Single-diffractive Drell-Yan pair production at the LHC

We present predictions for single-diffractive low-mass Drell-Yan pair production in $pp$ collisions at the LHC at $\sqrt{s}=13$ TeV. Predictions are obtained adopting a factorised form for the relevant cross sections and are based on a new set of diffractive parton distributions resulting from the QCD analysis of combined HERA leading proton data. We discuss a number of observables useful to characterise the expected factorisation breaking effects.


I. INTRODUCTION
The diffractive physics program pursued at the HERA ep collider in the recent past has substantially improved our knowledge on the dynamics of this class of processes. In the deep inelastic regime, the presence of a hard scale enables the derivation of a dedicated factorisation theorem [1,2] which allows the investigation of the partonic structure of the colour singlet exchanged in the t-channel. From scaling violations of the diffractive Deep Inelastic Scattering (DDIS) structure functions, quite precise diffractive parton distributions functions (dPDFs) have been extracted by performing QCD analysis [3][4][5][6] of available data.
With this tool available, factorisation tests have been conducted in order to investigate the range of validity of this hypothesis in processes other than DDIS. Factorisation has been shown to hold, as expected theoretically, in diffractive dijets production in DIS, where NLO predictions based on dPDFs well describe experimental cross sections [4,7] both in shape and normalisation. Factorisation breaking effects are expected to appear in diffractive photoproduction of dijets due to the resolved component of the quasi-real photon. In such a * Electronic address: federico.alberto.ceccopieri@cern.ch extracted from combined leading proton HERA data will be used for the calculation. In view of the expected factorisation breaking effects in hard, single-diffractive, measurements in hadronic collisions, the obtained values for the cross sections should be considered as upper bounds.
Given the explorative nature of the analysis, more intended as a feasibility study, theoretical predictions are calculated to leading order accuracy. We take into account, however, the virtual photon decay into leptons so that cross sections can be studied as a function of, measurable, final state leptons kinematics. This allows us to explore the phase space available for the process and to estimate the impact of typical experimental cuts on the transverse momenta and rapidities of the leptons.
From QCD analyses performed in DDIS and anticipating the results of the next section, we know that the colour-singlet exchanged in the t-channel is a gluon-enriched state. Since gluonic contributions to Drell-Yan production starts to O(α s ) in perturbation theory, an accurate estimation of the suppression factor will require the inclusion of higher order corrections. The impact of the latter and a detailed report on the extraction of diffractive parton distributions to NLO accuracy will be presented in a companion publication.
The paper is organised as follows. In Sec. II we report in some details the extraction of diffractive PDFs from combined HERA leading proton data. In Sec. III, making use of such distributions, we present results for single-diffractive Drell-Yan production in pp collisions at the LHC at √ s = 13 TeV. In Sec. IV we summarise our results.

II. FIT OVERVIEW
Diffractive DIS belongs to the Semi-Inclusive lepton-proton DIS process of the type where, along with the outgoing lepton, an additional proton p is detected in the final state. In eq. (1) X stands for the unobserved part of the hadronic final state and we indicate particles four-momenta in parenthesis. In the lp centre-of-mass system, diffractive DIS events are then characterised by outgoing protons with a large momentum fraction of the incident proton and quite small values of the transverse momentum measured with respect to the collision axis, i.e. in the target fragmentation region of the incident proton. The kinematic variables used to describe the DIS process are the conventional Lorentz invariants with q = k −k ′ . Final state protons are instead described by the fractional momentum of the singlet exchange with respect to the proton momentum, x IP , and the invariant momentum transfer t at the proton vertex: where typical DDIS selection requires x IP 0.1 and |t| 1 GeV 2 . In the following we will use the scaled fractional momentum variable β defined by β = x B /x IP . This is interpreted as the fractional momentum of interacting parton with respect to pomeron fractional momentum respectively. In the one-photon exchange approximation, it reads: According to the factorisation theorem [1,2], structure functions appearing in eq. (4), are factorised into perturbatively calculable short-distance cross sections and diffractive parton The index i runs on the flavour of the interacting parton. The hard-scattering coefficients C ki (k = 2, L) are pertubatively calculable as a power expansion in the strong coupling α s and depend upon µ 2 F and µ 2 R , the factorisation and renormalisation scales, respectively. The C ki coefficient functions are the same as in fully inclusive DIS. Diffractive PDFs f D i/p (β, µ 2 F , x IP , t) appearing in eq. (5) are proton-to-proton fracture functions [22] in the very forward kinematical limit and can be interpreted as the number density of interacting partons at a scale µ 2 F and fractional momentum β conditional to the detection of a final state proton with fractional momentum 1 − x IP and invariant momentum transfer t. The t-unintegrated diffractive PDFs appearing in eq. (5) obey standard DGLAP [25] evolution equations [19]. The same statement holds when they are integrated over t in a limited range [20]: In this paper we analyse the combined H1 and ZEUS diffractive DIS cross sections measurements [28] of the process in eq. (1) where leading protons are measured by dedicated forward spectrometers. The centre-of-mass energy for the e + p scattering is √ s = 318 GeV.
This data set covers the phase space region 2.5 < Q 2 < 200 GeV 2 and 0.0018 < β < 0.816 and it has widest coverage in the proton fractional energy loss, 0.00035 < x IP < 0.09, subdived in 10 bins in x IP , with an average of 20 points per-x IP bin for a total of 192 points.
At variance with all other DDIS cross sections measurements, the squared four-momentum transfer at the proton vertex, t, is integrated in the restricted range 0.09 < |t| < 0.55 GeV 2 in order to minimise systematic uncertainties originating from t-extrapolation of the various measurements outside their respective measured ranges. The reduced cross sections in eq. (4) are integrated over t in such a range and the diffractive PDFs in eq. (6) are defined accordingly. For x IP < 0.03 the data set overlaps with high-statistics LRG data set and for 0.03 < x IP < 0.09 it provides the best experimental information available on diffractive DIS cross sections. The combination procedure, in general, allows a reduction of the systematic uncertainties via cross-calibration of the various measurements. The direct detection of the forward proton allows to avoid any systematics associated with the large rapidity gap selection. By definition, these data are free from the proton dissociative background which has been found to contribute around 23% of the diffractive DIS cross sections based on LRG selection [3]. Therefore this set of data provides the most precise knowledge about the absolute normalisation of diffractive DIS cross sections. These advantages however come at the price of increased uncertainties relative to LRG data given the reduced statistics of the sample.
Diffractive parton distributions extracted form this data set will be used in the context of single hard diffraction in hadronic collisions in conjuction with ordinary parton distributions.
In order to avoid any mismatch between inclusive and diffractive PDFs we adopt leading order CTEQL1 parton distribution set [27] evolved in the zero-mass variable-flavour-number scheme (ZM-VFNS). The evolution of diffractive PDFs is performed within the same scheme and to the same accuracy by using QCDNUM17 [30] program. The QCD parameters are the ones quoted in Ref. [27]. In particular we set the charm and bottom masses to m c = 1.3 GeV and m b = 4.5 GeV, respectively, and the strong coupling is evaluated at one loop setting 130. In general factorisation theorem [1,2] for diffractive DIS in the form of eq. (5) holds at  fixed values of x IP and t so that the parton content of the color-singlet exchange described by f D i is uniquely controlled by the kinematics of the outgoing proton. Therefore, at least in principle, dPDFs may differ at different values of x IP and t. This idea has been successfully tested [5] in the analysis of LRG data from Ref. [3]. In the present context, given the limited number and accuracy of the data points in each x IP bin, we use a simpler approach, namely a fully factorised β − x IP ansatz for the flavour-symmetric singlet and gluon diffractive parton (momentum) distributions defined at the initial scale Q 2 0 : The initial conditions in eq. (7) are characterised by a common flux factor F (x IP ) controlled by a single power at low x IP . An extra modulation, controlled by parameters f 1 and f 2 , is introduced to accomodate the data at larger values of x IP . In order to guarantee the vanishing of the singlet distribution on the endpoint, we fix the large-β behaviour of the singlet by setting C q = 0.5 but additional freedom at intermediate values of β is allowed leaving D q and E q parameters free in the minimisation. Since the gluon distributions is x IP = 0.016   only indirectly fixed by the slope of the reduced cross section, the gluon parameters B g and C g are highly correlated and we decide to fix C g = 0.5 for a total of 9 free parameters.
Such distributions, once evolved, are used to calculate the diffractive structure functions F D 2,L with the help of the QCDNUM17 convolution engine and to reconstruct the diffractive reduced cross sections in eq. (4) which are then minimised against data [28] with the help of the MINUIT [29] program. The choice of Q 2 0 is optimised performing a scan giving the best χ 2 value for Q 2 0 = 1.5 GeV 2 . By using total errors quoted in the experimental analysis and the standard χ 2 definition, we obtain a total χ 2 /d.o.f.=0.913. We report in Table (I) the best parameters and the breakdown of the contributions to χ 2 function in each x IP bin. According to these numbers there is no misrepresentation of the data accross the x IP bins. The comparison of the best fit results and the reduced cross sections is presented in Fig. (1) for four representative values of x IP as a function of Q 2 or β. We supplement the best-fit parametrisations with an additional set of parametrisations obtained according to the Hessian method outlined in Refs. [31,32]  which allows to propagate experimental uncertainties to arbitrary observables. We note that our initial conditions assume a common β-shape for the diffractive PDF in all x IP bins. This theoretical hypothesis, in turn, determines an unrealistic precise determination of the diffractive PDFs if associated with the standard ∆χ 2 = 1 criterion, often exceeding the precision of the data. In order to correct for such an effect and to obtain a more conservative error estimate we choose a tolerance criterion ∆χ 2 = 10 (one unit for x IP bin) and dPDFs alternative parametrisations are obtained with this choice. We have checked by explicit evaluation that each parametrisation gives a consistent value for the χ 2 function, χ 2 best + ∆χ 2 . The error bands presented in the plots are obtained according to this criterion. In Fig. (2) we present the singlet and gluon momentum distributions in two x IP bins at different scales. The singlet shows a bump in the large β region (β 0.5) at the lower scale which is progressively washed away by evolution at higher scales. The rise of the gluon distribution at small β is accelerated by pQCD evolution and it is the dominant contribution for β 0.1.
We have further performed two consistency checks detailed below. The first one concerns the diffractive longitudinal structure function which contributes starting from O(α s ) and it is absent to the accuracy of the present calculation. Since its dominant contributions appear in the large-y region, the fit has been repeated with the cut y < 0.5 imposed. The second one addresses the issue, reported in previous analyses [3][4][5][6], of the inclusion in the fit of the lowest Q 2 points. For such a reason, the minimisation has been repeated by including only data points for which Q 2 > 6 GeV 2 . In both cases we observe a modest decrease in the χ 2 /d.o.f . However, as shown in Fig. (3), the resulting parametrisations are compatible, within uncertainties, with the ones obtained without imposing the cuts. Given the substantial stability of the results against variation of the phase space boundary of data included in the fit, we consider the "no cut" scenario as our default choice and use the corresponding best-fit parametrisations in the next Section.

III. SINGLE-DIFFRACTIVE DRELL-YAN PRODUCTION
The signature of hard diffraction in hadronic collisions is the presence of hard scattering process associated with the production of a leading proton. Among many others, we consider here the simplest hard scattering process, namely the Drell-Yan pair production. Therefore we consider the reaction The invariant mass of the pair q 2 = (p 3 + p 4 ) 2 = Q 2 is chosen to be large enough so that perturbative QCD can be applied. In hadronic collisions, the Lorentz-invariant variable z is used to characterise final state hadrons and is defined by In the hadronic centre-of-mass frame, where the second identity holds, z is just the observed proton energy, E * p , scaled down by the beam energy, √ s/2. Hard diffrative events are then characterised by low values of the invariant x IP and t, both in the same range of values as the one measured in DDIS.
Assuming factorisation to hold, one loop corrections to the process in eq. (8) have been reported in Refs [23,24], where a generalised procedure for the collinear factorisation is proposed. The latter is the same as the one proposed in particle production in the target fragmentation region in DIS [26] and requires the introduction of fracture functions. However we stress that the ability to consistently subtract collinear singularities in such a semiinclusive processes is a necessary but not a sufficient condition for factorisation to hold in hard diffractive processes in hadronic collisions. The one-loop calculation mentioned above, infact, does take into account only the so-called active partons. It completely ignores multiple soft parton exchanges between active and spectators partons, whose effects should be accounted for in any proof of QCD factorisation (as done in the inclusive Drell-Yan case).
In eq. (8), we assume that the proton with momentum P 1 is moving in the +z direction and the leading proton with momentum P is produced quasi-collinearly to P 1 at large and positive rapidities. At the cross section level, diffractive parton distributions for the proton with momentum P 1 will be used. The same process, of course, may occur also in the opposite emisphere and, since the hadronic initial state is symmetric, will be not considered here.
At the partonic level and to lowest order in the strong coupling, the process proceeds via the annihalition of a quark-antiquark pair into a massive virtual photon which subsequently decays into a opposite-sign lepton pair. To be definite we consider here the decay into muons: Before discussing our results we found useful to sketch some details of the calculation. The incoming parton momenta in the hadronic centre-of-mass-system are given by with s = (P 1 + P 2 ) 2 . We choose as final state variables the lepton rapidities y 3 , y 4 , and lepton transverse momentum, p t . In terms of the latter, the four momenta of the leptons are given by with p t = |p t | and q = p 3 + p 4 . The differential cross section, to leading order accuracy, involves appropriate products of diffractive and ordinary parton distributions functions. It where the sum runs over quark and antiquarks. In actual calculations we have set the factorisation scale to µ F = M µµ . We further introduce the leptons rapidity sum, Y , and differenceȳ: The former defines the rapidity of the virtual photon. The partonic Mandelstam invariants appearing in eq. (15) are then given bŷ In terms of these variables the momentum fractions are given by Since both momentum fractions can not exceed unity, the following bounds can be derived: with τ = M µµ / √ s. Given the kinematic constraint x 1 x IP , the pair-rapidity spans an increasingly asymmetric range as x IP decreases. For x IP < √ τ , the pair is entirely in the Y < 0 rapidity range. Formally, the rapidity range for the inclusive Drell-Yan case is recovered simply setting x IP = 1 in eq. (22). In the present analysis we focus on diffractive processes tagged with dedicated instrumentation [17]. We choose the proton fractional momentum loss to be in the range 10 −4 < x IP < 10 −1 , with maximal overlap with the range measured at HERA [28]. Predictions presented in the following are integrated over the t-range of the data [28] out of which dPDFs are estracted, i.e. 0.09 < |t| < 0.55 GeV 2 . We set the centre-of-mass energy of the pp collisions to √ s = 13 TeV. The invariant mass of the muon pair is required to be in the range 2 < M µµ < 20 GeV, a range of virtualities in line with those measured at HERA. We assume that the J/Ψ and Υ contributions, which both lie within this mass range, can be properly subtracted from the data sample. We require both muons to have rapidity |y µ | < 2.45 but we do not apply cuts either on the muons transverse or three momenta.
The resulting fiducial cross sections for single-diffractive Drell-Yan pair production is reported in Tab. (II). In the case that proton spectrometers are installed on both side of the interaction point, such a result for the fiducial cross section should be doubled. As already anticipated, the quoted result does not include any rapidity gap suppression factor and predictions refer to virtual photon decay in the muon channel. The first error represents the propagation of experimental uncertainties as obtained in the diffractive PDF fit. The second one, of theoretical nature, is obtained varying the factorisation scale µ 2 F appearing in both distributions in eq. (15) in the range 1/2M 2 µµ < µ 2 F < 2M 2 µµ . In this regime of relatively low Q 2 values where diffractive and inclusive parton distributions evolve faster, we find that the dominant error source, of theoretical nature, is associated with missing higher order corrections. The latter are known to high accuracy for a number of distributions and will be considered in a separate publication. In the present contest, predictions can be dσ/dp µ t [pb/GeV] momentum distribution shows a maximum (the Jacobian peak) at p t = M min µµ /2. The muon pair rapidity distribution, presented in the bottom left panel, is slightly asymmetric and indicates a preference for the virtual photon to populate the negative rapidity emisphere (the one containing the dissociated proton, in the chosen reference frame). We note that, despite phase space limitations introduced by eq. (22) and the difference between diffractive and ordinary parton distributions, the muon-pair populates the available rapidity range, as defined by the muon rapidity cuts and by eq. (16). In the bottom right panel we present the x IP distribution. In general, it is well known that such distribution behaves approximately as an inverse power of x IP at small x IP . In the present case, the flattening of the distribution at small x IP can be ascribed to the shrinkage of phase space for the production of massive pair, since the maximum partonic centre-of-mass energy is reduced to √ x IP s. The kinematic differential cross section as a function of Y is shown in four different ranges of the pair invariant mass and integrated over x IP . In all mass bins, the distributions show a maximum in the negative rapidity range, a signal that the interacting parton from the dissociated proton carries, on average, slightly more momentum with respect to the one originating from the scattered proton. In Fig. (7) we present single differential distributions as a function of x IP in four different invariant mass ranges. As the invariant mass increases, we observe a progressive flattening of the distributions at small x IP . This effect is due to the phase space reduction induced by the constrain M 2 µµ = βx IP x 2 s, which at low x IP disfavours the production of increasingly massive pair. In Fig. (8) we present single differential cross section as a function of β, the fractional momentum of the interacting parton with respect to the one of the colour singlet exchanged in the t-channel, integrated in various bins of M µµ and x IP .
Such distributions offer an insight to the sensitivity of the cross section to diffractive parton distributions, modulo kinematics effects. In the lowest x IP bin the distributions span all the allowed β range and progressively shrinks at large β as x IP increases, a natural consequence of momentum conservation. As already shown in Fig. (5) and Fig. (8), the distributions in the pair rapidity Y are asymmetric around Y = 0. The asymmetry decreases both as the mass of the pair increases and as x IP increases. Such an effect is absent in the inclusive Drell-Yan case initiated by a symmetric initial state. This effect is better appreciated considering the asymmetry A defined by and shown in the left panel of Fig. (9). The asymmetry, integrated over all masses and proton energy loss, the asymmetry reaches its maximum 3% at Y ≃ 1 and is always negative, implying that the muon-pair is produced mostly in the emisphere opposite to the one containing the scattered proton. The asymmetry, being normalised to the integrated single diffractive cross section, is not affected by uncertainties due to the rapidity gap survival factor and it is sensitive to the shape of diffractive PDFs. Depending on the accuracy of the data, this predicted behaviour, absent in the inclusive case, could be expolited to correlate the forward proton detection with the central Drell-Yan production. As discussed at the beginning of the Section, predictions are affected by large theoretical errors associated with scale variations. Such uncertainties can be conveniently reduced by considering the ratio R of diffractive to inclusive cross sections which also offers the advantage to reduce experimental systematics errors. In eq. (24) Y stands for the selected hard scattering process (DY this case) and X for the unobserved part of the final state. At Tevatron the ratio R has been measured in a variey of final state [12][13][14] and it shows a quite stable behaviour with a value close to 1%. For the single-diffractive Drell-Yan production in pp collisions at √ s = 13 TeV, the ratio R is presented in the right panel of Fig. (9). Given our leading order estimate of the inclusive Drell-Yan cross section, R varies between 6% and 8% and decreases mildly as a function of the invariant mass of the pair, M µµ . This prediction however does not take into account the RGS suppression factor.
With this respect it would be interesting to check whether the data follow at least the shape of the ratio as a function of M µµ .

IV. CONCLUSIONS
In this paper we have considered the single-diffractive production of low-mass Drell-Yan pair in pp collisions at the LHC at √ s = 13 TeV. Predictions are based on a fully factorised approach for the cross section which makes use of a set of diffractive parton distributions obtained from a QCD fit to combined leading proton DIS data from HERA. A number of distributions are presented both in terms of Drell-Yan pair and scattered proton variables.
Examples of asymmetries and ratio are constructed in order to minimise theoretical and experimental uncertanties. In view of the foreseen measurements of this type of process at the LHC Run-II, these results constitute a baseline for the characterisation of the expected factorisation breaking effects.