Measurement of the ttbar Production Cross Section using the Semileptonic Final States in pp Collisions at √s=7 TeV with 35.3 pb⁻¹ of ATLAS Data

Abstract

A measurement of the production cross section for top quark pairs (tt) in pp collisions at √s = 7 TeV is presented using data recorded with the ATLAS experiment at the Large Hadron Collider in 2010. Events are selected in the tt semileptonic decay channel by requiring a single lepton (electron e or muon μ), large missing transverse energy and at least four jets. In a data sample of 35:3 pb⁻¹, 396 e + jets events and 653 μ + 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 tt cross section separately in the e+ jets and μ + jets channel. A combined fit to the top likelihood distribution in both channels is employed to extract a combined cross section. The tt cross section, assuming a top mass of 172.5 GeV, is found to be: e + jets channel: σ(u) = 164.4^+16.2(-15.4)(stat)^31.2(-36.2)(syst)^5.8(-5.4)(lumi) pb; μ + jets channel: σ(u) = 167.8^+1.3.9(-13.4)(stat)^3.9(-34.8)(syst)^5.9(-5.5)(lumi) pb; e + jets and μ + jets channel combined: σ(u) = 166.4^+10.5(-10.2)(stat)^31.7(-33.6)(syst)^5.9(-5.5)(lumi) pb The measurement agrees with approximate NNLO perturbative QCD calculations.