Supersymmetric Z decays at the LHC
Searching for bosons, predicted in GUT-inspired U(1) gauge models and in the Sequential Standard Model, is one of the main challenges of the experiments carried out at the Large Hadron Collider. Such searches have so far focused on high-mass dilepton pairs, assuming that the has only Standard Model decay modes, and have set mass exclusion limits around 2.5-3 TeV. In this talk, I investigate supersymmetric decays at 14 TeV LHC, extending the MSSM in such a way to accommodate extra heavy gauge bosons. In particular, I study decays into pairs of sleptons, charginos and neutralinos, leading to final states with leptons and missing energy, and present results for few reference points of the parameter space, consistent with a SM-like Higgs boson with a mass around 125 GeV. I also discuss the feasibility to search for Dark Matter candidates, by analysing decays into the lightest MSSM neutralinos.
Supersymmetric Z decays at the LHC
Gennaro Corcella††thanks: Speaker.
INFN, Laboratori Nazionali di Frascati
Via E. Fermi 40, I-00044 Frascati (RM), Italy
Heavy neutral gauge bosons are predicted in extensions of the Standard Model (SM), based on U(1) gauge symmetries, typically inspired by Grand Unification Theories (see, e.g.,  for a review). Moreover, they are present in the so-called Sequential Standard Model (SSM), wherein and possibly bosons have the same couplings to fermions as the and .
The LHC experiments have so far searched for the assuming that it decays into SM channels, focusing on high-mass electron or muon pairs. The absence of this signal has led the ATLAS Collaboration to set the limits TeV in the SSM and TeV in U(1) models , and CMS to set TeV (SSM) and TeV (GUTs) . Hereafter, I study possible decays of the beyond the Standard Model (BSM), taking particular care about supersymmetric final states, within the Minimal Supersymmetric Standard Model (MSSM).
Supersymmetric decays were first considered in  and lately reconsidered in [5, 6, 7], in both GUT-inspired and SSM frameworks. Although the SM modes still dominate, the opening of BSM decay channels decreases the branching ratio into dilepton pairs and therefore modifies the LHC exclusion limits. Reference , using the same representative point of the MSSM parameter space as in , found that the LHC exclusion limits decrease by an amount 150-300 GeV, once including supersymmetric decays. However, Refs. [5, 6] chose a reference point which is consistent with the present limits on supersymmetry, but not with the observed Higgs mass, around 125 GeV. Reference  extended the investigation in  and, by fully including one-loop corrections to the Higgs mass, managed to identify points of the parameter space which are consistent with both supersymmetry searches and Higgs-boson properties.
From the viewpoint of supersymmetry, the production of sparticles in decays has the advantage that their invariant mass is fixed by the heavy boson mass: therefore, if one had to discover a , it would be a cleaner channel to search for supersymmetry, with respect to direct production in or annihilation. Moreover, decays into the lightest neutralinos are interesting processes to search for Dark Matter candidates, since they lead to mono-jet and mono-photon final states (see  for a recent study on Dark Matter at the LHC, within the U(1) extension of the MSSM).
In this talk, I shall highlight the main results of Ref. , investigating the feasibility to search for supersymmetry in decays at 14 TeV in U(1) models.
2 Theoretical framework
U(1) gauge groups and bosons arise in the framework of a rank-6 Grand Unification group . The is associated with U(1), coming from the breaking of into SO(10):
The subsequent breaking of SO(10) into SU(5) leads to the U(1) group and bosons:
A generic is then a mixture of and bosons, with a mixing angle :
Another scenario, typical of superstring theories, is a direct breaking of E in the SM and a U(1) group, leading to a boson, with a mixing angle :
As in , in this talk I shall mostly concentrate the analysis on the the and models, as they are the most interesting in the supersymmetric extension of the SM.
In fact, the MSSM gets a few novel features, due to presence of the boson. In addition to the scalar Higgs doublets and , an extra neutral singlet is necessary to break U(1) and give mass to the . After electroweak symmetry breaking, the Higgs sector consists of one pseudoscalar and three scalars , and , where is the new boson, due to the extra U(1). In the gaugino sector, two more neutralinos are present, associated with the supersymmetric partners of and .
Furthermore, as thoroughly debated in , the U(1) group leads to extra D- and F-term contributions to the sfermion masses. In particular, the D-term, depending on the sfermion and Higgs U(1) charges, can be large and negative, so that even the sfermion squared masses can become negative and thus unphysical (see  for a few examples of unphysical configurations).
Hereafter, the mass will always be set to and the coupling constants of U(1) and U(1), i.e. and , are assumed to be proportional, as often happens in GUTs:
The supersymmetric parameters , , , and , where and are the soft masses of the gauginos and , are fixed as follows:
Given , the wino mass can be obtained through GeV, being the Weinberg angle. As for the trilinear couplings of squarks and sleptons with the Higgs in the soft supersymmetric Lagrangian and , the soft trilinear coupling of the the three Higgs fields (, and ), they are fixed to the same value:
In this section I present a few results for the models U(1) and U(1). In each scenario, a point of the parameter space is chosen, in such a way that at least one decay mode into supersymmetric particles is substantial and leads to an observable signal at the LHC. Besides, in order to suppress QCD backgrounds, particular care will be taken about leptonic final states.
3.1 Phenomenology - U model
The squark and slepton masses, obtained by summing to the numbers in Eq. (3.1) the D- and F-terms, computed by means of the SARAH  and SPheno  codes, are quoted in Tables 1 and 2. The notation and refers to the mass eigenstates, determined from the weak states and , after diagonalizing the mass mixing matrices. Tables 3 and 4 contain the mass spectra of Higgs bosons and gauginos (charginos and neutralinos), respectively. In the Higgs sector, the lightest has a mass compatible with the SM Higgs, is roughly as heavy as the , the novel scalar , the pseudoscalar and the charged are all above 4 TeV and therefore too heavy to contribute to the decay width of a 2 TeV . As for the gauginos, with the exception of the heaviest neutralino , they are lighter than the .
|Final State||Branching ratio (%)|
|Final State||branching ratio (%)|
The branching ratios are quoted in Table 5, neglecting rates which are below 0.1%. The overall branching ratio into supersymmetric final states is 28.3%; the rate into chargino pairs accounts for about 10%, while decays into pairs of the lightest neutralinos, i.e. , possibly relevant for the searches for Dark Matter, for almost 5%.
Investigating in more detail the process, subsequent decays may lead to final states with charged leptons and missing energy, as in the following process ():
The decay rates of the charginos are reported in Table 6; the cross section of the process in Eq. (3.1), computed by MadGraph , reads: pb at 14 TeV. One may therefore expect about 80 events for a luminosity , almost 240 at 300 fb.
In the following, I will present some leptonic distributions and compare them with direct decays, , and direct chargino production, i.e.
Figure 1 presents the transverse momentum of leptons produced in all three processes, according to MadGraph interfaced to HERWIG  for parton showers and hadronization. In direct events, the two leptons get the full initial-state transverse momentum and the spectrum is relevant at high values; in processes (3.1) and (3.1), some (missing) transverse momentum is lent to neutrinos and neutralinos, which significantly decreases the of and . For direct charginos (3.1) there is no cutoff on the invariant mass and the leptons are quite soft; in , the leptonic is instead higher than in process (3.1).
Fig. 2 presents the invariant mass (left) and the angle between the two charged leptons in the laboratory frame (right). In the cascade (3.1), lies in the range 20 GeV 100 GeV and is maximum at GeV; for direct chargino production, is peaked about 5 GeV and rapidly decreases. As for the spectrum, in direct leptonic decays it exhibits a maximum about , i.e. and are almost back-to-back; in , the distribution is broader and peaked at a lower ; in the chain (3.1), and are produced at smaller .
Figure 3 displays the distributions of the sum of the transverse momenta of ‘invisible’ particles (neutrinos and neutralinos), i.e. the so-called MET (missing transverse energy), and the transverse mass of all final-state particles in Eqs. (3.1) and (3.1). In both processes, the MET spectrum is significant in the low range: in the chain (3.1) it is sharply peaked at MET GeV and smoothly decreases at larger MET values; for direct chargino production, the MET exhibits an even sharper peak at MET GeV. As for the transverse mass, in (3.1) it is substantial only at small ; in (3.1) it is instead relevant in the range and peaked just below the mass threshold, 2 TeV in the present reference point.
Decays into neutralino pairs , accounting for almost 5%, are relevant for the searches for Dark Matter candidates and yield a cross section pb at 14 TeV LHC. Therefore, about 640 events at fb and up to almost at 300 fb can be foreseen, with typical signatures given by mono-photon or mono-jet final states. Competing processes are decays into neutrino pairs, amounting to about pb at 14 TeV, with events at 100 and 300 fb. Figure 4 displays the total missing transverse energy (MET) spectrum and the contribution due to neutrino and neutralino pairs in decays; unlike previous distributions, they are normalized to the total LO cross section and not to unity, in such a way to appreciate the discrepancy between the two subprocesses. The shapes of all distributions are roughly the same, but the total number of events at any MET value is higher by about 60% if neutralinos contribute.
3.2 Phenomenology - U model
Table 11 presents the branching ratios of the : within supersymmetry, sneutrino pairs exhibit the highest rate, slightly below 10%. It is therefore worthwile investigating the cascade:
|Final State||Branching ratio (%)|
|Final state||branching ratio (%)|
|Final State||Branching ratio (%)|
In Fig. 5 the transverse momenta of in direct decays and of the softest and hardest lepton in the decay chain (3.2) are plotted. In the cascade, the hardest lepton has a broad spectrum, relevant in the 10 GeV GeV range, whereas the of the softest is a narrow distribution, substantial only for 8 GeV GeV; the spectrum in the direct production is roughly the same as in the case.
I investigated supersymmetric decays at the LHC, for TeV, in GUT-inspired U(1) and U(1) gauge models. The analysis was carried out for few reference points of the MSSM, extended to account for the new U(1) symmetry, and consistent with a 125 GeV Higgs boson. Both and models yield substantial supersymmetric event rates in the 14 TeV run of the LHC: decays into chargino, sneutrino or neutralino pairs lead to final states with leptons and missing energy, which can be discriminated from processes and from direct sparticle production. Once data on high-mass leptons at 14 TeV are available, it will be interesting comparing them with the theory predictions, as done in  at 8 TeV, and possibly determine the exclusion limits accounting for supersymmetry. Nevertheless, a full analysis should compare such signals with the backgrounds due to SM, other supersymmetric processes and non-supersymmetric decays, and account for the detector simulation. The inclusion of background and detector effects is in progress.
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