Phenomenology of onshell Higgs production in the MSSM with complex parameters
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Abstract
A computation of inclusive cross sections for neutral Higgs boson production through gluon fusion and bottomquark annihilation is presented in the MSSM with complex parameters. The predictions for the gluonfusion process are based on an explicit calculation of the leadingorder cross section for arbitrary complex parameters which is supplemented by higherorder corrections: massive top and bottomquark contributions at NLO QCD, in the heavy topquark effective theory the topquark contribution up to N\(^{3}\)LO QCD including a soft expansion for the \(\mathcal {CP}\)even component of the light Higgs boson. For its \(\mathcal {CP}\)odd component and the heavy Higgs bosons the contributions are incorporated up to NNLO QCD. Twoloop electroweak effects are also incorporated, and SUSY QCD corrections at NLO are interpolated from the MSSM with real parameters. Finite wave function normalisation factors ensuring correct onshell properties of the external Higgs bosons are incorporated from the code FeynHiggs. For the typical case of a strong admixture of the two heavy Higgs bosons it is demonstrated that squark effects are strongly dependent on the phases of the complex parameters. The remaining theoretical uncertainties for cross sections are discussed. The results have been implemented into an extension of the numerical code SusHi called SusHiMi.
1 Introduction
In 2012 the experimental collaborations ATLAS and CMS announced the discovery of a Higgslike boson [1, 2] produced in collisions of protons at the Large Hadron Collider (LHC). Apart from the precise measurement of its production and decay properties in order to test whether there are deviations from the expectations for a Standard Model (SM) Higgs boson, an essential part of the programme of the LHC experiments in the upcoming years will be the search for additional Higgs bosons. The observed state can be easily accommodated in extended Higgs sectors like a TwoHiggsDoublet Model (2HDM) or supersymmetric extensions, e.g. the Minimal Supersymmetric Standard Model (MSSM). For the search for additional Higgs bosons and the test of deviations from the SM expectations for the SMlike Higgs boson, the precise knowledge of production cross sections through gluon fusion and bottomquark annihilation for these Higgs bosons is a key ingredient. Current efforts in this direction are summarised in the reports of the LHC Higgs Cross Section Working Group, see Refs. [3, 4, 5, 6]. So far, the searches for additional Higgs bosons have been interpreted in various scenarios beyond the Standard Model, including several supersymmetric ones. However, those analyses do not yet cover the most general case where \(\mathcal {CP}\) is violated and leads to mixing between \(\mathcal {CP}\)even and odd eigenstates. The reason that an analysis for the general case taking account the possibility of \(\mathcal {CP}\)violation has not been possible so far has mainly been the lack of appropriate theoretical predictions for the Higgs production rates at the LHC for complex parameters in the MSSM and of a practical prescription for taking into account relevant interference effects in Higgs production and decay. A discussion of the latter has recently been given in Ref. [7]. Therefore it is our goal in the present paper to provide stateoftheart crosssection predictions in the MSSM, taking into account \(\mathcal {CP}\)violating effects, for the two main Higgs production channels at the LHC, which can be used as input for future experimental analyses in \(\mathcal {CP}\)violating Higgs scenarios. We present in this paper precise predictions for neutral Higgs boson production through gluon fusion and bottomquark annihilation in the MSSM with complex parameters, in which \(\mathcal {CP}\)even and \(\mathcal {CP}\)odd Higgs states form three admixed Higgs mass eigenstates \({h_a},a\in \lbrace 1,2,3\rbrace \). Complex parameters in the MSSM give rise to additional sources of \(\mathcal {CP}\) violation beyond the one induced by the mixing of the quarks of the SM, described by the Cabibbo–Kobayashi–Maskawa (CKM) matrix [8, 9]. In order to explain the baryon asymmetry of the universe, such additional sources of \(\mathcal {CP}\) violation beyond the CKM phase are actually needed, see e.g. Refs. [10, 11, 12] for reviews. It is thus of interest to investigate the MSSM with complex parameters. Its Higgs sector is influenced by the additional phases only beyond tree level. Still, these phases are of relevance in the Higgs boson collider phenomenology as they can induce a large mixing among the heavy Higgs bosons, and squark and gluino loop contributions also directly affect Higgs boson production and decay.
For a brief summary of higherorder corrections to the most important production processes – gluon fusion and bottomquark annihilation – in the SM and the MSSM with real parameters we refer to Sect. 3 and focus here on studies performed for the Higgs sector of the MSSM with complex parameters. Early investigations of Higgs production through gluon fusion at hadron colliders in the MSSM with complex parameters were carried out in Refs. [13, 14, 15]. A thorough analysis taking different production channels into account was presented in Ref. [16], and results for Higgsstrahlung can be found in Ref. [17]. Large effects of stops on the cross section for a \(\mathcal {CP}\)odd Higgs boson neglecting \(\mathcal {CP}\)even and odd Higgs mixing were discussed in Ref. [18]. References [19, 20] discuss the production of a light Higgs through gluon fusion including its decay into two photons in the MSSM with complex parameters. It should be noted that the mentioned references were published before the Higgs discovery in 2012 and mostly employ only the lowest order in perturbation theory for the production processes. It is therefore timely to improve these predictions by including uptodate higherorder corrections and to investigate the compatibility with the experimental results obtained for the observed signal at 125 GeV. For this purpose we incorporate the prediction within the MSSM with complex parameters into the numerical code SusHi [21, 22], which calculates Higgs production through gluon fusion and heavyquark annihilation [23] in the SM, the MSSM, the TwoHiggsDoubletModel (2HDM) and the NexttoMinimal Supersymmetric Standard Model (NMSSM) [24]. However until now, SusHi did not support complex parameters in the MSSM and thus did not provide predictions for \(\mathcal {CP}\)admixed Higgs bosons.
For the calculation of the masses and the wave function normalisation factors ensuring the correct onshell properties of external Higgs bosons, which involves the evaluation of Higgs boson selfenergies and their renormalisation, we use the code FeynHiggs [25, 26, 27, 28, 29]. It employs a Feynmandiagrammatic approach and includes the full oneloop [28] and the dominant twoloop corrections of \(\mathcal {O}(\alpha _t\alpha _s)\) [30] and \(\mathcal {O}(\alpha _t^2)\) [31, 32] in the MSSM with complex parameters.^{1} A detailed description of the prediction for the Higgs boson masses and the wave function normalisation factors as implemented in FeynHiggs can be found in Refs. [7, 28, 35, 36, 37, 38, 39]. Whereas the Higgs sector at tree level remains \(\mathcal {CP}\)conserving, at higher orders an admixture of all three neutral Higgs bosons, i.e. the two \(\mathcal {CP}\)even Higgs bosons h, H and the \(\mathcal {CP}\)odd Higgs boson A, is induced. The case where the light Higgs boson describes the SMlike Higgs at \({\sim }125\) GeV is typically accompanied with a strong admixture of the two heavy Higgs bosons. For a proper prediction in such a case interference effects need to be taken into account in the full process involving production and decay of the Higgs bosons, which requires going beyond the usual narrowwidth approximation (see also Refs. [40, 41, 42, 43, 44]). A convenient way to incorporate interference effects is a generalised narrowwidth approximation for the production and decay of onshell particles as described in Refs. [7, 45, 46], where in Ref. [46] only lowestorder contributions have been considered, while in Refs. [7, 45] also the inclusion of higherorder corrections has been addressed. The results for the cross sections for onshell Higgs boson production obtained in the present paper are suitable for direct incorporation into the framework of a generalised narrowwidth approximation.
Our paper is organised as follows: We start by outlining the relevant quantities in the Higgs, the gluino and the squark sector of the MSSM with complex parameters in Sect. 2. We move to the description of the gluonfusion cross section in Sect. 3, where we discuss the calculation of the cross section at LO and the applicability of higherorder corrections. Next we introduce in Sect. 4 the code SusHi and its extension SusHiMi, which we use for our phenomenological studies carried out in Sect. 5. We discuss the remaining theoretical uncertainties in Sect. 6. Lastly, we conclude in Sect. 7 and list Higgs(s)quark couplings in Appendix A.
2 The MSSM with complex parameters
In this section we discuss the relevant sectors of the MSSM with complex parameters, namely the gluino, the squark as well as the Higgs sector. While the discussion of the gluino and the squark sector at tree level is sufficient for our purposes, we will briefly describe the inclusion of higherorder corrections in the Higgs sector. The MSSM with complex parameters allows for 12 physical, independent phases of the complex parameters, once the phases of the wino softbreaking parameter \(M_2\) and the softbreaking parameter \(m_{12}^2\) are rotated away. Those independent phases are the ones of the softbreaking gaugino masses \(M_1\) and \(M_3\), the Higgsino mass parameter \(\mu \) and trilinear softbreaking couplings \(A_f, f\in \lbrace e,\mu ,\tau ,u,d,c,s,t,b\rbrace \). In the subsequent discussion we focus on these phases and their effect on the gluino, the squark and the Higgs sector as well as the Higgs boson cross sections.
2.1 Gluino and squark sector
2.2 Higgs sector
2.3 Higgs mixing at higher orders
\(\mathcal {CP}\)violating mixing between the neutral Higgs bosons \(\lbrace h,H,A\rbrace \) arises as a consequence of radiative corrections and results in the neutral mass eigenstates \(\lbrace h_1, h_2, h_3\rbrace \), where by convention \(m_{h_1} \le m_{h_2} \le m_{h_3}\). The full mixing in higher orders takes place not just between \(\lbrace h, H, A\rbrace \), but also with the Goldstone boson and the electroweak gauge bosons. In general, \((6 \times 6)\)mixing contributions involving the fields \(\left\{ h,H,A,G,Z,\gamma \right\} \) need to be taken into account. For the calculation of the Higgs boson masses and wave function normalisation factors at the considered order it is sufficient to restrict to a \((3 \times 3)\)mixing matrix among \(\left\{ h,H,A \right\} \), since mixing effects with \(\lbrace G,Z,\gamma \rbrace \) only appear at the subleading twoloop level and beyond. In processes with external Higgs bosons, on the other hand, mixing contributions with G and Z already enter at the oneloop level, but the numerical effect of these contributions has been found to be very small, see e.g. Refs. [35, 36, 37, 62]. In our numerical analysis of the Higgs production through gluon fusion and bottomquark annihilation below we will neglect these kinds of (electroweak) mixing contributions of the external Higgs bosons with Goldstone and gauge bosons. Concerning electroweak corrections, we only incorporate the potentially numerically large contributions to the Higgs boson masses and wave function normalisation factors as well as the electroweak contribution to the correction affecting the relation between the bottomYukawa coupling and the bottom quark mass (see above), while all other contributions considered here like e.g. electroweak corrections to gluon fusion involve at least one power of the strong coupling. For the contribution of the Z boson and the Goldstone boson to the gluonfusion process via \(gg\rightarrow \lbrace Z^*,G^*\rbrace \rightarrow h_i\) (the photon only enters at higher orders) it should be noted that contributions from massdegenerate quark weakisodoublets vanish and only top and bottomquark contributions proportional to their masses are of relevance, see the discussion of the Higgsstrahlung process in Refs. [63, 64]. This is a consequence of the fact that only the axial component of the quarkquarkZ boson coupling contributes to the loopinduced coupling of the Z boson to two gluons. Similarly, squark contributions in \(gg\rightarrow \lbrace Z^*, G^*\rbrace \) are completely absent at the oneloop level, even in case of \(\mathcal {CP}\) violation in the squark sector. The oneloop contributions to \(gg\rightarrow \lbrace Z^*, G^*\rbrace \) therefore have no dependence on the phases of complex parameters.
2.4 Wave function normalisation factors for external Higgs bosons
3 The gluonfusion cross section

\({\hat{\mathbf Z}}\) factors, which relate the amplitude for an external onshell Higgs \(h_a\) (in the mass eigenstate basis) to the amplitudes of both the \(\mathcal {CP}\)even lowestorder states h and H and the \(\mathcal {CP}\)odd state A, see Sect. 2.4.

Nonvanishing couplings of squarks \(g^A_{\tilde{f}ii}\) to the pseudoscalar component A.

Different left and righthanded quark couplings \(g^\phi _{q_L}\) and \(g^\phi _{q_R}\) with \(\phi \in \lbrace h,H,A\rbrace \), see Sect. 2.1.
3.1 Lowestorder cross section
3.2 Higherorder contributions
Gluon fusion receives sizeable corrections at higher orders in QCD. The NLO corrections for the SM quark contributions are known for arbitrary quark masses [70, 71, 72, 73, 74, 75]. NNLO (SM) QCD contributions were calculated in the limit of a heavy topquark mass [76, 77, 78], similar to the recently published N\(^{3}\)LO contributions for a \(\mathcal {CP}\)even Higgs boson in an expansion around the threshold of Higgs production [79, 80, 81, 82, 83].^{5} Finite topquark mass effects at NNLO are known in an expansion of inverse powers of the topquark mass [86, 87, 88, 89, 90, 91, 92, 93]. All of the previously mentioned corrections are implemented in SusHi [21, 22] and can be added in all supported models. We will later discuss in more detail for which Higgs mass ranges these corrections are applicable, which also explains why the above mentioned N\(^{3}\)LO contributions are only employed for the \(\mathcal {CP}\)even component of the light Higgs boson.
In the MSSM with real parameters analytical NLO virtual contributions involving squarks, quarks and gluinos are either known in the limit of a vanishing Higgs mass [98, 99, 100, 101] or in an expansion of heavy SUSY masses [102, 103, 104].^{6} Even NNLO corrections of stopinduced contributions to gluon fusion are known [108, 109]; SusHi can approximate these NNLO stop effects [110] in the \(\mathcal {CP}\)conserving MSSM. We neglect those contributions in our analysis for the MSSM with complex parameters.
Despite this fact, we expect that the interpolation of the virtual twoloop contributions involving squarks and gluinos to the gluonfusion amplitude provides a reasonable approximation, for the following reasons (we discuss the theoretical uncertainty associated with the interpolation in Sect. 6 and assign a conservative estimate of the uncertainty in our numerical analysis). We focus here on the gluon fusion amplitude without \({\hat{\mathbf Z}}\) factors, since in the \({\hat{\mathbf Z}}\) factors the full phase dependence is incorporated without approximations. Gluino contributions are generally suppressed for gluino masses that are sufficiently heavy to be in accordance with the present bounds from LHC searches, while gluonexchange contributions do not add an additional phase dependence compared to the dependence on the phases of \(A_q\) and \(\mu \) in the LO cross section, which is fully taken into account. The dependence of the NLO amplitude on the phases of \(A_q\) and \(\mu \) is therefore expected to follow a similar pattern as the LO amplitude, which is also what we find in the application of the interpolation method.
One can also compare the higherorder corrections to the gluonfusion process with the ones to the Higgs boson masses and \({\hat{\mathbf Z}}\) factors. In fact, a similar interpolation was probed in the prediction for Higgs boson masses in the MSSM with complex parameters, see e.g. Refs. [28, 30, 112, 113], where the phase dependence of subleading twoloop contributions beyond \(\mathcal{O}(\alpha _t \alpha _s)\) were approximated with an interpolation before the full phase dependence of the corresponding twoloop corrections at \(\mathcal{O}(\alpha _t^2)\) was calculated [31, 32]. Generally good agreement was found between the full result and the approximation [31, 32]. In order to investigate the interpolation of the phase of \(M_3\) that is associated with the gluino we performed a similar check concerning the phase dependence of twoloop squark and gluino loop contributions. We numerically compared the full result for the Higgs mass prediction at this order from FeynHiggs with an approximation where the phases at the twoloop level are interpolated. Despite the fact that also for the Higgs mass calculation new diagrams proportional to \(g_{\tilde{q}ii}^A\) arise away from phases 0 and \(\pi \), the phase dependence of the interpolated results generically follows the behaviour of the full results very well.
4 The program SusHi and the extension SusHiMi
As already mentioned N\(^{3}\)LO QCD corrections are only taken into account for the \(\mathcal {CP}\)even component of the light Higgs boson, which allows us to match the precision of the light Higgs boson cross section in the SM employed in uptodate predictions. This is motivated by the fact that the light Higgs boson that is identified with the observed signal at \(125~\text {GeV}\) is usually assumed to have a dominant \(\mathcal {CP}\)even component, which is also the case in the scenarios which are considered in our numerical discussion. For the \(\mathcal {CP}\)odd component of the light Higgs and the heavy Higgs bosons we employ the NNLO corrections for the topquark induced contributions to gluon fusion in the effective theory of a heavy topquark, i.e. we do not take into account topquark mass effects beyond NLO, but only factor out the LO QCD cross sections \(\sigma _{{\mathrm{LO}}}^{t,{\mathrm {e}}}\) and \(\sigma _{{\mathrm{LO}}}^{t,{\mathrm {o}}}\). The strategy to employ the EFT result at NNLO beyond the topquark mass threshold can be justified from the comparison of NLO corrections, which are known in the EFT approach and exactly with full quarkmass dependence and agree also beyond the topquark mass threshold. On the other hand, the N\(^{3}\)LO QCD corrections that were obtained for the topquark contribution are only known in the EFT approach and for an expansion around the threshold of Higgs production at \(x=m_{h_a}^2/s\rightarrow 1\), which we can take into account up to \(\mathcal {O}(1x)^{16}\). Since the combination of the EFT approach and the threshold expansion becomes questionable above the topquark mass threshold, we apply N\(^{3}\)LO QCD corrections only for the \(\mathcal {CP}\)even component of the light Higgs boson and thus match the precision of the SM prediction. The electroweak correction factor \(\delta _{\mathrm {EW}}^{\mathrm {lf}}\) multiplied in the “\(\mathcal {CP}\)even” run is obtained from Eq. (32).
SusHi also allows one to obtain differential cross sections as a function of the transverse momentum or the (pseudo)rapidity of the Higgs boson. These effects can be studied also in the MSSM with complex parameters. In the case of nonvanishing transverse momentum, which is only possible through additional radiation, i.e. real corrections, the precision for massive quark contributions in extended Higgs sectors is currently limited to the LO prediction [114, 115]. The predictions of the \(p_T\) distributions in SusHiMi have been obtained from the LO contributions with arbitrary complex parameters, and in contrast to the total cross sections are therefore not affected by additional interpolation uncertainties from higher orders in comparison to the case of the MSSM with real parameters.
5 Numerical results
For our numerical analysis we slightly modify two standard MSSM scenarios introduced in Ref. [116], namely the \(m_h^{\mathrm {mod}+}\) and the lightstop scenario. The scenarios have been chosen for illustration, featuring relatively large squark and gluino contributions to the gluon fusion process. The corresponding effects will be relevant in our discussion of the associated theoretical uncertainties.
Whereas for the \(m_h^{\mathrm {mod}+}\)inspired scenario we pick heavy Higgs bosons through \(m_{H^\pm }=900\) GeV with \(\tan \beta =10\) and 40 for the study of \(\Delta _b\) effects, we choose \(m_{H^\pm }=500\) GeV with \(\tan \beta =16\) for the lightstop inspired scenario. A detailed discussion of squark effects for the Higgs boson cross sections in the lightstop scenario can also be found in Ref. [119]. For the chosen parameter point the squark effects are sizeable, both for the light Higgs boson and in particular also for the heavy \(\mathcal {CP}\)even Higgs boson, where they reduce the gluonfusion cross section by about \({\sim }90\)%. The Higgs boson masses and the \({\hat{\mathbf Z}}\) factors are obtained from FeynHiggs 2.11.2. The cross sections are evaluated with SusHiMi, which is based on the latest release of SusHi, version 1.6.1. We will mostly focus on the gluonfusion cross section and present the bottomquark annihilation cross section only for the scenario with \(\tan \beta =40\).
For the parameter points associated with the mentioned scenarios in the MSSM with real parameters we vary the phases of \(A_t=A_te^{i\phi _{A_t}}\) and \(M_3=m_{\tilde{g}}e^{i\phi _{M_3}}\) leaving the absolute values constant in order to address various aspects in the phenomenology of Higgs boson production. The phases of \(A_b\) and \(\mu \) do not introduce new phenomenological features, and we do not display results for the variation of those phases. A variation of the phase of \(X_t\) leads to very similar cross sections for all Higgs bosons as observed for the variation of the phase of \(A_t\). This can be understood from the fact that we choose not too large values of \(\mu \) and \(\tan \beta \ge 10\), and so \(X_t \approx A_t\). Note that the stop masses are constant as a function of the phase of \(X_t\), if the absolute value of \(X_t\) is fixed. Before we proceed we want to briefly discuss experimental constraints on the phases: The most restrictive constraints on the phases arise from bounds on the electric dipole moments (EDMs) of the electron and the neutron, see Refs. [120, 121, 122] and references therein. EDMs from heavy quarks [123, 124] and the deuteron [125] also have an impact. MSSM contributions to these EDMs already contribute at the oneloop level and primarily involve the first two generations of sleptons and squarks. Thus, EDMs lead to severe constraints on the phases of \(A_{q}\) for \(q\in \{ u,d,s,c\}\) and \(A_l\) for \(l\in \{e,\mu \}\). Using the convention that the phase of the wino softbreaking mass \(M_2\) is rotated away, one finds tight constraints on the phase of \(\mu \) [126]. On the other hand constraints on the phases of the thirdgeneration trilinear couplings are significantly weaker. We refer the reader to Ref. [127] for a review. While recent constraints from EDMs [128] taking into account twoloop contributions [129] have the potential to rule out the largest values of the phase of \(A_t\), there is still significant room for variation of the phases of \(A_t\) and \(M_3\). We therefore display the full range of the phases of \(A_t\) and \(M_3\) in our considered scenarios without explicitly imposing EDM constraints, following the common approach in benchmark scenarios for Higgs phenomenology (see e.g. Ref. [130] for a recent discussion). It should be noted in this context that in particular the variation of \(A_t\) affects the value of the stop masses. Additionally, the Higgs boson masses are a function of the phases of the complex parameters. The impact is particularly pronounced for the mass of the light Higgs boson. In order to factor out the impact of phase space effects, we normalise the prediction for the cross section of the light Higgs boson in the MSSM to the cross section of a SM Higgs boson with identical mass as the light Higgs mass eigenstate \(m_{h_1}\). In case of the heavy Higgs bosons for which the phase space effects are much less severe, we stick to the inclusive cross sections without such a normalisation. The predicted value for the Higgs boson mass \(m_{h_1}\) deviates from 125 GeV by up to a few GeV in our illustrative studies. Deviations from the experimental value in this ballpark are still commensurate with the remaining theoretical uncertainties from unknown higherorder corrections of current stateoftheart calculations of the light Higgs boson mass in the MSSM [6].
Subsequently we discuss three aspects: We start with a discussion of squark effects for the Higgs boson production cross sections. They are of relevance both for the heavy Higgs bosons and the light Higgs boson. Secondly, we focus on the admixture of the two heavy Higgs bosons (described through \({\hat{\mathbf Z}}\) factors) and its effect on production cross sections. Lastly we discuss \(\Delta _b\) corrections in the context of the \(m_h^{\mathrm {mod}+}\)inspired scenario with large \(\tan \beta \), for which the bottomquark annihilation process for the heavy Higgs bosons is relevant as well.
Note that given the large admixture of the two heavy Higgs bosons in the MSSM with complex parameters, interference effects in the full processes of production and decay can be large. However, we restrict our discussion in the present paper to Higgs boson production. The results for the cross sections obtained in our paper can be employed in a generalised narrowwidth approximation as described in Ref. [7] in order to incorporate interference effects. We will address this issue elsewhere.
The prediction for Higgs boson cross sections is affected by various theoretical uncertainties, which we discuss in detail in Sect. 6. In order to demonstrate the improvement in precision through the inclusion of higherorder corrections, all subsequent figures which show the LO cross section and our best prediction cross section according to Eq. (38) include renormalisation and factorisation scale uncertainties. The procedure for obtaining these scale uncertainties is outlined in Sect. 6.
5.1 Squark contributions in the lightstop inspired scenario
In Fig. 4a, b we show the gluonfusion cross sections of the heavy Higgs bosons \(h_2\) and \(h_3\), respectively, as a function of \(\phi _{A_t}\). The colour coding is identical to Fig. 3 except for the fact that we show the Kfactor of our best prediction for the cross section with respect to the LO cross section, \(\sigma /\sigma _{\mathrm{LO}{}}\), rather than a cross section normalised to the SM Higgs boson cross section. In fact, the heavy Higgs masses change only slightly as a function of the phase \(\phi _{A_t}\), and therefore the associated phase space effect is small. For vanishing phase \(\phi _{A_t}=0\) it is known that squark effects are huge and reduce the cross section by \({\sim } 89\)% (\(h_2\)) and \({\sim } 22\)% (\(h_3\)) [119]. These squark effects are strongly dependent on the phase \(\phi _{A_t}\) and induce a large positive correction at phase \(\phi _{A_t}=\pi \) in case of \(h_2\). For \(h_3\) the effects are not as pronounced, but still sizeable. The Kfactor for both processes \(gg\rightarrow h_2\) and \(gg\rightarrow h_3\) remains within [1, 1.6], i.e. higherorder corrections mainly follow the phase dependence of the LO cross section. The dependence of the Kfactor on \(\phi _{A_t}\) follows the black, dotdashed curve, which shows the cross section with quark contributions only. The significant dependence of the cross section where only quark contributions are included on the phase \(\phi _{A_t}\) is induced by the admixture of the two Higgs bosons through \({\hat{\mathbf Z}}\) factors. We will discuss this feature in detail for the \(m_h^{\mathrm {mod}+}\)inspired scenario in Sect. 5.2.
5.2 Admixture of Higgs bosons in the \(m_h^{\mathrm {mod}+}\)inspired scenario
In this subsection we discuss the \(m_h^{\mathrm {mod}+}\)inspired scenario with \(\tan \beta =10\) and \(m_{H^{\pm }}=900\) GeV. Since the squark masses are at the TeV level in this scenario, the numerical effect of the squark loops in the gluon fusion vertex contributions is rather small for the production cross section of the light Higgs boson \(h_1\). We do not discuss the results for \(h_1\) in this section. The results for the two heavy Higgs bosons are displayed in Fig. 6. The effects from squark loops are at the level of about \({\pm } 20\)% in this case. The considered scenario is typical for the decoupling region of supersymmetric theories, where a light SMlike Higgs boson (that is interpreted as the signal observed at about 125 GeV) is accompanied by additional heavy Higgs bosons that are nearly massdegenerate. In the general case where the possibility of \(\mathcal {CP}\)violating interactions is taken into account, there can be a large mixing between the \(\mathcal {CP}\)even and \(\mathcal {CP}\)odd neutral Higgs states. This feature is clearly visible in Fig. 6. The dependence on the phase \(\phi _{A_t}\) is seen to be closely correlated to the mixing character of the two neutral heavy Higgs bosons.
Figure 6a depicts the masses of the two heavy Higgs bosons \(h_2\) and \(h_3\) as a function of \(\phi _{A_t}\) together with the \(\mathcal {CP}\)odd character of \(h_2\) and \(h_3\), being defined as \({\hat{\mathbf Z}}_{aA}^2\). For illustration here and in the following we call the mass eigenstates \(h_2\) and \(h_3\) either \(h_e\) or \(h_o\), depending on their mixing character: if \({\hat{\mathbf Z}}_{aA}^2\gtrsim 1/2\) the mass eigenstate \(h_a\) is called \(h_o\), otherwise it is called \(h_e\). It can be seen in Fig. 6b, c that the behaviour of the cross sections as a function of \(\phi _{A_t}\) closely follows the variation in the \(\mathcal {CP}\)even and \(\mathcal {CP}\)odd character of the Higgs states. A similar effect was already apparent in the top and bottomquark induced cross sections depicted in the lightstop inspired scenario, see Fig. 4, however there the effects of squark contributions are dominant. Also in this case our best prediction for the cross section is significantly reduced in comparison with the prediction in LO QCD. The variation of the Kfactors between about 1.2 and 1.5 with the phase \(\phi _{A_t}\) also follows the modification of the mixing character of the two neutral heavy Higgs bosons.
5.3 \(\Delta _b\) corrections in the \(m_h^{\mathrm {mod}+}\)inspired scenario
In Fig. 9 we separately analyse the squark contributions for the LO cross section, i.e. the prediction omitting the squark loop contributions (black dotdashed curves) is compared with the ones where first the pure LO squark contributions are added (depicted in cyan), and then the resummation of the \(\Delta _b\) contributions to the bottomquark Yukawa coupling is taken into account. For the latter both the results for the full (\(\Delta _{b2}\), blue) and the simplified (\(\Delta _{b1}\), red) resummation are shown. While the pure LO squark contributions are seen to have a moderate effect, it can be seen that the incorporation of the resummation of the \(\Delta _b\) contribution leads to a significant enhancement of the squark loop effects. We furthermore confirm that for the heavy neutral Higgs bosons considered here the simplified resummation approximates the full resummation of the \(\Delta _b\) contribution very well. The curves corresponding to \(\Delta _{b2}\) and \(\Delta _{b1}\) hardly differ from each other both for the variation of \(\phi _{A_t}\) and \(\phi _{M_3}\). As before all curves include the same \({\hat{\mathbf Z}}\) factors obtained from FeynHiggs. The results for \(h_o\), which are not shown here, are qualitatively very similar. The LO squark contributions are less relevant for the \(h_o\) cross section, since those contributions are absent in the MSSM with real parameters. We also note that the curves for \(h_o\) follow a similar behaviour as the ones for \(h_e\), which implies that there are no large cancellations expected in the sum of the cross sections for the two heavy Higgs bosons times their respective branching ratios. Thus, the phases entering \(\Delta _b\) could potentially lead to observable effects in the production of the two heavy Higgs bosons even if the two states cannot be experimentally resolved as separate signals.
Having discussed the three different sources for \(\mathcal {CP}\)violating effects relevant for Higgs boson production through gluon fusion in the MSSM – squark loop contributions, admixtures through \({\hat{\mathbf Z}}\) factors and resummation of \(\Delta _b\) contributions – for completeness we also briefly discuss the bottomquark annihilation cross section for the \(m_h^{\mathrm {mod}+}\)inspired scenario with \(\tan \beta =40\). The corresponding cross section is shown in Fig. 10 as a function of \(\phi _{A_t}\) and \(\phi _{M_3}\). For such a large value of \(\tan \beta \) this cross section exceeds the gluonfusion cross section by far. It shows a very significant dependence on the phases \(\phi _{A_t}\) and \(\phi _{M_3}\), which is mainly induced by the \(\Delta _b\) contribution.
6 Remaining theoretical uncertainties
In the previous section we analysed our cross section predictions regarding \(\mathcal {CP}\)violating effects entering via squark loop contributions, \({\hat{\mathbf Z}}\) factors and \(\Delta _b\) contributions. Therein, we included renormalisation and factorisation scale uncertainties, which as expected are reduced upon inclusion of higherorder corrections. However, the cross section predictions are also affected by other relevant theoretical uncertainties, which we want to discuss in detail in this section.

PDF\(+\alpha _s\) uncertainties: The fitted parton distribution functions (PDF) and the associated value of \(\alpha _s\) induce an uncertainty in the prediction of the gluonfusion cross section and, in particular, also the bottomquark annihilation cross section. In our calculation we employ the MMHT2014 PDF sets at LO, NLO and NNLO [118], which can be used for both gluon fusion and bottomquark annihilation. In Refs. [24, 119] it was observed that despite the effects of squarks in supersymmetric models, the PDF\(+\alpha _s\) uncertainties are mostly a function of the Higgs boson mass \(m_{h_a}\). We will therefore not discuss them in more detail, since – similar to the prescription for MSSM Higgs boson cross sections by the LHC Higgs Cross Section Working Group [6] – relative uncertainties can be taken over from tabulated relative uncertainties obtained for the SM Higgs boson or a pseudoscalar (in a 2HDM with \(\tan \beta =1\)) as a function of its mass. For Higgs masses in the range between 50 GeV and 1 TeV the typical size of PDF\(+\alpha _s\) uncertainties for gluon fusion is \({\pm } (3{}5)\)% following the prescription of Ref. [132]. They increase up to \({\pm } 10\)% for Higgs masses up to 2 TeV. For bottomquark annihilation they are in the range \(\pm (3{}8)\)% for Higgs masses between 50 GeV and 1 TeV and up to \({\pm } 16\)% for Higgs masses below 2 TeV.

Renormalisation of the bottomquark mass and definition of the bottom Yukawa coupling: In our calculation the bottomquark mass is renormalised onshell, and the bottomYukawa coupling is obtained from the bottomquark mass as described in Sect. 2.1. The renormalisation of the bottomquark mass and the freedom in the definition of the bottomYukawa coupling are known to have a sizeable numerical impact on the cross section predictions. This is in particular the case for large values of \(\tan \beta \) where the bottomYukawa coupling of the heavy Higgs bosons is significantly enhanced and the topquark Yukawa coupling is suppressed. On the other hand, in these regions of parameter space bottomquark annihilation is the dominant process, for which there is less ambiguity regarding an appropriate choice for the renormalisation scale. The described uncertainties in the MSSM with complex parameters are analogous to the case of real parameters. We therefore refer to the discussion in Ref. [119] and references therein for further details.
 We obtain the renormalisation and factorisation scale uncertainty as follows: The central scale choice is \((\mu _\text {R}^0,\mu _\text {F}^0)=(m_{h_a}/2,m_{h_a}/2)\) for gluon fusion and \((\mu _\text {R}^0,\mu _\text {F}^0)=(m_{h_a},m_{h_a}/4)\) for bottomquark annihilation. We obtain the scale uncertainty by taking the maximal deviation from the central scale choice \(\Delta \sigma \) obtained from the additional scale choices \((\mu _\text {R},\mu _\text {F})\in \{(2\mu _\text {R}^0,2\mu _\text {F}^0),(2\mu _\text {R}^0,\mu _\text {F}^0),(\mu _\text {R}^0,2\mu _\text {F}^0), (\mu _\text {R}^0,\mu _\text {F}^0/2),(\mu _\text {R}^0/2,\mu _\text {F}^0),(\mu _\text {R}^0/2,\mu _\text {F}^0/2)\}\). We perform this procedure individually for all three cross sections in Eq. (41) and then obtain the overall absolute uncertainty throughwhere we assume the two LO cross sections to be fully correlated. The uncertainty bands that we have displayed in the plots shown above correspond to the cross section range covered by \(\sigma \pm \Delta \sigma ^{\mathrm {scale}}\).$$\begin{aligned} \Delta \sigma ^{\mathrm {scale}}=\sqrt{\left( \Delta \sigma _{{{\mathrm{N}^{k}\mathrm{LO}}}}^{\Delta _{b1}}\right) ^2 +\left( \Delta \sigma _{{\mathrm{LO}{}}}^{\Delta _{b2}} \Delta \sigma _{{\mathrm{LO}{}}}^{\Delta _{b1}}\right) ^2}, \end{aligned}$$(45)

In order to display the propagation of an uncertainty arising from higherorder contributions to \(\Delta _b\) to our cross section calculation, we vary the value of \(\Delta _b\) obtained from FeynHiggs by \({\pm } 10\%\). This variation by \({\pm } 10\%\) roughly corresponds to the effect of a variation of the renormalisation scales, see the discussion in Ref. [119]. We label the obtained uncertainty as \(\Delta \sigma ^{\mathrm {resum}}\) and assign an uncertainty band of \(\sigma \pm \Delta \sigma ^{\mathrm {resum}}\).

The employed interpolation for the twoloop virtual squarkgluino contributions following Eq. (35) leads to a further uncertainty. A conservative estimate for it can be obtained as follows: We determine the cross section \(\sigma (\phi _z)\) following Eq. (38) not only for the correct phase \(\phi _z\) in Eq. (35), but also leave the phase within Eq. (35) constant, i.e. fixed to 0 and \(\pi \). We call the obtained cross sections \(\sigma (0)\) and \(\sigma (\pi )\). For each value of \(\phi _z\) we take the difference \(\Delta \sigma ^{\mathrm {int}}=\sin ^2(\phi _z)\sigma (0)\sigma (\pi )/2\). It is reweighted with \(\sin ^2(\phi _z)\), since we know that our result is correct at phases 0 and \(\pi \). The obtained uncertainty band is given by \(\sigma \pm \Delta \sigma ^{\mathrm {int}}\).
In the following we display the effects of the estimated uncertainties for certain scenarios, where we choose the displayed scenarios and the displayed cross sections such that the effect of the uncertainties is largest. While the scale uncertainties were included in all previous figures for the LO prediction as well as for our best prediction already, we will discuss the interpolation uncertainty for the lightstop inspired scenario with \(\tan \beta =16\) and the resummation uncertainty for the \(m_h^{\mathrm {mod}+}\)inspired scenario with \(\tan \beta =40\).
Figure 11 shows the renormalisation and factorisation scale uncertainties \(\Delta \sigma ^{\mathrm {scale}}\) as before and in addition the above described interpolation uncertainty \(\Delta \sigma ^{\mathrm {int}}\), which in case of the variation of \(\phi _{M_3}\) can be substantial. As can be seen in Fig. 11, the interpolation uncertainty obtained from our conservative estimate can in this scenario even exceed the scale uncertainty for the gluonfusion cross section of \(h_2\). It should be noted that this is an extreme case, while the interpolation uncertainty, which is an NLO effect related to the squark and gluino loop contributions, remains small for the other previously described scenarios (which we do not show here explicitly). This is simply a consequence of the fact that the relative impact of the squark and gluino contributions in the other scenarios is much smaller than in the lightstop inspired scenario. The interpolation uncertainty for the gluonfusion cross section of \(h_3\) in Fig. 11 is much less pronounced than for \(h_2\), since as discussed above the squark loop corrections are significantly smaller in this case and would vanish if \(h_3\) were a pure \(\mathcal {CP}\)odd state. The behaviour in the lower panels of Fig. 11 displays the fact that by construction the assigned interpolation uncertainty vanishes for the phases 0 and \(\pi \), where the interpolated result in the MSSM with complex parameters merges the known result of the MSSM with real parameters. For the variation of \(\phi _{A_t}\) the LO cross section incorporating squark contributions already includes the dominant effect on the cross section, such that the uncertainty due to the interpolated NLO contributions is also less pronounced than in case of the variation of \(\phi _{M_3}\).
The described \(\Delta _b\) uncertainties are depicted in Fig. 12. Since \(\Delta _b\) crosses 0 as a function of \(\phi _{M_3}\) twice, the uncertainty that we have associated to it according to the prescription discussed above also vanishes there, as can be seen in the lower panel of Fig. 12b. Even for the large value of \(\tan \beta \) chosen here the assigned \(\Delta _b\) uncertainty of \(\pm 10\%\) is much smaller than the scale uncertainty of the displayed cross sections. Despite the different behaviour with the phases \(\phi _{A_t}\) and \(\phi _{M_3}\) displayed in the lower panel of Fig. 12 the qualitative effect of the resummation uncertainties on the Higgs boson production cross sections is nevertheless rather similar. The latter is also true for the bottomquark annihilation cross section, which is not depicted here. The resummation uncertainties are of most relevance for large values of \(\tan \beta \), where the cross section of bottomquark annihilation exceeds the gluonfusion cross section.
7 Conclusions
In this paper we have presented theoretical predictions for inclusive cross sections for neutral Higgs boson production via gluon fusion and bottomquark annihilation in the MSSM with complex parameters, and demonstrated the relevance of the \(\mathcal {CP}\)violating phases on these cross sections.
The cross section predictions for the gluonfusion process at leadingorder are based on an explicit calculation taking into account the dependence on all complex parameters in the MSSM, and the complete form of the analytical formulae for the general \(\mathcal {CP}\)violating case including Higgs mixing has been presented in the literature for the first time. The wave function normalisation factors arising from the \((3\times 3)\)mixing of the lowestorder mass eigenstates of the Higgs bosons \(\lbrace h, H, A\rbrace \) into the loopcorrected mass eigenstates \(\lbrace h_1, h_2, h_3\rbrace \) have been described with full propagator corrections using the selfenergies of the neutral Higgs bosons as provided by FeynHiggs. Furthermore, the LO predictions for the gluonfusion process in the MSSM with complex parameters deviate from those of the MSSM with real parameters due to nonzero couplings of the squarks to the pseudoscalar A and potentially different left and righthanded bottomYukawa couplings arising from the resummation of \(\tan \beta \)enhanced sbottom contributions in \(\Delta _b\). We have supplemented the LO computation of the cross section by higherorder contributions: using for the treatment of the higherorder corrections a simplified version of the \(\Delta _b\) resummation we have included the full massive top and bottom quark contributions at NLO QCD and have interpolated the NLO SUSY QCD corrections from the amplitudes in the MSSM with real parameters. We have thoroughly discussed the uncertainties involved in using such an interpolation. The interpolation uncertainty at NLO, which is most relevant in scenarios where the squarks and the gluino are relatively light in view of the present limits from the LHC searches, could be avoided if an explicit result for the squarkgluino contributions at NLO QCD in the MSSM becomes available for the general case of complex parameters. For the topquark contribution in the effective theory of a heavy topquark we have added NNLO QCD contributions for all Higgs bosons, and N\(^{3}\)LO QCD contributions in an expansion around the threshold of Higgs production for the \(\mathcal {CP}\)even component of the light Higgs boson \(h_1\) to match the precision of the predictions for the SM Higgs boson. Electroweak effects, which include twoloop contributions with couplings of the heavy gauge bosons to the \(\mathcal {CP}\)even component of the Higgs bosons mediated by light quarks, have been added to the \(\mathcal {CP}\)even component of the gluonfusion cross section.
The results presented in this paper are currently the state of the art for neutral Higgs production in the MSSM with complex parameters. Our calculations have been implemented in an extension of the code SusHi called SusHiMi, which is linked to FeynHiggs. SusHiMi is available upon request. Using SusHiMi, we have investigated the phenomenological effects of \(\mathcal {CP}\)violating phases on the production of Higgs bosons in the MSSM with complex parameters in two slightly modified benchmark scenarios, lightstop and \(m_h^{\text {mod+}}\). We have found in our analysis of Higgs boson production through gluon fusion that a proper description of squark and gluino loop contributions is essential. This refers both to the loop contributions to the gluon–gluon–Higgs vertex and to the corrections entering through \(\Delta _b\). Squark and gluino loop contributions furthermore enter the wave function normalisation factors that are necessary to ensure the correct onshell properties of the produced Higgs boson. Where squark and gluino contributions are sizeable the production cross sections show a significant dependence on the \(\mathcal {CP}\)violating phases. We have discussed the remaining theoretical uncertainties in the cross section predictions taking into account renormalisation and factorisation scale uncertainties, a resummation uncertainty for \(\Delta _b\) and an uncertainty due to the performed interpolation of NLO SUSY QCD corrections. We have furthermore briefly commented on other uncertainties that can directly be taken over from the case of the MSSM with real parameters.
A further important feature that occurs in the production processes for the two heavy states \(h_2\) and \(h_3\) in the general case where \(\mathcal {CP}\)violating interactions are taken into account is the fact that there can be a large mixing between these often nearly massdegenerate states. Their mixing effects are incorporated in the wave function normalisation factors for the external Higgs bosons. For a proper interpretation of experimental exclusion limits arising from MSSM Higgs searches, which so far have only been analysed in the framework of the \(\mathcal {CP}\)conserving MSSM, it will be important to take into account interference effects in the full process of Higgs production and decay. Our results for the cross sections for onshell Higgs bosons can be directly used in the context of a generalised narrowwidth approximation to incorporate these interference effects. This topic will be addressed in a forthcoming publication.
Footnotes
 1.
 2.
The softbreaking parameter \(M_1\) associated with the bino can also be complex, but has a minor impact on the Higgs sector, and we neglect its phase dependence in the following.
 3.
We note that the convention differs from the convention employed by FeynHiggs by a different sign of \(\chi _1^0\) and \(\phi _1^\), which induces different signs in the corresponding elements of the matrices in Eqs. (11) and (12) and the \(\chi _1^0\) couplings to (s)quarks displayed in the Appendix.
 4.
In the MSSM with real parameters only couplings involving \(\tilde{f}_i\tilde{f}_jA\) with \(i\ne j\) are nonvanishing, and left and righthanded quark Yukawa couplings are identical, \(g^\phi _q\equiv g^\phi _{q_L}=g^\phi _{q_R}\).
 5.
 6.
 7.
The name is inspired by the mixing of the Higgs bosons. SusHiMi can be obtained upon request.
 8.
 9.
Indirect bounds from the effects of stops on the measured Higgs rates are much weaker, see e.g. Ref. [117].
Notes
Acknowledgements
We thank Sebastian Paßehr and Pietro Slavich for discussions and Elina Fuchs for discussions and comments on the manuscript. The authors acknowledge support by Deutsche Forschungsgemeinschaft through the SFB 676 “Particles, Strings and the Early Universe” and by the European Commission through the “HiggsTools” Initial Training Network PITNGA2012316704.
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