Measurement of the top quark pair production cross section with the ATLAS experiment at $\sqrt{s}$=7,TeV

The production cross section of top quark pairs with a semi-leptonic final state was measured in proton-proton collisions at $sqrt{s}=7,TeV$ with the ATLAS experiment at the LHC. A first measurement was performed with data corresponding to an integrated luminosity of $2.9,pb^{-1}$, collected during the early running phase of the LHC. Large efforts were made in order to cope with the low statistics and the restricted understanding of the detector at the early stage of the experiment. Events with exactly one electron or muon, high missing transverse energy and at least three jets were analyzed. Flavor tagging algorithms were used to further increase the purity of the signal in the final selection sample, requiring at least one jet to originate from a $b$~quark. The production cross section was extracted from a maximum likelihood fit to the invariant mass spectrum of the hadronically decaying top quark candidate, yielding $130 pm 44, extrm{(stat.)} ,,{}^{+34}_{-28}, extrm{(syst.)} ,,{}^{+16}_{-13}, extrm{(lumi.)},pb$. The analysis was improved using the complete data set recorded with the ATLAS detector in 2011, corresponding to an integrated luminosity of~$35,pb^{-1}$. An improved fitting method was used that allowed the data to constrain and reduce the impact of the systematic uncertainties on the measurement. In order to achieve this the data was categorized into 18 exclusive channels depending on the lepton flavor, the jet multiplicity, and the number of heavy-flavor tagged jets. In the improved analysis a top quark pair production cross section of $156,{}^{+20}_{-18}, extrm{(stat.+syst.)} pm 5, extrm{(lumi.)},pb$ was measured, representing one of the most precise measurements to date corresponding to~$35,pb^{-1}$.