| Top quark was discovered by CDF and Dφcollaborations in 1995. Since then, the measurement on the top production is a very interesting topic in the realm of high energy physics. The essential purpose of the thesis is to measure the top pair cross section in lepton+jets channels in the ATLAS experiment at Large Hadron Collider (LHC). The anal-ysis is using the proton-proton collision data at 7TeV collected in 2010 with an integrated luminosity of 33.7 pb-1.The event selection is based on the signature of top pair topology with one good isolated electron or muon, large missing transverse energy and at least 4 jets. After the selec-tion cuts, 709 events in the electron channel and 583 events in the muon channel suvive in real data. The QCD multijet background is measured with the data driven "Matrix Method", which gives 249.4±74.8(e-chan) and 40.3±12.9(μ-chan). Three data driven meth-ods are applied to measure the W+jets background:the charge asymmetry method, the Berends Scaling method and the method combining both. The charge asymmetry method has large statistical uncertainty at the current stage, while the Berends Scaling method is dominanted by systematic uncertainty. Finally the combined method gives lower syst.+stat. uncertainty, which is:W+jets(e-chan)=273.0±78.2, W+jets(μ-chan)=293.5±81.1. Other backgrounds are much smaller and estimated from MC simulation. The number of signal top pair events is obtained by fitting the distribution of the number of b-tagged jet, when the b-tagging efficiency is simultaneously fitted. The method is called "tag counting", which uses the information of b-tagging and avoids the input of b-tagging efficiency measured from other methods. The fitting is applied to the number of events as a function of the number of b-tagged jets, when the backgrounds are subtracted from real data events. The non b-tagged events are not used for its large contamination of backgrounds. Finally, the fitted number of signal events is 203.4±19.8 in the electron channel.221.5±19.6 in the muon channel and 424.6±27.8 in the combined channel, with the statistical uncertainty followed. The systematic uncertainties are considered and in total are relatively 27.8%,22.4% and 22.3% in the electron, muon and combined channels.The fitted number of signal events is applied to the equation below: where L is the integrated luminosity and A·∈is the acceptance times selection efficiency, which is given by MC. The scale factors measured in real data are applied to A·∈, which account for lepton trigger, reconstruction, identification and resolution. Since the statistical uncertainty in the electron and muon combined channel is lower, the top pair cross section measured in the combined channel is 148.1±34.9 pb, where systematic and statistical uncer-tainties are both included. The result ofσtt is consistent with the Standard Model prediction at NNLO calculated by Hathor tool. It also agrees with the measurements from other meth-ods. With more collision data in 2011, one can expect a more precise measurement on top pair cross section.
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