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

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 tt production cross section in data and compare the measurement with theoretical predictions. Within the Standard Model, a top quark almost exclusively decays to a W -boson and a b- quark. Using a data sample of 35 pb−1 , candidate events are selected in the dilepton topology (where both W -bosons from tt decay leptonically) with large missing trans- verse energy and at least two jets. The branching ratio of this process is 6.5%. The theoretical expectation for tt production cross section in proton-proton collisions at center-of-mass energy of 7 TeV is 165+11 pb. Backgrounds from Z/γ ∗ → ll and mis- −16 identified leptons are estimated from data. Results include detailed studies of system- atic uncertainties. A tt production cross section 181±22(stat.) ±14(syst.) +8 (lumi.)pb −7 is measured for an assumed top quark mass of 172.5 GeV.