Supersymmetric Z{}^{\prime} decays at the LHC

Supersymmetric Z decays at the LHC

Gennaro Corcella 
INFN, Laboratori Nazionali di Frascati
Via E. Fermi 40, I-00044 Frascati (RM), Italy

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 Corcellathanks: Speaker.

INFN, Laboratori Nazionali di Frascati

Via E. Fermi 40, I-00044 Frascati (RM), Italy



18th International Conference From the Planck Scale to the Electroweak Scale 25-29 May 2015 Ioannina, Greece

1 Introduction

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., [1] 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 [2], and CMS to set  TeV (SSM) and  TeV (GUTs) [3]. 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 [4] 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 [6], using the same representative point of the MSSM parameter space as in [5], 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 [7] extended the investigation in [5] 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 [8] 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. [7], 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 [7], 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 [4], 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 [5] 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:


3 Results

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 model corresponds to a mixing angle in Eq. (2). Following [6], the mass is set to  TeV, the MSSM parameters as in Eqs. (2) and (2), whereas the soft sfermion masses at the mass scale are given by:


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 [9] and SPheno [10] 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 .

5609.8 5609.4 5609.9 5609.5 2321.7 2397.2
5504.9 5508.7 5504.9 5508.7 2119.6 2036.3
Table 1: Squark masses in GeV the reference point of U.
1392.4 953.0 1398.9 971.1 1389.8 961.5 1395.9 961.5
Table 2: Masses of charged sleptons () and sneutrinos in the reference point of the U(1) model.
125.0 1989.7 4225.0 4225.0 4335.6
Table 3: Masses of neutral and charged Higgs bosons.
204.8 889.1 197.2 210.7 408.8 647.9 889.0 6193.5
Table 4: Masses of charginos and neutralinos in the reference point of the U(1) model.
Final State Branching ratio (%)
Table 5: decay rates for  TeV.
Final State branching ratio (%)
Table 6: Chargino decay rates in the reference point of the model.

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 [11], 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.


The cross section of (3.1) is roughly  pb. Furthermore, in (3.1) the invariant mass reproduces , while in (3.1) the charginos do not have this constraint and can be rather soft.

Figure 1 presents the transverse momentum of leptons produced in all three processes, according to MadGraph interfaced to HERWIG [12] 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).

Figure 1: Lepton transverse momentum for the model at  TeV and  TeV, for a direct decay (left) and chains initiated by or direct chargino production processes (right).

Figure 2: Left: dilepton invariant mass, with and originated from decays into charginos and from direct events. Right: angle between in the laboratory frame, in all three processes.

Figure 3: Left: missing transverse energy due to the neutrinos and neutralinos in the cascade initiated by a primary decay (solid histogram) and for direct chargino production (dashes). Right: transverse mass for the final-state particles (leptons, neutrinos and neutralinos) in the same processes.

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.

Figure 4: MET spectrum in decays: plotted are the neutralino (dashes), neutrino (dots) and total (solid) contributions to the missing transverse energy.

3.2 Phenomenology - U model

Hereafter, I investigate the U(1) model, i.e. in (2), for  TeV, MSSM parameters as in Eqs. (2) and (2) and soft sfermion masses fixed to the following values:


After adding D- and F-terms, the masses of squarks and sleptons are those quoted in Tables 7 and 8, whereas Table 9 and 10 contain the masses of Higgs bosons and gauginos, respectively.

3130.8 3129.8 3130.8 3129.8 3130.8 3175.5
3065.9 2863.6 3065.9 2863.6 3065.9 2823.5
Table 7: Squark masses in GeV in the model.
1194.6 1364.5 1208.8 1307.7 1361.8 456.0 1368.0 456.0
Table 8: Slepton masses in the scenario.
124.9 2004.2 4229.4 4229.4 4230.0
Table 9: Higgs bosons in the model, with masses expressed in GeV.
206.5 882.4 199.3 212.5 408.2 882.3 1562.8 2569.2
Table 10: Masses in GeV of charginos and neutralinos in the 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:


The branching ratios of sneutrinos and neutralinos are given in Tables 12 and 13.

Final State Branching ratio (%)
Table 11: decay rates in the MSSM reference point for a mass  TeV.
Final state branching ratio (%)
Table 12: Sneutrino branching ratios, in the representative point of the model.
Final State Branching ratio (%)
Table 13: Decay rates for the lighter neutralino .

The cross section of the cascade (3.2) is  pb [7].

Figure 5: Left: Transverse momentum of the hardest (solid) and softest (dashes) lepton in the cascade initiated by a primary decay into sneutrinos. Right: Lepton transverse momentum in processes.

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.

Figure 6: Left: transverse mass of final states initiated by a decay of the into sneutrino pairs and leading to a final state with four leptons and MET. Right: missing transverse energy due to neutrinos and neutralinos.

Figure 6 presents the spectrum of the missing transverse energy and of the transverse mass of the final state (3.2): the MET distribution is similar to the one; the transverse mass is relevant in the range and is overall a broader and smoother distribution.

4 Conclusions

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 [6] 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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