Measurement of top pair production in l + jets channels in ATLAS at √(s) = 7 TeV

Top quark was discovered by CDF and DO collaborations in 1995. Since then, the measurement on the top production is a very interesting topic in the realm of high energy physics. This thesis describes a measurement of the tt-bar production cross section at √(s) equals 7 TeV using the first LHC data. This measurement depends on the cut and count method. The requirements of one good lepton, large EmissT, at least four jets are applied to suppress the backgrounds. It also requires in particular the knowledge of the W+Jets background, which will be measured using two methods, and a measurement of the b-jet tagging efficiency, which is measured using tt-bar events. The QCD background is measured by applying a data driven method. Finally the systematic error are estimated by considering the methods used in the measurement. The measurement gives a result of tt-b cross section σ(tt-bar) = (148.1 ± 34.9) pb which is consistent with the results from different top pair decay channels and agrees with the theoretical prediction in the Standard Model. (author)