inclsqgl_summarypaper.bib \addbibresourceatlas-paper.bib \AtlasTitleSummary of the searches for squarks and gluinos using collisions with the ATLAS experiment at the LHC \AtlasRefCodeSUSY-2014-06 \PreprintIdNumberCERN-PH-EP-2015-162 \AtlasAbstractA summary is presented of ATLAS searches for gluinos and first- and second-generation squarks in final states containing jets and missing transverse momentum, with or without leptons or -jets, in the = 8 TeV data set collected at the Large Hadron Collider in 2012. This paper reports the results of new interpretations and statistical combinations of previously published analyses, as well as a new analysis. Since no significant excess of events over the Standard Model expectation is observed, the data are used to set limits in a variety of models. In all the considered simplified models that assume R-parity conservation, the limit on the gluino mass exceeds 1150 GeV at 95% confidence level, for an LSP mass smaller than 100 GeV. Furthermore, exclusion limits are set for left-handed squarks in a phenomenological MSSM model, a minimal Supergravity/Constrained MSSM model, R-parity-violation scenarios, a minimal gauge-mediated supersymmetry breaking model, a natural gauge mediation model, a non-universal Higgs mass model with gaugino mediation and a minimal model of universal extra dimensions. \pdfstringdefDisableCommands\pdfstringdefDisableCommands
Supersymmetry (SUSY) [Miyazawa:1966, Ramond:1971gb, Golfand:1971iw, Neveu:1971rx, Neveu:1971iv, Gervais:1971ji, Volkov:1973ix, Wess:1973kz, Wess:1974tw] is a generalization of space-time symmetries that predicts new bosonic partners for the fermions and new fermionic partners for the bosons of the Standard Model (SM). If R-parity is conserved [Fayet:1976et, Fayet:1977yc, Farrar:1978xj, Fayet:1979sa], SUSY particles (called sparticles) are produced in pairs and the lightest supersymmetric particle (LSP) is stable. The scalar partners of the left- and right-handed quarks, the squarks ( and which mix to form two mass eigenstates and , ordered by increasing mass), and the fermionic partners of the gluons, gluinos (), could be produced in strong interaction processes at the Large Hadron Collider (LHC) [LHC:2008] and decay via cascades ending with a stable LSP. The rest of the cascade would yield final states with multiple jets and possibly leptons arising from the decay of sleptons (), the superpartners of leptons, or , and Higgs () bosons originating from the decays of charginos () or neutralinos (), where the charginos and neutralinos are the mass eigenstates formed from the linear superpositions of the superpartners of the charged and neutral electroweak and Higgs bosons. In the Minimal Supersymmetric extension of the Standard Model (MSSM) [Fayet:1976et, Fayet:1977yc, Farrar:1978xj, Fayet:1979sa, Dimopoulos:1981zb], there are four charginos, and , and four neutralinos, ( to 4, ordered by increasing mass); unless stated otherwise, this is assumed in the following. In a large variety of models, the LSP is the lightest neutralino (), which interacts weakly and is a possible candidate for dark matter [Goldberg:1983nd]. Undetected LSPs would result in substantial missing transverse momentum (, with magnitude ). Significant can also arise in R-parity-violating (RPV) scenarios in which the LSP decays to final states containing neutrinos or in scenarios where neutrinos are present in the cascade decay chains of the produced SUSY particles. Significant mass splitting between the top squark (stop) mass eigenstates and is possible due to the large top Yukawa coupling.111The masses of the and are the eigenvalues of the stop mass matrix. The stop mass matrix involves the top quark Yukawa coupling in the off-diagonal elements, which typically induces a large mass splitting. Because of the SM weak isospin symmetry the mass of the left-handed bottom squark (sbottom, ) is tied to the mass of the left-handed stop (), and as a consequence the lightest sbottom () and stop () could be produced via the strong interaction with relatively large cross-sections at the LHC, either through direct pair production or in the decay of pair-produced gluinos.
The ATLAS experiment [Aad:2008zzm] performed several searches for supersymmetric particles in Run 1. No statistically significant excesses of events compared to the predictions of the Standard Model were observed. Therefore the results were expressed as model-independent limits on the production cross-sections of new particles and limits in the parameter space of supersymmetric or simplified models.
The large cross-sections of squark and gluino production in strong interaction processes offer sensitivity to a broad range of SUSY models. This paper provides a summary of the results from inclusive searches for gluinos and first- and second-generation squarks performed by ATLAS, using data from proton–proton () collisions at a centre-of-mass energy of 8 \TeV collected during Run 1 of the LHC. The results for direct production of third-generation squarks are reported elsewhere [3rdGen-summarypaper]. In addition to summarizing already published searches for squarks and gluinos, this paper presents new signal regions, new interpretations and statistical combinations of those searches, as well as an additional search using the Razor variable set [RazorVariables], thus improving the sensitivity to supersymmetric models. In order to differentiate strongly produced SUSY events from the SM background, the searches typically require high due to the presence of the LSP and possibly neutrinos, several high- jets and large deposited transverse energy. They are further classified according to the presence of leptons and -jets. A first class of searches applies a veto on leptons [0-leptonPaper, MonojetPaper, multijetsPaper], a second considers final states containing electrons and muons [1lepPaper, dilepton-edgePaper, SS3LPaper], and a third requires tau leptons in the final state [TauStrongPaper]. A fourth class of searches concentrates on final states containing multiple -jets [3bjetsPaper].
The paper is organized as follows. Section 2 summarizes the SUSY signals in the strong production of gluinos and light-flavour squarks. Section LABEL:sec:detector describes the ATLAS experiment and the data sample used, and section LABEL:sec:mcsamples the Monte Carlo (MC) simulation samples used for background and signal modelling. The physics object reconstruction and identification are presented in section LABEL:sec:objects. A description of the analysis strategy is given in section LABEL:sec:strategy, and the experimental signatures are presented in section LABEL:sec:signatures. A summary of systematic uncertainties is presented in section LABEL:sec:sysuncert. Results obtained using the new signal regions with selections similar to those used in previous publications as well as the new analysis using the Razor variable set are reported in section LABEL:sec:results. The strategy used for the combination of the results from different analyses is discussed in section LABEL:sec:combination. Limits in phenomenological and simplified models are presented in section LABEL:sec:limits. Section LABEL:sec:conclusion is devoted to a summary and conclusions.
2 SUSY models
Since no superpartners of any of the SM particles have been observed, SUSY, if realized in nature, must be a broken symmetry with a mechanism for breaking the symmetry taking place at a higher energy scale. It is difficult to construct a realistic model of spontaneously broken low-energy supersymmetry where the SUSY breaking arises solely as a consequence of the interactions of the particles of the MSSM [Fayet:1974jb, Fayet:1974pd, O'Raifeartaigh:1975pr]. Therefore, it is often assumed that the SUSY breaking originates in a “hidden” sector, and its effects are transmitted to the MSSM by some unknown mechanism. Various such mechanisms have been proposed, such as gravity-mediated SUSY breaking (SUGRA) [Chamseddine:1982jx, Barbieri:1982eh, Ibanez:1982ee, Hall:1983iz, Ohta:1982wn, Kane:1993td], gauge-mediated SUSY breaking (GMSB) [Dine:1981gu, AlvarezGaume:1981wy, Nappi:1982hm, Dine:1993yw, Dine:1994vc, Dine:1995ag] and anomaly-mediated SUSY breaking (AMSB) [Giudice:1998xp, Randall:1998uk]. As a result, these models consider only a small part of the parameter space of the more general MSSM. In such SUSY models, the particle spectrum is typically specified by fixing parameters at the high scale. In order to translate this set of parameters into physically meaningful quantities that describe physics near the electroweak scale, it is necessary to evolve them using their renormalization group equations.
Another approach to constraining SUSY at the electroweak scale is to use simplified models [Alwall:2008ag, Alves:2011wf] which are based on an effective Lagrangian that only describes a small set of kinematically accessible particles, interactions, production cross-sections and branching ratios. The simplest case corresponds to considering one specific SUSY production process with a fixed decay chain.
Several classes of phenomenological and simplified models, as well as a minimal Universal Extra Dimensions (mUED) scenario [Cheng:2000, Cheng:2002], covering different combinations of physics objects in the final state, are considered in this paper. Unless otherwise specified, R-parity is assumed to be conserved and the lightest neutralino, \ninoone, is taken to be the LSP. The phenomenological models include a scenario for the phenomenological MSSM (pMSSM) [Djouadi:1998di, Berger:2008cq, CahillRowley:2012cb], minimal Supergravity/Constrained MSSM (mSUGRA/CMSSM) [Chamseddine:1982jx, Barbieri:1982eh, Ibanez:1982ee, Hall:1983iz, Ohta:1982wn, Kane:1993td], bilinear R-parity violation (bRPV) [brpv], a minimal gauge-mediated supersymmetry breaking model (mGMSB) [Dine:1981gu, AlvarezGaume:1981wy, Nappi:1982hm, Dine:1993yw, Dine:1994vc, Dine:1995ag], natural gauge mediation (nGM) [ngm], and a non-universal Higgs mass model with gaugino mediation (NUHMG) [Covi:2007xj]. The simplified models presented here include the pair production of gluinos or first- and second-generation squarks with various hypotheses for their decay chains (direct, one-step or two-step decay), as well as gluino decays via real or virtual third-generation squarks. Direct decays are those where the considered SUSY particles decay directly into SM particles and the LSP, e.g., . One-step (two-step) decays refer to the cases where the decays occur via one (two) intermediate on-shell SUSY particle(s), e.g., . In gluino decays via third-generation squarks, gluinos undergo a one-step decay to a stop or sbottom such as