Measurement of the top quark pair production cross section with the ATLAS detector at LHC in <em>pp</em> collisions at √<em>s</em> =7 TeV in dilepton final states

Abstract

The production cross section of top quark pairs in proton-proton collisions at center-of-mass energy of 7 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider is measured. The goal of the analysis is to provide a benchmark test of perturbative QCD and the Standard Model by measuring the <i>t<span style="text-decoration:overline">t</span></i> production cross section in data and compare the measurement with theoretical predictions. Within the Standard Model, a top quark almost exclusively decays to a <i>W</i>-boson and a <i>b</i>-quark. Using a data sample of 35 pb^-1, candidate events are selected in the dilepton topology (where both <i>W</i>-bosons from <i>t<span style="text-decoration:overline">t</span></i> decay leptonically) with large missing transverse energy and at least two jets. The branching ratio of this process is 6.5%. The theoretical expectation for <i>t</i><i style="text-decoration:overline">t</i> production cross section in proton-proton collisions at center-of-mass energy of 7 TeV is <span style="white-space:nowrap">165<span style="white-space:nowrap;margin-left:0.3em;"><span style="display:inline-block; margin-bottom:-0.3em; vertical-align:-0.4em; line-height:1.2em; font-family:monospace,'Courier New'; font-size:65%; text-align:right;">+11<br />-16</span></span></span> pb. Backgrounds from Z/γ^* → <i>ll</i> and mis-identified leptons are estimated from data. Results include detailed studies of systematic uncertainties. A <i>t<span style="text-decoration:overline">t</span></i> production cross section 181 ± 22(stat.) ± 14(syst.)<span style="white-space:nowrap;margin-left:0.3em;"><span style="display:inline-block; margin-bottom:-0.3em; vertical-align:-0.4em; line-height:1.2em; font-family:monospace,'Courier New'; font-size:75%; text-align:right;">+8<br />-7</span></span>(lumi) pb is measured for an assumed top quark mass of 172.5 GeV.