Fewer analyses exist for Z bosons in supersymmetric U (1) models [5, 18, 24] . In this case, an additional problem arises. The mass of the Z boson is, as usual, proportional to the VEV of the singlet Higgs boson S. But this parameter also determines the scale of the chargino/neutralino sector, thus a heavy Z implies a heavy electroweakino sector, reducing further the interest in such models at the LHC. To avoid this link, we work in a secluded scenario [25, 26] , where the scalar sector of the U (1) model is augmented by three additional singlet scalars 1 , whose role is to decouple the mass of the Z from the scale of chargino and neutralino masses. Thus in this model, one can still preserve a large Z mass while allowing light charginos/neutralinos, and in particular, a light dark matter candidate, which is the lightest supersymmetric particle (LSP) 2 . In the secluded model, Z decays into (light) chargino and neutralino pairs, or into sfermions can be significant, and affect the mass limits, albeit slightly.
There have been studies of the Z boson where the mass constraints were considerably reduced, assuming the model to be leptophobic [27] [28] [29] [30] or quark-phobic [31] . For these, Z couplings to the leptons or quarks are tuned (by assuming family non-universality [32] , by using a specific value of the kinetic mixing [30] , or by a choice of U (1) charges [29] ). What is lacking is a comprehensive study of a heavy generic Z , which satisfies rather than evades the mass bounds imposed by the data, concentrating on its production, decays and observability at high-luminosity (HL), and/or high-energy (HE) frontier of a future LHC, as well as the hadronic mode of a Future Circular Collider (FCC-hh). We remedy this here.
A heavy gauge boson, with mass M Z > 4 TeV cannot always be treated as a narrow width, as the ratio Γ Z /M Z could be > 10%. In this case, interference effects are important [33] [34] [35] and must be included. New physics contributions to the Z width may significantly decrease the branching ratios into SM particles, and therefore the mass limits quoted by the experiments may have to be revisited. Furthermore, Z decays into supersymmetric particles represent an excellent tool to investigate the electroweak interactions at the LHC in a phase-space corner that cannot be explored by employing the usual techniques. Therefore, the possible discovery of supersymmetry in Z mediated processes would help to understand the role of Z in the SUSY breaking and open the path to additional investigations, since one would need to formulate a consistent scenario accommodating both sparticles and heavy gauge bosons.
The scope of this paper is indeed the investigation of the phenomenology of Z bosons at the LHC, assuming that they are heavy, and that they can decay into both SM and supersymmetric particles. We will analyze the decay channels of the Z boson, including decays to neutralinos, charginos, sleptons and Higgs bosons, which are normally neglected.
In our detailed study, we will allow the U (1) parameters to run within suitable ranges, taking into account the recent experimental limits. Throughout this work, we will focus especially on the decay of the Z into slepton, chargino and neutralino pairs, eventually leading to multilepton final states 2 + E T or 4 + E T or 6 + E T . To test the observability of such signals at the LHC, we devise benchmark scenarios and test their features at high integrated luminosity and at the center-of-mass √ s = 14, 27 and 100 TeV. We include calculations of possible SM backgrounds and we present a complete simulation analysis for the HL-LHC and future hadron colliders, indicating the significance of each scenario, and most promising observable for each signal. Our paper is organized as follows. In Sec. II we describe the secluded U (1) model with particular emphasis on its neutral gauge (in II.1) and neutralino (in II.2) sectors. We then proceed to analyze the implications of the model at colliders in Sec. III, focusing first in choosing three benchmarks, which obey experimental constraints, and which are able to reproduce the correct relic density, while maximizing Z decays into supersymmetric particles, in III.1. Then we proceed with the analysis of Z production and decays. We concentrate our analysis on multilepton signals III.2, looking at 2 + E T (in III.2.1), 4 + E T (in III.2.2), and 6 + E T (in III.2.3) signals. We summarize our findings and conclude in Sec. IV.
We present here the main ingredients of the secluded U (1) model, with particular emphasis on the Z boson. The model is based on the gauge group SU
The additional Abelian group introduces, in addition to the MSSM superfields, three right-handed neutrino superfieldsN c i by four scalar singletsŜ,Ŝ 1 ,Ŝ 2 andŜ 3 and an additional neutral gauge boson and gaugino. While S is needed to break the symmetry, three additional singletsŜ 1 ,Ŝ 2 andŜ 3 (the secluded sector) are introduced to split the mass scale of the additional gauge boson from that of electroweakinos. Unfortunately, anomaly cancelation requires the presence of additional superfields, with exotic quantum numbers, which are assumed to be heavy and decoupled form the rest of the spectrum. The superpotential in this model including the exotic fields is given by
where the fields Q, L are the exotic fermions, M R is a large mass scale and h ν is the Yukawa coupling responsible for generating neutrino masses. In this form, neutrinos are Dirac particles, whose masses imply, for the Yukawa coupling
The effective µ term is generated dynamically as µ = h s S . The scalar potential includes the F -term, given by
while the D-term scalar potential is
where g 1 , g 2 and g 1 are the coupling constants for the U (1) Y , SU (2) L and U (1) gauge groups while Q φ is the U (1) charge of the field φ. Finally, the potential includes the SUSY-breaking soft terms, expressed in terms of soft-SUSY breaking masses M 2 i and triple scalar couplings A i as
The symmetry-breaking sector of the model is very complex, and finding an acceptable minimum of the Higgs potential, even at the tree level, is non-trivial [26] . Once a minimum is found, the mass of the lightest Higgs boson can be fine tuned to 125 GeV by small variations in the parameterh s . Setting masses for the additional scalars in the TeV range insures that the mixing with the lightest Higgs boson is small, and thus this Higgs will obey mass [37] and signal bounds [38] consistent with the SM-like Higgs found at the LHC. Additional Higgs states, in particular the lightest pseudoscalar, being heavy, will also satisfy constraints from B s → µ + µ − branching ratio [39] . The U (1) charges of the fields satisfy conditions arising the requirement of cancellation of gauge and gravitational anomalies. For instance, the charges for Higgs fields in the model are chosen so that Q S = −Q S1 = −Q S2 = Q S3 /2, Q Hu + Q H d + Q S = 0. The U (1) charge of the quark doublet Q is kept as a free parameter after the normalization Q Hu = −2, Q H d = 1, Q S = 1, Q S1 = −1, Q S2 = −1, Q S3 = 2. A complete list of conditions for anomaly cancellations in the model, and a choice of charge assignments of the SM and exotic quarks and leptons in the model can be found in [36] .
Through spontaneous breakdown of the group SU (2) L ⊗ U (1) Y ⊗ U (1) to U (1) em the Higgs acquire the VEVs
Here the first two VEVs are required to break the SM, and the third to break U (1) . After symmetry breaking, one massless state (the photon) and two massive states (the Z, Z bosons) arise as orthonormal combinations of W 3 µ , Y µ and Y µ gauge bosons. The W 1 µ and W 2 µ combine to form W ± µ , the charged vector bosons in the model. Unlike in the MSSM, the Z boson is not a physical state by itself but mixes with the Z boson. This mass mixing arises from the fact that the Higgs doublets H u,d are charged under each factor of SU (2) L ⊗ U (1) Y ⊗ U (1) , and the associated mass-squared matrix is given by
in the Z µ , Z µ basis, where the matrix elements are
where g 2 Z = g 2 2 + g 2 1 . The physical neutral vector bosons, Z 1,2 , are obtained by diagonalizing M 2 ZZ :
is their mass mixing angle, and
are their squared masses of the eigenstates. The collider searches plus various indirect observations require the Z-Z mixing angle θ ZZ to be at most a few times 10 −3 [13] , where unavoidable model dependence arises from Z couplings. This bound requires either M Z2 to be large enough (well in the TeV range) or ∆ 2 to be sufficiently suppressed by the vacuum configuration, that is,
Which of these options is realized depends on the U (1) charge assignments and the soft-breaking masses in the Higgs sector. In our calculation, large Z masses insure a small mixing angle. 
and diagonalized by N * M N N † =diag(mχ0 1 , ....., mχ0 9 ), 0 ≤ mχ0 1 ≤ ....... ≤ mχ0 9 . The matrix elements in Eq. 2.11 are defined as
The gaugino masses are free parameters of the model, and we introduce the ratios
These parameters, representing mixing of U (1) Y and U (1) gauginos, and mass of the U (1) gaugino, measured relative to the U (1) Y gaugino mass, will be seen to be important in scanning over the parameter space, as the underlying physics will be sensitive to their variation.
We now proceed to the main analysis in this work, looking at the consequences of a heavy neutral gauge boson at the collider. As we shall see, and as found before, Z bosons satisfying all collider, cosmological and low energy constraints, do not offer promising prospects for observability at the present LHC, even operating at 3 ab −1 . Thus, we will also analyze the prospects of observing a signal at the HE-LHC operating at 27 TeV as well as at the FCC-hh. As the parameter space is large, choosing realistic benchmarks is a more transparent method to show physics results than a scan.
In order to give definite predictions for the production and decay rates of the Z boson, we scan the parameter space for benchmark scenarios to showcase the salient points of the model.
The benchmark points chosen must obey five important conditions:
• The parameters chosen had to insure the stability of the vacuum;
• The points had to satisfy relic density constraints from WMAP of cold dark matter [40] for the LSP, assumed here to be the lightest neutralino;
• The mass of the Z boson has to satisfy mass constraints from ATLAS and CMS, as discussed in the next subsection;
• Of the parameter points satisfying the above two conditions, benchmarks were chosen to enhance the supersymmetric decay signals of the Z boson; and
• In each scenario, the lightest Higgs boson is SM-like and has m H 0 1 = 125 GeV. To analyze the model, we used CalcHEP [41] , micrOMEGAs-4.3.1 [42] , PYTHIA8 [43, 44] , Delphes [45] [46] [47] , and MadAnalysis [48] to prepare the model, calculate the mass spectrum and branching ratios, calculate the relic density, generate events and eventually carry out the simulation. Our goal was to find benchmarks that satisfy cosmological constraints on dark matter and satisfy collider constraints (the invisible width of the Z boson, limits on charged sparticle masses, charginos mass, first and second-generation squark masses, lightest Higgs boson mass, Br(B s,d → µ + µ − ), Br(B → X s γ), ∆M B s,d and various others), as outlined in [49, 50] , and choose those exhibiting distinct decay features, while offering some promise for collider observability.
The three benchmark points, and all the parameters associated with them, are given in Table I . We give VEVs, Yukawa couplings, trilinear couplings, mass ratios and mixings for the gauginos and soft scalar fermion masses. The low value of tan β ≈ 1 is consistent with constraints from B s → µ + µ − branching ratio [39] . For each benchmark scenario, the mass spectra for the supersymmetric partners obtained are given in Table II . The mass of the additional Z boson is > ∼ 4 TeV, and consistent with the ATLAS [19] analyses on Z dilepton decays. As seen in Table I , the VEVs of the additional scalars (S 1 , S 2 and S 3 ) v si , i = 1, 2, 3 are mostly taken above the TeV scale so that the Z mass bound is satisfied independent of the value of the chosen VEV of the scalar field S. For convenience, the parameters µ eff and h s are taken as free parameters and the VEV of S is determined using the relation
The differences between the benchmarks are the following. In BP1, the gaugino masses MỸ lightest slepton soft masses ∼ 500 GeV are much lighter than those of the lightest sneutrinos (Mν = 3000 GeV). In BP3, the gaugino masses are both light MỸ (260 GeV) MW , R Y Y = 0.01 is very small, while the lightest sneutrinos and sleptons have soft masses 500 GeV and 200 GeV, respectively. The neutralino parameters affect the LSP and its composition, while the slepton and sneutrino masses affect branching ratios of Z into sfermions. The calculation of the relic density is performed importing the model files from CalcHEP [41] into the MicrOmegas package [42] . All the numbers obtained are within the 1σ range of the WMAP result obtained from the Sloan Digital Sky Survey [40] 
The relic density of dark matter Ω DM h 2 is very sensitive to the parameter R Y . The value of the relic density is [55] [56] [57] and muon [58, 59] :
In our benchmarks, the contributions are too small to saturate these differences, as these were chosen to instead yield interesting Z phenomenology. In general, the Z boson in this model can decay into all SM fermions, into supersymmetric particles: squark, slepton, sneutrino, neutralino, chargino, in addition to Higgs-boson pairs, W -boson pairs and ZH.
Both ATLAS and CMS collaborations have searched for Z bosons. The assumption is that they are produced in pp, then decay into SM particles. The decay channels explored are jj [60] [61] [62] , bb [63] , tt [64, 65] , e + e − [19, 66] , µ + µ − [19, 66] , τ + τ − [22, 67] , W + W − [68, 69] , and ZH 0 1 [69] , within a variety of models with extended U (1) and SU (2) gauge groups. Of these channels, the leptonic decays e + e − and µ + µ − impose the most stringent constraints on the Z mass, normally M Z > ∼ 4.3 TeV. However, all these analyses assumed non-supersymmetric scenarios. It has been shown that, including supersymmetry, these bounds can be reduced by ∼ 300 GeV [30] . Hence, as we wish to explore the largest parameter space possible, we shall assume that M Z ≥ 4.0 TeV.
We calculated the branching ratios of the Z decaying into various final states for the three selected benchmark points, and show the results for the dominant ones in Table IV . As expected, branching ratios for decays into quarks (BP2 and BP3) or neutrinos (BP1) dominate over those into supersymmetric particles. The benchmarks were chosen for non-negligible decays into SUSY particles pairs, and are dominated by decays into sneutrinos and chargino pairs (BP1), chargino and neutralino pairs (BP2), and into slepton pairs (BP3). In Table IV we also test the width/mass ratio. For all benchmarks considered, Γ Z /M Z remains safely under 10%, justifying treating Z as a narrow resonance. The branching ratios of Z are very sensitive to variations in Q Q , the U (1) charge for the left-handed quark doublet. To highlight Z decays into supersymmetric channels, we fixed all parameters for the chosen benchmarks BP1, BP2 and BP3 as in Table I , except Q Q , which is allowed to vary freely. In Fig.1 we plot the branching ratios of the Z as a function of Q Q for each benchmark scenario. The particular choice for Q Q for each benchmark as given in Table I , is obtained by requiring that some branching ratios into supersymmetric particles be maximal. We indicated these choices in each panel of Fig. 1 signature consisting of charged-leptons and large missing energies, which are particularly well suited for observation at the LHC. To determine and classify all possible signals for the three scenarios we look into the decay topology of these particles. We classify signals according to the final number of leptons present in the signal events. The generic Feynman diagrams contributing dominantly to channels leading to signals with leptons and missing energy are shown in Fig. 2 for BP1 (a), BP2 (b) and BP3 (c).
The events are generated at the partonic level with CalcHEP [41] and they are subsequently passed to PYTHIA8 [43, 44] for decay, showering and hadronization. Events which are saved in HepMC format [70] are then passed to MadAnalysis [48] for applying cuts and further data analysis. Delphes [45] [46] [47] is used for fast detector simulations. We simulated events for the 2 + E T , 4 + E T and 6 + E T signals at the LHC with 14 TeV center-of-mass energy, the HE-LHC with 27 TeV, and as well as the FCC-hh with 100 TeV. We used different PDF sets to model parton distributions for the colliders at different center of mass energies. While CTEQ6l1 PDF set [71] was used for the 14 TeV LHC case, the PDF set from the PDF4LHC15 collaboration [72] was used for both HE-LHC and FCC-hh. In the numerical study, for the calculation of signal significance, we have taken the integrated luminosities L = 3 ab −1 , 15 ab −1 , and 30 ab −1 for the HL-LHC, HE-LHC, and FCC-hh, respectively. Table I except for Q Q , which is allowed to vary. The choice of Q Q for each benchmark, chosen to maximize decays into supersymmetric particles, is indicated in each panel as a vertical grey line. For each benchmark, we use two different formulas for the significance of the signals [73, 74] , where the first expression corresponds to the usual definition, and the second is more useful for smaller number of background events:
where S is the number of signal events, B the number of background events, and σ 2 B the standard deviation for background events. We generated the events for the signal in each scenario, and we also simulated the SM background for the three benchmarks, separately for 14, 27 and 100 TeV.
We concentrate on leptonic final states, considered as golden channels in experimental searches at LHC. To exploit these features, this study will be focused on the decays of the Z boson into supersymmetric particles, leading to final states with leptons and missing energy, due to the presence of neutralinos or neutrinos. In the following, we present a study of Z decays into multileptonic final states for a given set of the secluded sector U (1) model parameters (BP1, BP2 and BP3), dividing our analysis into 2 + E T , 4 + E T and 6 + E T signals.
In this analysis, for each final state, we impose cuts on the kinematical observables to suppress the SM background, as given in Table V . Given the event topologies, stricter cuts on the leading lepton transverse momentum favor events with 2 + E T and 4 + E T . While the cuts on the angular variables and lepton separation remain the same, the kinematic cuts increase (in general), as expected going from 14 TeV to 27 TeV and eventually to 100 TeV. We proceed in turn to analyze each of the final states, 2 + E T , 4 + E T and 6 + E T signals and discuss their potential for observability. The main decay modes of Z giving rise to dilepton final states are:
In the following figures, we first show the relevant kinematic variables for signals and background at 14, 27 and 100 TeV, before any cuts were imposed. We plot the differential cross-section, normalized to unity, with individual bin contents divided by the sum of all the data in the available bins. This way, the uncertainties are correlated across the bins, such that the uncertainties on the total integrated luminosity cancel. The resulting normalized differential fiducial cross-section is plotted as a function of various representative kinematic variables [75, 76] 
for a system composed of a lepton and the invisible transverse momentum available in each event.
Here Fig. 3 , M T of the leading lepton (top panels) and next-to-leading lepton (bottom panels), and in Fig.  4 , the leading lepton transverse momentum (top panels) and next-to-leading lepton transverse momentum (bottom panels). The left-hand side panels in both figures correspond to signals and backgrounds at 14 TeV, the middle at 27 TeV, and the right-hand side for 100 TeV. The main backgrounds (di-bosons) are indicated in solid lines while the signals are plotted in dotted lines, color coded: green for BP1, black for BP2 and orange for BP3. Both of these graphs show clearly that at large M T ( ) and p T ( ), the signal dominates the background, and the graphs justify our choice of kinematic cuts. Table VI gives values for signal and background cross-sections after each cut. We also show the signal significance, for both σ A and σ B , for each benchmark, at proposed total integrated luminosity: for 14 TeV at L = 3 ab −1 , for 27 TeV at L = 15 ab −1 and for 100 TeV at L = 30 ab −1 . While BP3 appears to be most promising, the significance for all benchmarks at 14 TeV is very low, dispelling any hope for observing the Z boson in the 2 + E T final state. This is not surprising, and in complete agreement with other findings [29, 78] . However, at 27 TeV the cuts imposed are especially effective for BP3, and we obtain large significances for both σ A and σ B . At 100 TeV, the cuts imposed to reduce the background wiped out the signal, and we were not able to gain any predictable features. 
The main decay modes of the Z boson yielding 4 + E T signals are
In the following figures, we first plot the relevant kinematic variables signals and background at 14, 27 and 100 TeV, before any cuts were imposed. We show, in Fig. 5 , H T , top row, the scalar sum of transverse hadronic energy [79] 
used as a way to show the hadronic activity and in Fig 5, bottom row, the total missing transverse energy E T . For each figure we plot, in the left-hand columns, signals and backgrounds at 14 TeV, the middle columns at 27 TeV, and right-hand columns, at 100 TeV. Both these plots indicate that the signal over background, before cuts, seems most promising at higher energies, and that E T is overall a better variable to differentiate between signal and background.
We also note that, as for the 2 + E T case, the scenario BP3 is the most promising. In Fig. 6 we plot the transverse mass M T for the leading lepton 1 (top row panels) and next-to-leading lepton The transverse momenta of the third and fourth leptons have similar distributions to the next-to leading lepton, and thus we do not plot them. The left-hand side panels in both figures correspond to signals and backgrounds at 14 TeV, the middle at 27 TeV, and the right-hand side for 100 TeV. The main backgrounds (three-bosons) are indicated in solid lines while the signals are plotted in dotted lines: green for BP1, and orange for BP3 3 . These graphs show clearly that at large M T ( ) and p T ( ), the signal dominates the background, and the graphs justify our choice of kinematic cuts. The distribution is rather similar for the leading leptons, and the p T observable is better than the M T at distinguishing signals from the backgrounds. In all the plots, the signal from BP3 is most promising, particularly at high energy/momenta. Table VII gives values for signal and background cross sections after each cut. We also show the signal significance, for both σ A and σ B , for each benchmark, at proposed total integrated luminosity: for 14 TeV at L = 3 ab −1 , for 27 TeV at L = 15 ab −1 and for 100 TeV at L = 30 ab −1 . Unlike the case of 2 + E T , we keep the cuts constant for different centre-of-mass energies. Again, the significance for all benchmarks at 14 TeV for observing the Z boson in the 4 + E T final state is low. However, at 27 TeV both BP2 and BP3 show some promise, and we obtain large significances of ∼ 3σ or more for σ A . At 100 TeV, though there are many uncertainties and unknowns, and our results should be interpreted as estimates only, both BP2 and BP3 show significant promise for observability.
The dominant decay model of the Z gauge boson, yielding a 6 + E T signal is
Quite clearly, the 6 + E T signal requires that this decay have a non-negligible branching ratio, which occurs for the BP3 scenario, as seen from Table IV , where Z →˜ R˜ R ∼ 3%. Thus as expected, this will be the only signal of interest for the 6 + E T signal. In Fig. 8 we plot the missing transverse energy E T , for the signal and background for the 6 + E T signal: (left-hand) signals and backgrounds at 14 TeV, (middle) at 27 TeV, and (right-hand) at 100 TeV. We show the main (four-bosons) backgrounds in solid lines while the the signal BP3 is given by a dotted green line. In Fig. 9 we plot transverse momentum p T for the leading lepton 1 (top panels), the next-to-leading lepton 2 (second panels), and the next-to-next to leading lepton 3 (bottom panels) for the signal and background for the 6 + E T signal. The transverse momentum p T plots for the fourth, fifth, and sixth lepton are similar and we do not show them here. As before the left side panels indicate signals and backgrounds at 14 TeV, the middle at 27 TeV, and the right side at 100 TeV. The main backgrounds (four-bosons) are indicated in solid lines while the dotted green line represent the signal BP3. As expected, the leading lepton p T distribution is most promising in distinguishing this signal from background, with other lepton p T distributions slightly less so.
Similar to the 2 + E T and 4 + E T signals, the effects of various cuts on cross sections are listed in Table VIII . We also show the signal significance, for both σ A and σ B , for BP3, at proposed total integrated luminosity: for 14 TeV at L = 3 ab −1 , for 27 TeV at L = 15 ab −1 and for 100 TeV at L = 30 ab −1 . The signal significance can be around 3σ at 27 TeV and even greater than 8σ at 100 TeV. 
In this work we have analyzed the LHC, HE-LHC and FCC-hh discovery prospects of a new neutral gauge boson (Z ) through its supersymmetric decay modes, as a promising signal for supersymmetry in an extended gauge structure. We have assumed that the Z originates from an additional U (1) symmetry, and we decoupled its mass scale from that of supersymmetry breaking, assumed to be the same as the scale of breaking U (1) . This allows the Z boson to be heavy, as indicated by lower limits M Z > ∼ 4 TeV from the experimental searches, while the electroweakinos remain light. For this, we relied on the secluded U (1) model, where three additional singlet superfields are added to the model. Unlike the VEV of the singlet scalar field which breaks U (1) and affects the mass of supersymmetric particles, the VEVs of the additional scalars enter only in the expression for the Z mass.
This scenario provides a fertile ground for analyzing Z decays into chargino, neutralino, slepton and sneutrino pairs. As LHC is particularly sensitive to events containing one or more leptons, we looked for production of Z followed by decays into multileptons plus missing energy. For this, we devised three benchmarks (BP1, BP2 and BP3) where branching ratios into some supersymmetric particles are enhanced. For instance in BP1, the decay into sneutrinos and chargino is enhanced, in BP2 it is the decay into chargino and neutralino pairs, while for BP3, the decay into right sleptons and lightest staus is important. The benchmarks are chosen also so they satisfy collider and relic density constraints.
We proceed by analyzing the observability of the signals at √ s = 14, 27 and 100 TeV, looking separately at 2 + E T , 4 + E T and 6 + E T signals. Throughout our benchmarks, the ratio Γ Z /M Z remains under 10%, so we can treat Z as a narrow resonance. For each signal, we perform a Monte Carlo simulation of the signal and background, and devise cuts to disproportionately suppress the latter. We present the results before and after the cuts, and calculate the significance in two different ways. Overall, our findings indicate that the probability of observing Z through supersymmetric decays at 14 TeV is not good, even at high total integrated luminosity L = 3 ab −1 . This occurs across all 2 + E T , 4 + E T and 6 + E T signals and for all benchmarks. This confirms past analyses for 2 + E T , which indicated that, unless Z is leptophobic, and thus much lighter, the signal significance is small. However, that is not necessarily so at 27 or 100 TeV and across all signals. At 27 TeV, benchmark BP3 gives a significance well above 5σ in 2 + E T signal. While for the 4 + E T signal, we obtain significances of 3-4σ for both BP2 and BP3, and much higher at 100 TeV. For the 6 + E T signal, only BP3, were the Z decay into right sleptons is important, gives any significant contributions. The significance at 27 TeV with total integrated luminosity L = 15 ab −1 is 3-4σ, and can reach 8-9σ at 100 TeV with total integrated luminosity L = 30 ab −1 .
Of course, analyses at 27 TeV are plagued by uncertainties, and those at 100 TeV can be interpreted as merely estimates. However, our analysis shows that HE/HL-LHC and FCC-hh can be promising grounds for observing consequences of both supersymmetry and extended gauge symmetry, of which an additional neutral gauge boson is one of the simplest examples.
The work of M.F. has been partly supported by NSERC through grant number SAP105354. M.F. is grateful to a TUBITAK/BIDEB 2221 grant which enabled this collaboration, even if the spread of COVID-19 prevented a visit to METU.