Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Abstract

Abstract:The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy √ s = 8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb −1 . No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c ) of approximately 30. Keywords: Hadron-Hadron Scattering JHEP04(2015)116The ATLAS collaboration 58 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] postulates the existence of particles (sparticles) which differ by half a unit of spin from their Standard Model (SM) partners. The squarks (q L andq R ) and sleptons (˜ L and˜ R ) are the scalar partners of the left-handed and right-handed quarks and leptons, the gluinos (g) are the fermionic partners of the gluons, and the charginos (χ ± i with i = 1, 2) and neutralinos (χ 0 i with i = 1, 2, 3, 4) are the mass eigenstates (ordered from the lightest to the heaviest) formed from the linear superpositions of the SUSY partners of the Higgs and electroweak gauge bosons. An attractive feature of SUSY is that it can solve the SM hierarchy problem [10][11][12][13][14][15] if the gluino, higgsino and top squark masses are not much higher than the TeV scale.If strongly interacting sparticles exist at the TeV scale, they should be accessible at the Large Hadron Collider (LHC). In the minimal supersymmetric extension of the SM such particles decay into jets, possibly leptons, and the lightest sparticle (LSP). If the LSP is stable owing to R-parity conservation [15][16][17][18][19] and only weakly interacting, it escapes detection, leading to missing transverse momentum (p miss T and its magnitude E miss T ) in the final state. In this scenario, the LSP can be a dark-matter candidate. Significant E miss T can also arise in R-parity-violating 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 sparticles.This paper presents a search with the ATLAS detector [20,21] for SUSY in final states containing jets, at least one isolated lepton (electron or muon) and large E miss T . Different search channels are used in order to cover a broad parameter space: the events are selected by different requirements on the transverse momentum (p T ) of the leptons, either using low-p T leptons (referred to as the "soft" lepton selection), or high-p T leptons (referred to as the "hard" lepton selection). Each of these categories is further subdivided i...

METing SUSY on the Z peak

“…In this point, the cross section increases by a factor ∼15 in going from 8 to 13 TeV center of mass energy. In any case, for both points this would have unambiguous signatures, both on the Z -peak with a scaling of the signal found at LHC8 and in direct searches for gluinos using jets plus missing E T in the extension of the analysis in [13][14][15].…”

“…Naively, the first option to consider in a hadron collider would be strong production of squarks or gluinos (assuming they produce Z bosons in their decays). However, current experimental searches of jets plus missing energy at LHC force the masses of these colored particles to be high [13][14][15]. Nevertheless, as we will see below, in some cases we can still find cross sections of the required size.…”

“…2. However, we see that we would need both squark and gluino to be light, which is not possible if we take into account the bounds from LHC searches [13][14][15]. In summary, the best option seems to be gluino pair production with mg 1200 GeV with relatively heavy squarks mq 3000 GeV, if we can get at least one Z -boson in every gluino decay.…”

“…We have to emphasize here that it is not difficult to obtain the observed excess for light gluino masses, and a gluino mass in between the two presented examples, mg ∈ [900, 1100] GeV, could reproduce the observed signal. However, these points may be in conflict with direct searches of jets plus E miss T [13][14][15]. There is a tension between this excess and the bounds from gluino searches in jets plus E miss T .…”

“…In the case of gluino production, the needed cross section is obtained only for mg 1200 GeV and favors heavy squark masses. In fact, these gluino masses are in the boundary of the allowed region obtained from jets plus missing E T searches at LHC [13][14][15] and would contribute significantly only if everyg produces at least a Z -boson in its decay. For the production of squark pairs, present limits are mq 1400 GeV for heavy gluinos and mq 1650 GeV for degenerate squarks and gluinos [13][14][15].…”