Search for direct top-squark pair production in final states with two leptons in pp collisions at $ \sqrt{s} $ = 8 TeV with the ATLAS detector

Abstract

A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3 fb −1 of pp collision data at √ s = 8 TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino. A Generator-level object and event selection 44 Keywords: Hadron-Hadron ScatteringThe ATLAS collaboration 50 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] is an extension to the Standard Model (SM) which introduces supersymmetric partners of the known fermions and bosons. For each known boson or fermion, SUSY introduces a particle with identical quantum numbers except for a difference of half a unit of spin (S). The introduction of gauge-invariant and renormalisable interactions into SUSY models can violate the conservation of baryon number (B) and lepton number (L), resulting in a proton lifetime shorter than current experimental limits [10]. This is usually solved by assuming that the multiplicative quantum number R-parity (R),-1 - JHEP06(2014)124defined as R = (−1) 3(B−L)+2S , is conserved. In the framework of a generic R-parityconserving minimal supersymmetric extension of the SM (MSSM) [11][12][13][14][15], SUSY particles are produced in pairs where the lightest supersymmetric particle (LSP) is stable, and is a candidate for dark matter. In a large variety of models, the LSP is the lightest neutralino (χ 0 1 ). The scalar partners of right-handed and left-handed quarks (squarks),q R andq L , mix to form two mass eigenstates,q 1 andq 2 , withq 1 defined to be the lighter one. In the case of the supersymmetric partner of the top quark (top squark,t), large mixing effects can lead to one top-squark mass eigenstate,t 1 , that is significantly lighter than the other squarks. Consideration of naturalness and its impact on the SUSY particle spectrum, suggests that top squarks cannot be too heavy, to keep the Higgs boson mass close to the electroweak scale [16,17]. Thust 1 could be pair-produced with relatively large cross-sections at the Large Hadron Collider (LHC).The top squark can decay into a variety of final states, depending, amongst other factors, on the hierarchy of the mass eigenstates formed from the linear superposition of the SUSY partners of the Higgs boson and electroweak gauge bosons. I...

Third generation SUSY searches in CMS

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“…Both plots are similar, however the ATLAS exclusion limits are more stringent in the low stop mass region of interest. Both are reproduced in Figure 5.2 [155][156][157][158][159][160][161][162][163].…”

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