Lightest Neutralino Research Articles

Search for Heavy Stops with Merged Top Jets

We study an interesting region of phase space at the LHC for pair-produced stops decaying into hadronic top quarks and light neutralinos. After imposing a sizeable cut on the missing transverse energy, which is the key variable for reducing backgrounds, we have found that the two hadronic tops are likely to merge into a single fat jet. We develop a jet-substructure-based strategy to tag the two merged top-jets and utilize the MT2 variable to further reduce the backgrounds. We obtain about a 50% increase to the ratio of the signal over background and a mild increase on the signal discovery significance, based on a signal with a 1.2 TeV stop and a 100 GeV neutralino, for the 13 TeV LHC with 100 fb$^{-1}$. The general event kinematics could also occur and be explored for other new physics signatures with large missing transverse energy.

Search for supersymmetry in events with b-tagged jets and missing transverse momentum in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

A search for the supersymmetric partners of the Standard Model bottom and top quarks is presented. The search uses 36.1 fb−1 of pp collision data at sqrt{s}=13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Direct production of pairs of bottom and top squarks ( {overline{b}}_1 and {overline{t}}_1 ) is searched for in final states with b-tagged jets and missing transverse momentum. Distinctive selections are defined with either no charged leptons (electrons or muons) in the final state, or one charged lepton. The zero-lepton selection targets models in which the {overline{b}}_1 is the lightest squark and decays via {overline{b}}_1to b{overline{chi}}_1^0 , where {overline{chi}}_1^0 is the lightest neutralino. The one-lepton final state targets models where bottom or top squarks are produced and can decay into multiple channels, {overline{b}}_1to b{overline{chi}}_1^0 and {overline{b}}_1to t{overline{chi}}_1^{pm } , or {overline{t}}_1to t{overline{chi}}_1^0 and {overline{t}}_1to b{overline{chi}}_1^{pm } , where {overline{chi}}_1^{pm } is the lightest chargino and the mass difference {m}_{{overline{chi}}_1^{pm }}-{m}_{{overline{chi}}_1^0} is set to 1 GeV. No excess above the expected Standard Model background is observed. Exclusion limits at 95% confidence level on the mass of third-generation squarks are derived in various supersymmetry-inspired simplified models.

Probing squeezed bino-slepton spectra with the Large Hadron Collider

We consider a Minimal Supersymmetric Standard Model scenario in which the only light superparticles are a bino-like dark matter candidate and a nearly-degenerate slepton. It is notoriously difficult to probe this scenario at the Large Hadron Collider, because the slepton pair-production process yields a final state with soft leptons and small missing transverse energy. We study this scenario in the region of parameter space where the mass difference between the lightest neutralino and the lightest slepton ($\Delta m$) is $\lesssim 60~{\rm GeV}$, focusing on the process in which an additional radiated jet provides a transverse boost to the slepton pair. We then utilize the angular separation of the leptons from each other and from the missing transverse energy, as well as the angular separation between the jet and the missing transverse energy, to distinguish signal from background events. We also use the reconstructed ditau mass, the $\cos \theta^*_{\ell^+ \ell^-}$ variable, and for larger $\Delta m$, a lower bound on the lepton $p_T$. These cuts can dramatically improve both signal sensitivity and the signal-to-background ratio, permitting discovery at the Large Hadron Collider with reasonable integrated luminosity over the interesting region of parameter space. Using our search strategy the LHC will be able to exclude $m_{\tilde{\mu}} \approx 200$ GeV for $\Delta m \lesssim 60$ GeV at $1.5-3 \sigma$ with 1000 fb$^{-1}$ of integrated luminosity. Although we focus on a particular model, the results generalize to a variety of scenarios in which the dark matter and a leptonic partner are nearly degenerate in mass, and especially to scenarios featuring a scalar mediator.

Sneutrino DM in the NMSSM with inverse seesaw mechanism

In supersymmetric theories like the Next-to-Minimal Supersymmetric Standard Model (NMSSM), the lightest neutralino with bino or singlino as its dominant component is customarily taken as dark matter (DM) candidate. Since light Higgsinos favored by naturalness can strength the couplings of the DM and thus enhance the DM-nucleon scattering rate, the tension between naturalness and DM direct detection results becomes more and more acute with the improved experimental sensitivity. In this work, we extend the NMSSM by inverse seesaw mechanism to generate neutrino mass, and show that in certain parameter space the lightest sneutrino may act as a viable DM candidate, i.e. it can annihilate by multi-channels to get correct relic density and meanwhile satisfy all experimental constraints. The most striking feature of the extension is that the DM-nucleon scattering rate can be naturally below its current experimental bounds regardless of the higgsino mass, and hence it alleviates the tension between naturalness and DM experiments. Other interesting features include that the Higgs phenomenology becomes much richer than that of the original NMSSM due to the relaxed constraints from DM physics and also due to the presence of extra neutrinos, and that the signatures of sparticles at colliders are quite different from those with neutralino as DM candidate.

Constraining sleptons at the LHC in a supersymmetric low-scale seesaw scenario

We consider a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. We extend the Minimal Supersymmetric Standard Model by adding light right-handed neutrinos and their superpartners, the R-sneutrinos, and consider the lightest neutralinos to be higgsino-like. We consider the possibilities of having either an R-sneutrino or a higgsino as lightest supersymmetric particle. Assuming that squarks and gauginos are heavy, we systematically evaluate the bounds on slepton masses due to existing LHC data.

Search for supersymmetry in final states with two same-sign or three leptons and jets using 36 fb\u22121 of sqrt{s}=13 TeV pp collision data with the ATLAS detector

A search for strongly produced supersymmetric particles using signatures involving multiple energetic jets and either two isolated same-sign leptons (e or μ), or at least three isolated leptons, is presented. The analysis relies on the identification of b-jets and high missing transverse momentum to achieve good sensitivity. A data sample of proton-proton collisions at sqrt{s}=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fb−1, is used for the search. No significant excess over the Standard Model prediction is observed. The results are interpreted in several simplified supersymmetric models featuring R-parity conservation or R-parity violation, extending the exclusion limits from previous searches. In models considering gluino pair production, gluino masses are excluded up to 1.87 TeV at 95% confidence level. When bottom squarks are pair-produced and decay to a chargino and a top quark, models with bottom squark masses below 700 GeV and light neutralinos are excluded at 95% confidence level. In addition, model-independent limits are set on a possible contribution of new phenomena to the signal region yields.

DifferentiatingU(1)′supersymmetric models with right sneutrino and neutralino dark matter

We perform a detailed analysis of dark matter signals of supersymmetric models containing an extra $U(1)^\prime$ gauge group. We investigate scenarios in which either the right sneutrino or the lightest neutralino are phenomenologically acceptable dark matter candidates and we explore the parameter spaces of different supersymmetric realisations featuring an extra $U(1)^\prime$. We impose consistency with low energy observables, with known mass limits for the superpartners and $Z^\prime$ bosons, as well as with Higgs boson signal strengths, and we moreover verify that predictions for the anomalous magnetic moment of the muon agree with the experimental value and require that the dark matter candidate satisfies the observed relic density and direct and indirect dark matter detection constraints. For the case where the sneutrino is the dark matter candidate, we find distinguishing characteristics among different $U(1)^\prime$ mixing angles. If the neutralino is the lightest supersymmetric particle, its mass is heavier than that of the light sneutrino in scenarios where the latter is a dark matter candidate, the parameter space is less restricted and differentiation between models is more difficult. We finally comment on the possible collider tests of these models.

NMSSM Higgs boson search strategies at the LHC and the mono-Higgs signature in particular

We study the collider phenomenology of the extended Higgs sector of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The region of NMSSM parameter space favored by a 125\,GeV SM-like Higgs and naturalness generically features a light Higgs and neutralino spectrum as well as a large $\mathcal{O}(1)$ coupling between the Higgs doublets and the NMSSM singlet fields. In such regimes, the heavier Higgs bosons can decay dominantly into lighter Higgs bosons and neutralinos. We study the prospects of observing such decays at the 13\,TeV LHC, focusing on mono-Higgs signatures as probes of such regions of parameter space. We present results in a model-independent framework as well as in the NMSSM. In the NMSSM, we find that the mono-Higgs channel can probe TeV scale Higgs bosons and has sensitivity even in the low tan$\beta$, large $m_A$ regime that is difficult to probe in the MSSM. Unlike for many conventional Higgs searches, the reach of the mono-Higgs channel will improve significantly with the increased luminosity expected to be collected at the LHC in the ongoing and upcoming runs.

Diphoton signal of the light Higgs boson in natural NMSSM

Natural Next-to-Minimal Supersymmetric Standard Model (nNMSSM) is featured by predicting one CP-even Higgs boson satisfying $m_{h_1} \lesssim 120 \,{\rm GeV}$ and Higgsinos lighter than about 300 GeV, and consequently the cross section for DM-nucleon scattering in this scenario is usually quite large. We study the diphoton signal of the light Higgs boson in nNMSSM by considering the tight constraints from the latest LUX and PandaX-II experiments, and we conclude that the optimal value of the signal rate at 8 TeV LHC is greatly reduced in comparison with earlier predictions. For example, previous studies indicated that the rate may exceed $120 \,{\rm fb}$ for $m_{h_1} \simeq 80 \,{\rm GeV}$, while it is at most $25 \,{\rm fb}$ if the lightest neutralino in the scenario is fully responsible for the measured DM relic density. We also investigate the case of $m_{h_1} \simeq 98 \,{\rm GeV}$ which is hinted by the excesses of the LEP analysis on $Z \bar{b} b$ signal and the CMS analysis on the diphoton signal. We conclude that nNMSSM can explain simultaneously the excesses at $1\sigma$ level without violating any known constraints.

Invisible decay of the Higgs boson in the context of a thermal and nonthermal relic in MSSM

We study the decay of 125 GeV Higgs boson to light LSP neutralino in the phenomenological minimal supersymmetric standard model in the context of collider searches and astrophysical experiments. We consider the parameter space for light neutralinos that can be probed via the invisible Higgs decays and higgsino searches at the ILC. We consider the cases where the light neutralino is compatible with the observed relic density or where the thermal relic is over-abundant, pointing to non-standard cosmology. In the former case, when the neutralino properties give rise to under-abundant relic density, the correct amount of relic abundance is assumed to be guaranteed by either additional DM particles or by non-thermal cosmology. We contrast these different cases. We assess what astrophysical measurements can be made, in addition to the measurements made at the ILC, which can provide a clue to the nature of the light neutralino. We find that a number of experiments, including Xenon-nT, PICO-250, LZ in conjunction with measurements made at the ILC on invisible Higgs width can pin down the nature of this neutralino, along with its cosmological implications. Additionally, we also point out potential LHC signatures that could be complementary in this region of parameter space.

Low-mass neutralino dark matter in supergravity scenarios: phenomenology and naturalness

The latest experimental results from the LHC and dark matter (DM) searches suggest that the parameter space allowed in supersymmetric theories is subject to strong reductions. These bounds are especially constraining for scenarios entailing light DM particles. Previous studies have shown that light neutralino DM in the Minimal Supersymmetric Standard Model (MSSM), with parameters defined at the electroweak scale, is still viable when the low energy spectrum of the model features light sleptons, in which case, the relic density constraint can be fulfilled. In view of this, we have investigated the viability of light neutralinos as DM candidates in the MSSM, with parameters defined at the grand unification scale. We have analysed the optimal choices of non-universalities in the soft supersymmetry-breaking parameters for both, gauginos and scalars, in order to avoid the stringent experimental constraints. We show that light neutralinos, with a mass as low as 25 GeV, are viable in supergravity scenarios if the gaugino mass parameters at high energy are very non universal, while the scalar masses can remain of the same order. These scenarios typically predict a very small cross section of neutralinos off protons and neutrons, thereby being very challenging for direct detection experiments. However, a potential detection of smuons and selectrons at the LHC, together with a hypothetical discovery of a gamma-ray signal from neutralino annihilations in dwarf spheroidal galaxies could shed light on this kind of solutions. Finally, we have investigated the naturalness of these scenarios, taking into account all the potential sources of tuning. Besides the electroweak fine-tuning, we have found that the tuning to reproduce the correct DM relic abundance and that to match the measured Higgs mass can also be important when estimating the total degree of naturalness.

Searches for pair production of third-generation squarks in sqrt{s}=13,text {TeV} pp collisions

Searches are presented for direct production of top or bottom squark pairs in proton–proton collisions at the CERN LHC. Two searches, based on complementary techniques, are performed in all-jet final states that are characterized by a significant imbalance in transverse momentum. An additional search requires the presence of a charged lepton isolated from other activity in the event. The data were collected in 2015 at a centre-of-mass energy of 13,text {TeV} with the CMS detector and correspond to an integrated luminosity of 2.3,text {fb}^{-1}. No statistically significant excess of events is found beyond the expected contribution from standard model processes. Exclusion limits are set in the context of simplified models of top or bottom squark pair production. Models with top and bottom squark masses up to 830 and 890,text {GeV}, respectively, are probed for light neutralinos. For models with top squark masses of 675,text {GeV}, neutralino masses up to 260,text {GeV} are excluded at 95% confidence level.

Status of CMSSM in light of current LHC Run-2 and LUX data

Motivated by the latest results of the LHC Run-2 and LUX experiments, we examine the status of the constrained minimal supersymmetric standard model by performing a global fit. We construct a likelihood function including the electroweak precision observables, B-physics measurements, LHC Run-1 and -2 data of SUSY direct searches, Planck observation of the dark matter relic density and the combined LUX Run-3 and -4 detection limits. Based on the profile likelihood functions of 1 billion samples, we obtain the following observations: (i) The stau coannihilation region has been mostly excluded by the latest LHC Run-2 data; (ii) The focus point region has been largely covered by the LUX-2016 limits while the A-funnel region has been severely restricted by flavor observables like Bs→μ+μ−. The remaining parts of both regions will be totally covered by the future LZ dark matter experiment; (iii) Part of the stop coannihilation region may be detected with higher integrated luminosity LHC; (iv) The StauC&AF hybrid region still survives considering both LHC and dark matter experiments; (v) The masses of the stop, the lightest neutralino and the gluino have been pushed up to 363 GeV, 328 GeV and 1818 GeV, respectively.

Exploring the nearly degenerate stop region with sbottom decays

A light stop with mass almost degenerate with the lightest neutralino has important connections with both naturalness and dark matter relic abundance. This region is also very hard to probe at colliders. In this paper, we demonstrate the potential of searching for such stop particles at the LHC from sbottom decays, focusing on two channels with final states mathbf{2}mathbf{ell }+{boldsymbol{mathsf{E}}}_{mathbf{mathsf{T}}}^{mathbf{mathsf{miss}}} and mathbf{1}boldsymbol{mathsf{b}}mathbf{mathsf{1}}mathbf{ell}+{boldsymbol{E}}_{mathbf{T}}^{mathbf{miss}} . We found that, if the lightest sbottom has mass around or below 1 TeV and has a significant branching ratio to decay to stop and Wleft(tilde{b}to tilde{t}Wright) , a stop almost degenerate with neutralino can be excluded up to about 500-600 GeV at the 13 TeV LHC with 300 fb−1 data. The searches we propose are complementary to other SUSY searches at the LHC and could have the best sensitivity to the stop-bino coannihilation region. Since they involve final states which have already been used in LHC searches, a reinterpretation of the search results already has sensitivity. Further optimization could deliver the full potential of these channels.

Dark Matter in B–L supersymmetric Standard Model with inverse seesaw

We show that the B–L Supersymmetric Standard Model with Inverse Seesaw (BLSSMIS) provides new Dark Matter (DM) candidates (lightest right-handed sneutrino and lightest B–L neutralino) with mass of order few hundreds GeV, while most of other SUSY spectrum can be quite heavy, consistently with the current Large Hadron Collider (LHC) constraints. We emphasize that the thermal relic abundance and direct detection experiments via relic neutralino scattering with nuclei impose stringent constraints on the B–L neutralinos. These constraints can be satisfied by few points in the parameter space where the B–L lightest neutralino is higgsino-like, which cannot explain the observed Galactic Center (GC) gamma-ray excess measured by Fermi-LAT. The lightest right-handed sneutrino DM is analysed. We show that for a wide region of parameter space the lightest right-handed sneutrino, with mass between 80 GeV and 1.2 TeV, can satisfy the limits of the relic abundance and the scattering cross section with nuclei. We also show that the lightest right-handed sneutrino with mass \U0001d4aa(100) GeV can account for the observed GC gamma-ray results.

Cornering compressed gluino at the LHC

We discuss collider search strategies of gluinos which are highly degenerate with the lightest neutralino in mass. This scenario is fairly difficult to probe with conventional search strategies at colliders, and thus may provide a hideaway of supersymmetry. Moreover, such a high degeneracy plays an important role in dark matter physics as the relic abundance of the lightest neutralino is significantly reduced via coannihilation. In this paper, we discuss ways of uncovering this scenario with the help of longevity of gluinos; if the mass difference between the lightest neutralino and gluino is ≲ 100 GeV and squarks are heavier than gluino, then the decay length of the gluino tends to be of the order of the detector-size scale. Such gluinos can be explored in the searches of displaced vertices, disappearing tracks, and anomalously large energy deposit by (meta)stable massive charged particles. We find that these searches are complementary to each other, and by combining their results we may probe a wide range of the compressed gluino region in the LHC experiments.

Leptonic mono-top from single stop production at the LHC

Top squark (stop) can be produced via QCD interaction but also the electroweak interaction at the LHC. In this paper, we investigate the observability of the associated production of stop and chargino, ppto {tilde{t}}_1{tilde{chi}}_{overline{1}} , in compressed electroweakino scenarios at 14 TeV LHC. Due to small mass splitting between the lightest neutralino ( {tilde{chi}}_1^0 ) and chargino ( {tilde{chi}}_1^{-} ), the single stop production can give the mono-top signature through the stop decay {tilde{t}}_1to t{tilde{chi}}_1^0 . We analyze the leptonic mono-top channel of the single stop production and propose a lab-frame observable cos θbℓ to reduce the SM backgrounds. We find that such leptonic mono-top events from the single stop production can be probed at 2σ level at the HL-LHC if {m}_{{tilde{t}}_1}<760 GeV and {m}_{{tilde{chi}}_1^0}<150 GeV. Given a discovery of the stop and a measurement of the single stop production cross section, the stop mixing angle can also be determined from the single stop production at the HL-LHC.

Search for top squark pair production in compressed-mass-spectrum scenarios in proton–proton collisions at s=8 TeV using the αT variable

An inclusive search is performed for supersymmetry in final states containing jets and an apparent imbalance in transverse momentum, pt[miss], due to the production of unobserved weakly interacting particles in pp collisions at a centre-of-mass energy of 8 TeV. The data, recorded with the CMS detector at the CERN LHC, correspond to an integrated luminosity of 18.5 inverse femtobarns. The dimensionless kinematic variable alpha[T] is used to discriminate between events with genuine pt[miss] associated with unobserved particles and spurious values of pt[miss] arising from jet energy mismeasurements. No excess of event yields above the expected standard model backgrounds is observed. The results are interpreted in terms of constraints on the parameter space of several simplified models of supersymmetry that assume the pair production of top squarks. The search provides sensitivity to a broad range of top squark decay modes, including the two-body decay top squark to c chi0, where c is a charm quark and chi0 is the lightest neutralino, as well as the four-body decay top squark to b f f' chi0, where b is a bottom quark and f and f' are fermions produced in the decay of an intermediate off-shell W boson. These modes dominate in scenarios in which the top squark and lightest neutralino are nearly degenerate in mass. For these modes, top squarks with masses as large as 260 and 230 GeV are excluded, respectively, for the two- and four-body decays.

Quark-gluon discrimination in the search for gluino pair production at the LHC

We study the impact of including quark- and gluon-initiated jet discrimination in the search for strongly interacting supersymmetric particles at the LHC. Taking the example of gluino pair production, considerable improvement is observed in the LHC search reach on including the jet substructure observables to the standard kinematic variables within a multivariate analysis. In particular, quark and gluon jet separation has higher impact in the region of intermediate mass-gap between the gluino and the lightest neutralino, as the difference between the signal and the standard model background kinematic distributions is reduced in this region. We also compare the predictions from different Monte Carlo event generators to estimate the uncertainty originating from the modelling of the parton shower and hadronization processes.

Alignment without decoupling: The portal to light dark matter in the MSSM

We study light, thermal neutralino dark matter in the sub-GeV to $\ensuremath{\sim}65\text{ }\text{ }\mathrm{GeV}$ mass range in the minimal supersymmetric extension of the Standard Model. We consider realizations of the limit of alignment without decoupling in the Higgs sector where the heavier $CP$-even Higgs impersonates the observed Higgs state at 125 GeV, while the lighter $CP$-even Higgs is the mediator of dark matter annihilation. We single out three distinct and novel possibilities for light dark matter: (i) a neutralino with mass around half the light Higgs mass, in the sub-GeV to $\ensuremath{\sim}30\text{ }\text{ }\mathrm{GeV}$ mass range; (ii) a neutralino with a mass around half the pseudoscalar Higgs boson mass, in our examples around $(60--65)\text{ }\text{ }\mathrm{GeV}$; (iii) a very light neutralino with mass around the light Higgs mass, pair annihilating to Higgs pairs. We discuss the implications of all these possibilities for indirect and direct dark matter detection experiments, and we demonstrate that all scenarios will be tested by next generation direct detection experiments. We also emphasize that the unique Higgs phenomenology of this setup warrants a dedicated search program at the LHC.