Compressed electroweakino spectra at the LHC

Pedro Schwaller 1 2 3 Jose Zurita 0 0 Institut fur Physik (THEP) Johannes Gutenberg-Universitat , D-55099, Mainz, Germany 1 HEP Division, Argonne National Laboratory , 9700 Cass Ave., Argonne IL 60439, U.S.A 2 Physics Department, University of Illinois at Chicago , Chicago, IL 60637, U.S.A 3 CERN, Theory Division , CH1211 Geneva 23, Switzerland In this work, we examine the sensitivity of monojet searches at the LHC to directly produced charginos and neutralinos (electroweakinos) in the limit of small mass splitting, where the traditional multilepton plus missing energy searches loose their sensitivity. We first recast the existing 8 TeV monojet search at CMS in terms of a SUSY simplified model with only light gauginos (winos and binos) or only light Higgsinos. The current searches are not sensitive to MSSM-like production cross sections, but would be sensitive to models with 2-20 times enhanced production cross section, for particle masses between 100 GeV and 250 GeV. Then we explore the sensitivity in the 14 TeV run of the LHC. Here we emphasise that in addition to the pure monojet search, soft leptons present in the samples can be used to increase the sensitivity. Exclusion of electroweakino masses up to 200 GeV is possible with 300 fb1 at the LHC, if the systematic error can be reduced to the 1% level. Discovery is possible with 3000 fb1 in some regions of parameter space. Contents 1 Introduction 2 Phenomenology and limits on electroweakinos 3 Search strategy and parameter space 4 CMS monojet analysis and Monte Carlo validation 5 Recast of 8 TeV data 6 14 TeV projection 7 Conclusions 1 Introduction The discovery of a particle with a mass of about 125 GeV [1, 2], consistent with the Standard Model (SM) Higgs boson, dawns a new era in particle physics. After such an achievement, now the attention is turned to understand if this resonance is indeed the SM Higgs boson, or if there is room for Beyond the Standard Model (BSM) phenomena. Several open issues require new physics to be introduced at some scale. Among those, the hierarchy problem and dark matter suggest that new physics as light as the weak scale could be present. A weakly interacting, neutral and stable particle with mass of order of the weak scale is among the leading candidates for dark matter, while most solutions to the hierarchy problem require weakly interacting partners for the electroweak gauge bosons. This strongly motivates a search for weakly-interacting new physics, which, in contrast to strongly coupled new physics, can still be present at or very close to the weak scale. In the minimal supersymmetric standard model (MSSM), the superpartners of the weak gauge and Higgs bosons play both an important role in the solution of the hierarchy problem and in providing a candidate for dark matter. We will therefore adopt the MSSM as our BSM benchmark model. A typical MSSM signature is given by jets plus a large amount of transverse missing energy (ETmiss), originating from gluino and/or squark production, and the subsequent chain decays. Generically these decay chains end with the lightest supersymmetric particle (usually the neutralino) which constitutes a very good dark matter candidate. Current constraints MSSM superpartners can roughly be summarised as follows: gluinos and squarks of the first and second generation have to be heavier than about a TeV [311], stops heavier than 500-700 GeV [1216] and electroweakinos (charginos and neutralinos) heavier than 200-300 GeV [1720]. However, these constraints do often rely on certain simplified assumptions that can be relaxed. For instance, if the mass splittings between sparticles is small, then the amount of missing energy as well as the transverse momentum of the associated jets is reduced, and many of these searches become severely less sensitive to the new physics. In this article we investigate the possibility to use monojet plus Emiss searches to T look for chargino pair production decaying into neutralinos, leptons and neutrinos (and possibly more jets), in the case where there is a small mass gap between the chargino and the neutralino. In the particular context of the MSSM such a spectrum can be obtained by taking, for instance, the gaugino soft masses M1, M2 of the order of 100 GeV, while 0 1 1 TeV. The spectrum then contains two light neutralinos 1,2 and one chargino which are gaugino-like and whose mass splitting is controlled by M2 M1, and heavier states at the scale which are higgsino-like. In the context of Natural SUSY [2125] the opposite limit is achieved, namely O(100) GeV and M1, M2 & 1 TeV. Qualitatively the same spectrum is obtained, but with the light (heavy) states being Higgsino (gaugino)-like. Such an scenario is also motivated from the fact that in the MSSM the minimization of the Higgs potential requires of the order of the electroweak scale, to avoid a large fine-tuning. When the mass splittings are small, visible decay products from the decays of the heavier 02 and 1 states become too soft and do not pass the trigger requirements employed in most BSM searches. Therefore we will require an additional hard jet from ISR radiation to boost the missing energy, such that at least in principle, the signal can be recorded. This is the basic idea behind the monojet search for dark matter. We will analyse both the sensitivity of the existing 8 TeV monojet searches and present projections for the sensitivity of the future 14 TeV high and very high luminosity runs of the LHC. Furthermore we will attempt to improve the sensitivity of the monojet searches to MSSM-like scenarios by using soft leptons that are likely to be present in the samples. This paper is organized as follows. In section 2, we review the phenomenology of the electroweakino sector in the MSSM, discuss the current exclusions from collider experiments and bounds from dark matter. In section 3 we elaborate on our strategy and discuss the parameter space under study. In section 4 we explain our setup and validate our Monte Carlo simulation against the results of the CMS analysis. In section 5 we recast the current experimental results for the 8 TeV LHC, while in section 6 we present the reach of the 14 TeV LHC, for two benchmark luminosities of 300 and 3000 fb1. We conclude in section 7. Phenomenology and limits on electroweakinos Charginos and neutralinos are the superpartners of the weakly interacting bosonic fields in the SM. The partners of the electroweak gauge bosons, the winos and binos, and the partners of the two MSSM Higgs doublets, the Higgsinos, mix under the influence of electroweak symmetry breaking. The chargino mass matrix is given by [26] where M2 and are the supersymmetry breaking wino and Higgsino masses, respectively, MW is the W -boson mass and tan = vu/vd is the ratio of the vacuum expectation values of the two Higgs doublets. The neutral states mix according to the neutralino mass matrix, given by where sW and cW are the sin (...truncated)