Search for supersymmetry in pp collisions at sqrt{s} = 7TeV in final states with missing transverse momentum and b-jets

TL;DR

The paper reports the first ATLAS search for supersymmetry in final states with missing transverse momentum and at least one $b$-jet using 7 TeV pp collisions corresponding to 35 pb$^{-1}$. Employing zero- and one-lepton channels, the analysis targets gluino-mediated and direct production of third-generation squarks, testing MSSM, MSUGRA/CMSSM with $\tan\beta=40$, and SO(10) models. No excess over SM expectations is observed, leading to 95% CL exclusions: $m_{\tilde{g}}<590$ GeV for sbottom scenarios and $m_{\tilde{g}}<520$ GeV for stop scenarios, plus significant constraints in MSUGRA/CMSSM and SO(10) parameter spaces. These results broaden the SUSY exclusion reach at the LHC and illustrate ATLAS's capability to probe light gluinos and third-generation squarks in early data.

Abstract

Results are presented of a search for supersymmetric particles in events with large missing transverse momentum and at least one heavy flavour jet candidate in sqrt{s} = 7 TeV proton-proton collisions. In a data sample corresponding to an integrated luminosity of 35 pb-1 recorded by the ATLAS experiment at the Large Hadron Collider, no significant excess is observed with respect to the prediction for Standard Model processes. For R-parity conserving models in which sbottoms (stops) are the only squarks to appear in the gluino decay cascade, gluino masses below 590 GeV (520 GeV) are excluded at the 95% C.L. The results are also interpreted in an MSUGRA/CMSSM supersymmetry breaking scenario with tan(beta)=40 and in an SO(10) model framework.

Figures (4)

• Figure 1: Distributions of the effective mass, $m_{\mathrm{eff}}\xspace$, (left) and the $E_{\mathrm{T}}^{\mathrm{miss}}\xspace$, (right) for data and for the expectations from Standard Model processes after the baseline selections in the zero-lepton(top) and one-lepton channel (bottom). The data correspond to an integrated luminosity of 35 pb$^{-1}$. Black vertical bars show the statistical uncertainty of the data. The yellow band shows the full systematic uncertainty on the SM expectation. The $E_{\mathrm{T}}^{\mathrm{miss}}\xspace$ distributions are shown after a cut on $m_{\mathrm{eff}}\xspace$ at 600 GeV (zero-lepton) and 500 GeV (one-lepton). For illustration, the distributions for one reference SUSY signal, relevant for each channel, are superimposed.
• Figure 2: Observed and expected 95% C.L. exclusion limits, as obtained with the zero-lepton channel, in the ($m^{}_{\tilde{g}},m^{}_{\tilde{b}^{}_{1}}$) plane. The neutralino mass is assumed to be 60 GeV and the NLO cross sections are calculated using PROSPINO in the hypothesis of $m^{}_{\tilde{q}^{}_{1,2}} \gg m^{}_{\tilde{g}}$. The result is compared to previous results from CDF searches which assume the same gluino-sbottom decays hypotheses, a neutralino mass of 60 GeV and $m^{}_{\tilde{q}^{}_{1,2}}=500$ GeV ($\gg m^{}_{\tilde{g}}$ for the Tevatron kinematic range). Exclusion limits from the CDF and D0 experiments on direct sbottom pair production sbottomCDFsbottomD0 are also reported.
• Figure 3: Observed and expected 95% C.L. upper limits, as obtained with the one-lepton analysis, on the gluino-mediated and stop pair production cross section as a function of the gluino mass for two assumed values of the stop mass and BR($\tilde{t}^{}_{1} \rightarrow b\tilde{\chi}^{\pm}_{1})=1$. The chargino is assumed to have twice the mass of the neutralino ($=60$ GeV) and NLO cross sections are calculated using PROSPINO in the hypothesis of $m^{}_{\tilde{q}^{}_{1,2}} \gg m^{}_{\tilde{g}}$. Theoretical uncertainties on the NLO cross sections are included in the limit calculation.
• Figure 4: Observed and expected 95% C.L. exclusion limits as obtained from the zero- and one-lepton analyses, separately and combined, on MSUGRA/CMSSM scenario with tan$\beta$ = 40, $A_0$ = 0, $\mu>0$. The light-grey dashed lines are the iso-mass curves for gluinos and sbottom -- stop masses are 15% lower than sbottom masses, across the ($m_0,m^{}_{1/2}$) parameter space. The results are compared to previous limits from the LEP experiments lep2.