Measurement of the ttbar Production Cross Section using the Semileptonic Final States in $pp$ Collisions at sqrt{s} = 7 TeV with 35.3 pb^{-1} of ATLAS Data

A measurement of the production cross section for top quark pairs (ttbar) in $pp$ collisions at sqrt{s}=7 TeV is presented using data recorded with the ATLAS experiment at the Large Hadron Collider in 2010. Events are selected in the ttbar semileptonic decay channel by requiring a single lepton (electron e or muon mu), large missing transverse energy and at least four jets. In a data sample of 35.3 pb^{-1}, 396 e+jets events and 653 mu+jets events are observed. A multivariate top likelihood is built from four variables including a b-tagging variable. A fit to the top likelihood distribution is employed to extract the ttbar cross section separately in the e+jets and mu+jets channel. A combined fit to the top likelihood distribution in both channels is employed to extract a combined cross section. The ttbar cross section, assuming a top mass of 172.5 GeV, is found to be: e+jets channel: sigma_{ttbar} = 164.4_{-15.4}^{+16.2}(stat)_{-36.2}^{+31.2}(syst)_{-5.4}^{+5.8}(lumi) pb mu+jets channel: sigma_{ttbar} = 167.8_{-13.4}^{+13.9}(stat)_{-34.8}^{+30.9}(syst)_{-5.5}^{+5.9}(lumi) pb e+jets and mu+jets combined: sigma_{ttbar} = 166.4_{-10.2}^{+10.5}(stat)_{-33.6}^{+31.7}(syst)_{-5.5}^{+5.9}(lumi) pb The measurement agrees with approximate NNLO perturbative QCD calculations.