Abstract
A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the Z boson mass, jets tagged as originating from b-quarks and missing transverse momentum. The analysis is performed with proton–proton collision data at sqrt{s}=8{hbox { TeV}} collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb^{-1}. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state (tilde{t}_2) followed by the decay to the lighter top squark state (tilde{t}_1) via tilde{t}_2 rightarrow Ztilde{t}_1, and for tilde{t}_1 pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino (tilde{chi }^0_1) is the next-to-lightest supersymmetric particle and decays producing a Z boson and a gravitino (tilde{G}) via the tilde{chi }^0_1 rightarrow Ztilde{G} process.
Highlights
Supersymmetry (SUSY) [1,2,3,4,5,6,7,8,9] is an extension of the Standard Model (SM) which predicts new bosonic partners for the existing fermions and fermionic partners for the known bosons
This paper presents the first result on t2 direct pair production and extends the results of a previous ATLAS analysis, carried out using 7 TeV data corresponding to an integrated luminosity of 2.05 fb−1 [34], that excluded stop masses up to 310 GeV for 115 GeV < mχ10 < 230 GeV in natural gauge-mediated SUSY breaking (GMSB) scenarios
Samples generated with Alpgen 2.14 [52] interfaced with Herwig 6.510 [53], including Jimmy 4.3 [54] for the underlying event description, are used to evaluate generator systematic uncertainties, while Powheg Box r2129 interfaced to Herwig 6.510 and AcerMC 3.8 [55] interfaced to Pythia 6.426 are used for hadronisation and initial/final state radiation (ISR/FSR) uncertainty estimation respectively
Summary
Supersymmetry (SUSY) [1,2,3,4,5,6,7,8,9] is an extension of the Standard Model (SM) which predicts new bosonic partners for the existing fermions and fermionic partners for the known bosons. Sensitivity to direct t2 pair production can be obtained with this three-lepton signature even in models where additional decay modes of the t2, such as t2 → tχ or via the lightest Higgs boson (h) in t2 → ht, are significant. In this paper a search for stop pair production is reported in final states characterised by the presence of a Z boson with or without additional leptons, plus jets originating from b-quarks (b-jets) produced in the stop decay chain and significant missing transverse momentum from the undetected. This paper presents the first result on t2 direct pair production and extends the results of a previous ATLAS analysis, carried out using 7 TeV data corresponding to an integrated luminosity of 2.05 fb−1 [34], that excluded stop masses up to 310 GeV for 115 GeV < mχ10 < 230 GeV in natural GMSB scenarios
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