apparatus, the decay modes \(B ^{0}_{ s } \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} K ^{0}_{\mathrm{S}} \) and \(B ^{0}_{ s } \rightarrow { J / \psi } \phi\phi\) will also become accessible. The latter channel is interesting as the low Q-value will allow a precision determination of the \(B ^{0}_{ s } \) mass.
Branching ratios and expected yields for selected \(B _{ c } ^{+} \) decays to final states containing a J/ψ or ψ(2S) meson. The branching ratios for the J/ψ modes are taken from Ref. [573], with the additional constraint of the ratio of the \(B _{ c } ^{+} \rightarrow { J / \psi } 3\pi^{+}\) to \(B _{ c } ^{+} \rightarrow { J / \psi } \pi^{+}\) reported in Ref. [572]. The ψ(2S) mode branching ratios are estimated assuming that they are 0.5 of the J/ψ values, as observed in many modes (see for example Ref. [574]). Only dimuon modes are considered for the J/ψ and ψ(2S), and only the K^{+}K^{−}π^{+} (K^{+}π^{−}π^{+}) modes are considered for the \(D^{+}_{s}\) (D^{+}) modes. The \(B _{ c } ^{+} \rightarrow K^{+} K^{*0}\) numbers are taken from Ref. [575]
Mode | Branching ratio | Expected yield [50 fb^{−1}] |
---|---|---|
\(B _{ c } ^{+} \rightarrow { J / \psi } \pi^{+}\) | 2×10^{−3} | 52 000 |
\(B _{ c } ^{+} \rightarrow { J / \psi } 3 \pi^{+}\) | 5×10^{−3} | 17 000 |
\(B _{ c } ^{+} \rightarrow { J / \psi } K^{+}\) | (1–2)×10^{−4} | 3000–4000 |
\(B _{ c } ^{+} \rightarrow { J / \psi } K_{1}^{+}\) | 3×10^{−5} | 1000 |
\(B _{ c } ^{+} \rightarrow \psi {(2S)} \pi^{+}\) | 1×10^{−3} | 3000 |
\(B _{ c } ^{+} \rightarrow \psi {(2S)} 3 \pi^{+}\) | 2.5×10^{−3} | 1000 |
\(B _{ c } ^{+} \rightarrow { J / \psi } D^{+}_{s}\) | (2–3)×10^{−3} | 1400–1900 |
\(B _{ c } ^{+} \rightarrow { J / \psi } D^{+}\) | (5–13)×10^{−4} | 8–100 |
\(B _{ c } ^{+} \rightarrow K^{+} K^{*0}\) | 10^{−6} | 500 |
The large \(B _{ c } ^{+} \) data set will open possibilities for many other studies. Decay modes of the \(B _{ c } ^{+} \) meson to a \(B ^{0}_{ s } \) or B^{0} meson together with a pion or kaon will also be accessible. Studies of the \(B _{ c } ^{+} \rightarrow B ^{0}_{ s } \pi^{+}\) decay have been started with the data collected in 2011 where a handful of events are expected. As discussed in Ref. [573], semileptonic \(B _{ c } ^{+} \) decays to \(B ^{0}_{ s } \) can be used to provide a clean tagged decay source for CP violation studies. Finally, signals of the currently unexplored excited \(B^{+}_{c}\) meson states are expected to be observed [576, 577, 578, 579]. As discussed in Ref. [575] observation of the \(B_{c}^{*+}\) decay is extremely challenging due to the soft photon produced in the decay to the ground state. The prospects for observation of the first P-wave multiplet decays decaying radiatively to the ground state are more promising.
Baryonic states containing two heavy quarks will also be observable. The lightest of these, the Ξ_{cc} isodoublet, have an estimated cross-section of \(\mathcal{O}(10^{2}) \mbox{~nb}\) [584, 585] and so should be visible with 5 fb^{−1} collected with the current detector. However, the statistics may be marginal for follow-on analyses: measurements of the lifetime and ratios of branching fractions, searches for excited states, and so forth. They will certainly be insufficient for angular analyses aimed at confirming the quark model predictions for the spin-parity of these states. These studies will require the statistics and improved triggering of the LHCb upgrade. Heavier states such as the Ω_{cc}, Ξ_{bc}, and Ξ_{bb} have still smaller production cross-sections [585]. First studies towards Ξ_{bc} detection are in progress. These indicate that at best a handful of events can be expected in 5 fb^{−1}, but that this state should be observable with the upgrade.
5.4 Measurements with electroweak gauge bosons
Two of the most important quantities in the LHC electroweak physics programme are the sine of the effective electroweak mixing angle for leptons, \(\sin^{2}\theta^{\rm lept}_{\rm eff}\), and the mass of the W-boson, m_{W}. Thanks to its unique forward coverage, an upgraded LHCb can make important contributions to this programme. The forward coverage of LHCb also allows a probe of electroweak boson production in a different regime from that of ATLAS and CMS, and the range of accessible physics topics is not limited to electroweak bosons. For example, \(t\bar{t}\) production proceeds predominantly by gluon–gluon fusion in the central region, but has a significant contribution from quark–antiquark annihilation in the forward region, giving a similar production regime to that studied at the Tevatron.
5.4.1 \(\sin^{2}\theta^{\rm lept}_{\rm eff}\)
The value of \(\sin^{2}\theta^{\rm lept}_{\rm eff}\) can be extracted from \(A_{\rm FB}\), the forward–backward asymmetry of leptons produced in Z decays. The raw value of \(A_{\rm FB}\) measured in dimuon final states at the LHC is about five times larger than at an e^{+}e^{−} collider, due to the initial state couplings, and so, in principle, it can be measured with a better relative precision, given equal amounts of data. The measurement however requires knowledge of the direction of the quark and antiquark that created the Z boson, and any uncertainty in this quantity results in a dilution of the observed value of \(A_{\rm FB}\). This dilution is very significant in the central region, as there is an approximately equal probability for each proton to contain the quark or anti-quark that is involved in the creation of the Z, leading to an ambiguity in the definition of the axis required in the measurement. However, the more forward the Z boson is produced, the more likely it is that it follows the quark direction; for rapidities y>3, the Z follows the quark direction in around 95 % of the cases. Furthermore, in the forward region, the partonic collisions that produce the Z are nearly always between u-valence and \(\bar{u}\)-sea quark or d-valence and \(\bar{d}\)-sea quark. The \(s \bar{s}\) contribution, with a less well-known parton density function, is smaller than in the central region. Consequently, the forward region is the optimum environment in which to measure \(A_{\rm FB}\) at the LHC. Preliminary studies [586] have shown that with a 50 fb^{−1} data sample collected by the LHCb upgrade, \(A_{\rm FB}\) could be measured with a statistical precision of around 0.0004. This would give a statistical uncertainty on \(\sin^{2} \theta^{\rm lept}_{\rm eff}\) of better than 0.0001, which is a significant improvement in precision on the current world average value. It is also worth remarking that the two most precise values entering this world average at present, the forward–backward \(b\bar{b}\) asymmetry measured at LEP (\(\sin^{2} \theta^{\rm lept}_{\rm eff} = 0.23221 \pm 0.00029\)), and the left-right asymmetries measured at SLD with polarised beams (\(\sin^{2} \theta^{\rm lept}_{\rm eff}=0.23098 \pm0.00026\)), are over 3σ discrepant with each other [587]. LHCb will be able to bring clarity to this unsatisfactory situation.
5.4.2 m_{W}
Decreasing the uncertainty on m_{W} from its present error of 15 MeV/c^{2} is one of the most challenging tasks for the LHC (it may also be reduced further at the Tevatron). Although no studies have yet been made of determining m_{W} with LHCb itself, it is evident that the experiment can give important input to the measurements being made at ATLAS and CMS [591]. A significant and potentially limiting external uncertainty on m_{W} will again come from the knowledge of the parton density functions. These are less constrained in the kinematic range accessible to LHCb, so that precise measurements of W^{+}, W^{−}, Z and Drell–Yan production in this region can be used to improve the global picture. Improved determinations of the shapes of the differential cross-sections are particularly important. One specific area of concern arises from the knowledge of the heavy quarks in the proton. Around 20–30 % of W production in the central region is expected to involve s and c quarks, making the understanding of this component very important for the m_{W} measurement. LHCb can make a unique contribution to improving the knowledge of the heavy-quark parton density functions by exploiting its vertexing and particle identification capabilities to tag the relatively low-p_{T} final-state quarks produced in processes such as gs→Wc, gc→Zc, gb→Zb, gc→γc and gb→γb. These processes provide direct probes of the strange, charm and bottom partons, and can be probed at high and low values of Bjorken x inside the LHCb acceptance.
5.4.3 \(t\bar{t}\) production
Understanding the nature of top production, and in particular the asymmetry in \(t \bar{t}\) events reported by Fermilab [592, 593, 594, 595, 596], is of prime concern. As for the measurement of \(\sin^{2}\theta^{\rm lept}_{\rm eff}\), identifying the forward direction of events is crucial. The LHCb acceptance for identifying both leptons from \(t \bar{t}\) decays is far smaller than that of ATLAS and CMS (typically 2 % rather than 70 %, according to PYTHIA generator level studies). However, the higher \(q \bar{q}\) production fraction and better determined direction in the LHCb forward acceptance combine to suggest that competitive measurements can be achieved. With the integrated luminosity offered by the upgrade, statistical precision will no longer be an issue, and LHCb measurements of the \(t \bar{t}\) asymmetry will offer a competitive and complementary test of Tevatron observations [597].
5.5 Searches for exotic particles with displaced vertices
Different theoretical paradigms have been proposed to solve the so-called “hierarchy problem”, the most discussed being SUSY. There are, however, many other ideas including various models involving extra dimensions, Technicolour and little Higgs models. These ideas approach the hierarchy problem from the direction of strong dynamics [598].
A growing subset of models features new massive long-lived particles with a macroscopic distance of flight. They can be produced by the decay of a single-produced resonance, such as a Higgs boson or a Z′ [599, 600], from the decay chain of SUSY particles [601], or by a hadronisation-type mechanism in models where the long-lived particle is a bound state of quarks from a new confining gauge group, as discussed in Ref. [599]. In the last case, the multiplicity of long-lived particles in an event can be large, while only one long-lived particle is expected to be produced in other models. The decay modes may also vary depending on the nature of the particle, from several jets in the final state [600] to several leptons [602] or lepton plus jets [603]. A comprehensive review of the experimental signatures is given in Ref. [604].
The common feature amongst these models is the presence of vertices displaced from the interaction region. Such signatures are well suited to LHCb, and in particular to the upgraded experiment, which will be able to select events with displaced vertices at the earliest trigger level.
The potential of LHCb to search for such exotic Higgs decays at \(\sqrt {s}=14 ~\mathrm{TeV} \) has been discussed in Ref. [25], and is briefly summarised here. The benchmark model uses m_{H}=120 GeV/c^{2}, \(m_{\pi^{0}_{v}} = 35 ~\mathrm {GeV}/c^{2} \) and \(\tau_{\pi^{0}_{v}} = 10 \mbox{~ps}\). By combining vertex and jet reconstruction, the capacity to reconstruct this final state is shown using full simulation of the detector, assuming 0.4 interactions per crossing. Backgrounds to this signal from other processes, such as the production of two pairs of \(b\bar{b}\) quarks, have been considered and found to be negligible.
During 2010 and 2011 data taking, an inclusive displaced vertex trigger has been introduced in the second level of the software trigger. Preliminary studies [605] have demonstrated that for an output rate below 1 % of the overall trigger bandwidth, the efficiency of the whole trigger chain on events with two offline reconstructible \(\pi^{0}_{v}\) vertices with a minimum mass of 6 GeV and good vertex quality is of the order of 80 %. This strategy has been tested up to on average two visible interactions per crossing which is what is expected for the upgraded experiment.
In the upgraded detector, the track fake rate in the vertex detector is expected to be below one percent [26], compared to 6 % in the present detector. Other upgrades to the tracking detectors will also help to reduce the fake rate. Moreover the use of an improved description for the complex RF foil shape will give a better control on the background arising from hadronic interactions. It will enable the use of the true shape of the RF foil, rather than the loose fiducial volume cut used at present, which depending on the considered lifetime, rejects 10–30 % of the long-lived particles. Those improvements would allow to decrease the thresholds on the single candidates trigger and therefore increase the reach of such searches.
As discussed in Ref. [25] the coupling of vertex information to jet reconstruction will allow to reduce the physical backgrounds. Studies are on-going on this matter. Assuming a Higgs production cross-section at \(\sqrt{s} = 14 ~\mathrm{TeV} \) of 50 pb, an integrated luminosity of 50 fb^{−1} and a geometric efficiency of 10 %, 250 000 Higgs bosons will be produced in LHCb. If \(H^{0} \rightarrow \pi_{v}^{0}\pi_{v}^{0}\) is a dominant decay mode, then LHCb will be in an excellent position to observe this signal, taking advantage of the software trigger’s ability to select high-multiplicity events with good efficiency.
5.6 Central exclusive production
Although not part of the baseline for the LHCb upgrade, additional instrumentation is being considered which could improve the potential of LHCb to study CEP processes. For example, the inclusion of forward shower counters (FSCs) on both sides of the interaction point, as proposed in Ref. [612], would be able to detect showers from very forward particles interacting in the beam pipe and surrounding material. The absence of a shower would indicate a rapidity gap and be helpful in increasing the purity of a CEP sample. More ambitiously, the deployment of semi-conductor detectors very close to the beam, within Roman pots, several hundred meters away from the interaction point, as proposed for other LHC experiments [613] would also be beneficial for LHCb. The ability to measure the directions of the deflected protons in the CEP interaction provides invaluable information in determining the quantum numbers of the centrally produced state.
Accumulation and characterisation of large samples of exclusive \(c\bar {c}\) and \(b\bar{b}\) events. A full measurement programme of these ‘standard candles’ will be essential to understand better the QCD mechanism of CEP [614], and may provide vital input if CEP is used for studies of Higgs and other new particles [615].
Searches for structure in the mass spectra of decay states such as K^{+}K^{−}, 2π^{+}2π^{−}, K^{+}K^{−}π^{+}π^{−} and \(p\bar{p}\). A particular interest of this study would be the hunt for glueballs, which are a key prediction of QCD.
Observation and study of exotic particles in CEP processes. For example, a detailed study of the CEP process pp→p+X(3872)+p would provide a valuable new tool to aid understanding of this state. This and other states could be searched for in, for example, decays containing \(D\overline{D}\), which if observed would shed light onto the nature of the parent particle [614].
Even when running at a luminosity of 10^{33} cm^{−2} s^{−1} LHCb will have low pileup compared to ATLAS and CMS. This will be advantageous in triggering and reconstructing low mass CEP states.
The higher integrated luminosity that will be collected by the upgraded detector will allow studies to be performed on states that are inaccessible with only a few fb^{−1}. This is true, for example, of central exclusive χ_{b} production, which is expected to be a factor of ∼1000 less than that of χ_{c} mesons [614].
The particle identification capabilities of the LHCb ring-imaging Cherenkov detector system allow centrally produced states to be cleanly separated into decays involving pions, kaons and protons.
The low \(p_{\rm T}\) acceptance of LHCb, and high bandwidth trigger, will allow samples of relatively low mass states to be collected and analysed.
6 Summary
As described in the previous sections, LHCb has produced world-leading results across its physics programme, using the 1.0 fb^{−1} data sample of \(\sqrt{s} = 7 ~\mathrm{TeV} \)pp collisions collected in 2011. The inclusion of the data collected at \(\sqrt{s} = 8 ~\mathrm{TeV} \) during 2012 will enable further improvements in precision in many key flavour physics observables. However, an upgrade to the detector is needed to remove the bottleneck in the trigger chain that currently prevents even larger increases in the collected data sample. The upgraded detector with trigger fully implemented in software is to be installed during the 2018 long shutdown of the LHC, and will allow a total data set of 50 fb^{−1} to be collected. With such a data sample, LHCb will not only reach unprecedented precision for a wide range of flavour physics observables, but the flexible trigger will allow it to exploit fully the potential of a forward physics experiment at a hadron collider.
In this section, some highlights of the LHCb physics output so far, and their implications on the theoretical landscape, are summarised. The sensitivity of the upgraded detector to key observables is then given, before a concluding statement on the importance of the LHCb upgrade to the global particle physics programme.
6.1 Highlights of LHCb measurements and their implications
6.1.1 Rare decays
The measurement of the forward–backward asymmetry in B^{0}→K^{∗0}μ^{+}μ^{−} [15] has to be viewed as the start of a programme towards a full angular analysis of these decays. The full analysis will allow determination of numerous NP-sensitive observables (see, for example, Refs. [53, 54]). The measurements that will be obtained from such an analysis, as well as similar studies of related channels, such as \(B ^{0}_{ s } \rightarrow \phi \mu ^{+} \mu ^{-} \) [69], allow model-independent constraints on NP, manifested as limits on the operators of the effective Hamiltonian (see, for example, Refs. [42, 43]). Indeed, the first results already impose important constraints. Studies of radiative decays such as \(B ^{0}_{ s } \rightarrow \phi \gamma\) [16, 17] provide additional information since they allow to measure the polarisation of the emitted photon, and are therefore especially sensitive to models that predict new right-handed currents. Similarly, studies of observables such as isospin asymmetries [77] are important since they allow to pin down in which operators the NP effects occur.
Several new opportunities with rare decays at LHCb are becoming apparent. The observation of B^{+}→π^{+}μ^{+}μ^{−} [86], the rarest B decay yet discovered, enables a new approach to measure the ratio of CKM matrix elements |V_{td}/V_{ts}|. Decays to final states containing same-sign leptons [197] allow searches for Majorana neutrinos complementary to those based on neutrinoless double beta decay. LHCb can also reach competitive sensitivity for some lepton flavour violating decays such as τ^{+}→μ^{+}μ^{−}μ^{+} [191].
6.1.2 CP violation in the B sector
In addition, to understand the origin of the anomalous dimuon asymmetry seen by D0 [159], improved measurements of semileptonic asymmetries in both \(B ^{0}_{ s } \) and B^{0} systems are needed. LHCb has just released its first results on the \(B ^{0}_{ s } \) asymmetry [248], demonstrating the potential to search for NP effects with more precise measurements. Moreover, a constraint on, or a measurement of, the rate of the decay \(B ^{0}_{ s } \rightarrow \tau^{+}\tau^{-}\) is important to provide knowledge of possible NP contributions to Γ_{12} (see, for example, Refs. [153, 155]).
Among the B^{0} mixing parameters, improved measurements of both ϕ_{d} (i.e., sin2β) and ΔΓ_{d} are needed. Reducing the uncertainty on the former will help to improve the global fits to the CKM matrix [252, 266], and may clarify the current situation regarding the tension between various inputs to the fits (see, for example, Ref. [267]). Another crucial observable is the angle γ, which, when measured in the tree-dominated B→DK processes, provides a benchmark measurement of CP violation. The first measurements from LHCb already help to improve the uncertainty on γ [6, 7]: further improvements are both anticipated and needed.
Comparisons of values of γ from loop-dominated processes with the SM benchmark from tree-dominated processes provide important ways to search for new sources of CP violation. In particular, the study of \(B ^{0}_{ s } \rightarrow K ^{+} K ^{-} \) and B^{0}→π^{+}π^{−} decays [356], which are related by U-spin, allows a powerful test of the consistency of the observables with the SM [355, 357]. Similarly, the U-spin partners \(B ^{0}_{ s } \rightarrow K ^{*0} \overline{ K }{} ^{*0} \) [303] and \(B ^{0} \rightarrow K ^{*0} \overline{ K }{} ^{*0} \) are among the golden channels to search for NP contributions in \(b \rightarrow s q\bar{q}\) penguin amplitudes [308]. Another important channel in this respect is \(B ^{0}_{ s } \rightarrow \phi \phi\) [304], for which the CP-violating observables are predicted with low theoretical uncertainty in the SM. Studies of CP violation in multibody b hadron decays [376, 377] offer additional possibilities to search for both the existence and features of NP.
6.1.3 Charm mixing and CP violation
The SM predictions are somewhat cleaner for indirect CP violation effects, and therefore it is also essential to search for CP violation in charm mixing. New results from time-dependent analyses of D^{0}→K^{+}K^{−} [19] and \(D ^{0} \rightarrow K ^{0}_{\mathrm{S}} \pi ^{+} \pi ^{-} \) will improve the current knowledge, and additional channels will also be important with high statistics.
Several authors have noted correlations between CP violation in charm and various other observables (for example, Refs. [469, 484]). These correlations appear in, and differ between, certain theoretical models, and can therefore be used to help identify the origin of the effects. Observables of interest in this context include those that can be measured at high-\(p_{\rm T}\) experiments, such as \(t\bar{t}\) asymmetries, as well as rare charm decays. Among the latter, it has been noted that CP asymmetries are possible in radiative decays such as D^{0}→ϕγ [485], and that searches for decays involving dimuons, such as D^{0}→μ^{+}μ^{−} [178] and D^{+}→π^{+}μ^{+}μ^{−} are well motivated.
6.1.4 Measurements exploiting the unique kinematic acceptance of LHCb
The unique kinematic region covered by the LHCb acceptance enables measurements that cannot be performed at other experiments, and that will continue to be important in the upgrade era. These include probes of QCD both in production, such as studies of multi-parton scattering [531, 616], and in decay, such as studies of exotic hadrons like the X(3872) [562] and the putative Z(4430)^{+} state. Conventional hadrons can also be studied with high precision: one important goal will be to establish the existence of doubly heavy baryons. Central exclusive production of conventional and exotic hadrons can also be studied; the sensitivity of the upgraded experiment will be significantly enhanced due to the software trigger.
Measurements of production rates and asymmetries of electroweak gauge bosons in the LHCb acceptance are important to constrain parton density functions [588]. With high statistics, LHCb will be well placed to make a precision measurement of the sine of the effective electroweak mixing angle for leptons, \(\sin^{2}\theta^{\rm lept}_{\rm eff}\), from the forward–backward asymmetry of leptons produced in the Z→μ^{+}μ^{−} decay. Improved knowledge of parton density functions, as can be obtained from studies of production of gauge bosons in association with jets [617], will help to reduce limiting uncertainties on the measurement of the W boson. These studies are also an important step towards a top physics programme at LHCb, which will become possible once the LHC energy approaches the nominal 14 TeV.
The importance of having a detector in the forward region can be illustrated with the recent discovery by ATLAS and CMS of a new particle that may be the Higgs boson. It is now essential to determine if this particle has the couplings to bosons, leptons and quarks expected in the SM. In particular, at the observed mass the highest branching ratio is expected to be for \(H \rightarrow b\bar{b}\)—however this is a difficult channel for ATLAS and CMS due to the large SM background. LHCb with its excellent b-hadron sensitivity will be able to search for such decays. The forward geometry of LHCb is also advantageous to observe new long-lived particles that are predicted in certain NP models, including some with extended Higgs sectors. Although limits can be set with the current detector [605], this is an area that benefits significantly from the flexible software trigger of the upgraded experiment. Models with extended Higgs sectors also produce characteristic signals in flavour physics observables, which emphasises the need for the LHCb upgrade as part of the full exploitation of the LHC.
6.2 Sensitivity of the upgraded LHCb experiment to key observables
As mentioned in Sect. 1, the LHCb upgrade is necessary to progress beyond the limitations imposed by the current hardware trigger that, due to its maximum output rate of 1 MHz, restricts the instantaneous luminosity at which data can most effectively be collected. To overcome this, the upgraded detector will be read out at the maximum LHC bunch-crossing frequency of 40 MHz so that the trigger can be fully implemented in software. The upgraded detector will be installed during the long shutdown of the LHC planned for 2018. A detailed description of the upgraded LHCb experiment can be found in the Letter of Intent (LoI) [25], complemented by the recent framework technical design report (FTDR) [26], which sets out the timeline and costing for the project. A summary has been prepared for the European Strategy Preparatory Group [618].
Statistical sensitivities of the LHCb upgrade to key observables. For each observable the current sensitivity is compared to that which will be achieved by LHCb before the upgrade, and that which will be achieved with 50 fb^{−1} by the upgraded experiment. Systematic uncertainties are expected to be non-negligible for the most precisely measured quantities. Note that the current sensitivities do not include new results presented at ICHEP 2012 or CKM2012
Type | Observable | Current precision | LHCb 2018 | Upgrade (50 fb^{−1}) | Theory uncertainty |
---|---|---|---|---|---|
\(B ^{0}_{ s } \) mixing | \(2\beta_{s}( B ^{0}_{ s } \rightarrow { J / \psi } \phi)\) | 0.10 [139] | 0.025 | 0.008 | ∼0.003 |
\(2\beta_{s}( B ^{0}_{ s } \rightarrow { J / \psi } f_{0}(980))\) | 0.17 [219] | 0.045 | 0.014 | ∼0.01 | |
\(a_{\rm sl}^{s}\) | 6.4×10^{−3} [44] | 0.6×10^{−3} | 0.2×10^{−3} | 0.03×10^{−3} | |
Gluonic penguins | \(2\beta_{s}^{\rm eff}( B ^{0}_{ s } \rightarrow \phi\phi)\) | – | 0.17 | 0.03 | 0.02 |
\(2\beta_{s}^{\rm eff}( B ^{0}_{ s } \rightarrow K^{*0} \overline{K}^{*0})\) | – | 0.13 | 0.02 | <0.02 | |
\(2\beta^{\rm eff}( B ^{0} \rightarrow \phi K^{0}_{S})\) | 0.17 [44] | 0.30 | 0.05 | 0.02 | |
Right-handed currents | \(2\beta_{s}^{\rm eff}( B ^{0}_{ s } \rightarrow \phi\gamma)\) | – | 0.09 | 0.02 | <0.01 |
\(\tau^{\rm eff}( B ^{0}_{ s } \rightarrow \phi\gamma)/\tau _{ B ^{0}_{ s } }\) | – | 5 % | 1 % | 0.2 % | |
Electroweak penguins | S_{3}(B^{0}→K^{∗0}μ^{+}μ^{−};1<q^{2}<6 GeV^{2}/c^{4}) | 0.08 [68] | 0.025 | 0.008 | 0.02 |
\(s_{0} A_{\rm FB}( B ^{0} \rightarrow K^{*0}\mu^{+} \mu^{-})\) | 25 % [68] | 6 % | 2 % | 7 % | |
\(A_{\rm I}(K\mu^{+}\mu^{-}; 1 < q^{2} < 6 ~\mathrm{GeV}^{2}/c^{4} )\) | 0.25 [77] | 0.08 | 0.025 | ∼0.02 | |
\(\mathcal{B}( B ^{+} \rightarrow \pi ^{+} \mu ^{+} \mu ^{-} )/\mathcal{B}( B ^{+} \rightarrow K ^{+} \mu ^{+} \mu ^{-} )\) | 25 % [86] | 8 % | 2.5 % | ∼10 % | |
Higgs penguins | \(\mathcal{B}( B ^{0}_{ s } \rightarrow \mu^{+} \mu^{-})\) | 1.5×10^{−9} [13] | 0.5×10^{−9} | 0.15×10^{−9} | 0.3×10^{−9} |
\(\mathcal{B}( B ^{0} \rightarrow \mu^{+} \mu^{-})/\mathcal{B}( B ^{0}_{ s } \rightarrow \mu ^{+} \mu^{-})\) | – | ∼100 % | ∼35 % | ∼5 % | |
Unitarity triangle angles | γ(B→D^{(∗)}K^{(∗)}) | 4^{∘} | 0.9^{∘} | negligible | |
\(\gamma( B ^{0}_{ s } \rightarrow D_{s} K)\) | – | 11^{∘} | 2.0^{∘} | negligible | |
\(\beta( B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} )\) | 0.8^{∘} [44] | 0.6^{∘} | 0.2^{∘} | negligible | |
Charm CP violation | A_{Γ} | 2.3×10^{−3} [44] | 0.40×10^{−3} | 0.07×10^{−3} | – |
\(\Delta \mathcal{A}_{ \mathit{CP} } \) | 2.1×10^{−3} [18] | 0.65×10^{−3} | 0.12×10^{−3} | – |
In LHCb measurements to date, the CP-violating phase in \(B ^{0}_{ s } \) mixing, measured in both J/ψϕ and J/ψf_{0}(980) final states, has been denoted ϕ_{s}. In the upgrade era it will be necessary to remove some of the assumptions that have been made in the analyses to date, related to possible penguin amplitude contributions, and therefore the observables in \(b \rightarrow c\bar{c}s\) transitions are denoted by 2β_{s}=−ϕ_{s}, while in \(b \rightarrow q\bar{q}s\) (q=u,d,s) transitions the notation \(2\beta^{\rm eff}_{s}\) is used. This parallels the established notation used in the B^{0} system (the α,β,γ convention for the CKM unitarity triangle angles is used). The penguin contributions are expected to be small, and therefore a theory uncertainty on \(2\beta_{s} ( B ^{0}_{ s } \rightarrow { J / \psi } \phi ) \sim0.003\) is quoted, comparable to the theory uncertainty on \(2\beta( B ^{0} \rightarrow { J / \psi } K ^{0}_{\mathrm{S}} )\). However, larger effects cannot be ruled out at present. Data-driven methods to determine the penguin amplitudes are also possible [246, 277, 284]: at present these given much larger estimates of the uncertainty, but improvement can be anticipated with increasing data samples. The flavour-specific asymmetry in the \(B ^{0}_{ s } \) system, \(a_{\rm sl}^{s}\) in Table 16, probes CP violation in mixing. The “sl” subscript is used because the measurement uses semileptonic decays.
Sensitivity to the emitted photon polarisation is encoded in the effective lifetime, \(\tau^{\rm eff}\) of \(B ^{0}_{ s } \rightarrow \phi \gamma\) decays, together with the effective CP-violation parameter \(2\beta_{s}^{\rm eff}\). Two of the most interesting of the full set of angular observables in B^{0}→K^{∗0}μ^{+}μ^{−} decays [62], are S_{3}, which is related to the transverse polarisation asymmetry [63], and the zero-crossing point (s_{0}) of the forward–backward asymmetry. As discussed above, isospin asymmetries, denoted A_{I}, are also of great interest.
In the charm sector, it is important to improve the precision of \(\Delta \mathcal{A}_{ \mathit{CP} } \), described above, and related measurements of direct CP violation. One of the key observables related to indirect CP violation is the difference in inverse effective lifetimes of D^{0}→K^{+}K^{−} and \(\overline{ D }{} ^{0} \rightarrow K ^{+} K ^{-} \) decays, A_{Γ}.
The extrapolations in Table 16 assume the central values of the current measurements, or the SM where no measurement is available. While the sensitivities given include statistical uncertainties only, preliminary studies of systematic effects suggest that these will not affect the conclusions significantly, except in the most precise measurements, such as those of \(a_{\rm sl}^{s}\), A_{Γ} and \(\Delta \mathcal{A}_{ \mathit{CP} } \). Branching fraction measurements of \(B ^{0}_{ s } \) mesons require knowledge of the ratio of fragmentation fractions f_{s}/f_{d} for normalisation [145]. The uncertainty on this quantity is limited by knowledge of the branching fraction of \(D ^{+}_{ s } \rightarrow K ^{+} K ^{-} \pi ^{+} \), and improved measurements of this quantity will be necessary to avoid a limiting uncertainty on, for example, \(\mathcal{B}( B ^{0}_{ s } \rightarrow \mu ^{+} \mu^{-})\). The determination of 2β_{s} from \(B ^{0}_{ s } \rightarrow { J / \psi } \phi \) provides an example of how systematic uncertainties can be controlled for measurements at the LHCb upgrade. In the most recent measurement [139], the largest source of systematic uncertainty arises due to the constraint of no direct CP violation that is imposed in the fit. With larger statistics, this constraint can be removed, eliminating this source of uncertainty. Other sources, such as the background description and angular acceptance, are already at the 0.01 rad level, and can be reduced with more detailed studies.
Experiments at upgraded e^{+}e^{−}B factories and elsewhere will study flavour-physics observables in a similar timeframe to the LHCb upgrade. However, the LHCb sample sizes in most exclusive B and D final states will be far larger than those that will be collected elsewhere, and the LHCb upgrade will have no serious competition in its study of \(B ^{0}_{ s } \) decays, b-baryon decays, mixing and CP violation. Similarly the yields in charmed-particle decays to final states consisting of only charged tracks cannot be matched by any other experiment. On the other hand, the e^{+}e^{−} environment is advantageous for inclusive studies and for measurements of decay modes including multiple neutral particles [619, 620, 621, 622, 623], and therefore enables complementary measurements to those that will be made with the upgraded LHCb experiment.
6.3 Importance of the LHCb upgrade
The study of deviations from the SM in quark flavour physics provides key information about any extension of the SM. It is already known that the NP needed to stabilize the electroweak sector must have a non-generic flavour structure in order to be compatible with the tight constraints of flavour-changing processes, even if the precise form of this structure is still unknown. Hopefully, ATLAS and CMS will detect new particles belonging to these models, but the couplings of the theory and, in particular, its flavour structure, cannot be determined only using high-\(p_{\rm T}\) data.
Therefore, the LHCb upgrade will play a vital role in any scenario. It allows the exploration of NP phase space that a priori cannot be studied by high energy searches. Future plans for full exploitation of the LHC should be consistent with a co-extensive LHCb programme.
Footnotes
- 1.
Throughout the document, the inclusion of charge conjugated modes is implied unless explicitly stated.
- 2.
It is anticipated that any detectors that need replacement for the LHCb upgrade will be designed such that they can sustain a luminosity of \(\mathcal{L}_{\rm inst} = 2 \times10^{33}~\mathrm{cm} ^{-2}\,\mathrm{s}^{-1}\) [26]. Operation at instantaneous luminosities higher than the nominal value assumed for the estimations will allow the total data set to be accumulated in a shorter time.
- 3.
In principle there are also tensor operators, \(O_{T(5)} = (\bar {q}\sigma_{\mu\nu}b)(\bar{\ell}\sigma^{\mu\nu}(\gamma_{5})\ell )\), which are relevant for some observables.
- 4.
In radiative and semileptonic decays, the chromomagnetic operator O_{8} enters at higher order in the strong coupling α_{S}.
- 5.
Light resonances at q^{2} below 1 GeV^{2} cannot be treated within QCDF, and their effects have to be estimated using other approaches. In addition, the longitudinal amplitude in the QCDF/SCET approach generates a logarithmic divergence in the limit q^{2}→0, indicating problems in the description below 1 GeV^{2} [