Mesure de la section efficace de production de paires de quarks, top-antitop à l’aide des premières données du détecteur ATLAS auprès du LHC

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 analysis is using the proton-proton collision data at 7 TeV collected in 2010 with a luminosity of 33.7 pb-1. The event selection is based on the signature of top pair topology with 1 electron or muon, large missing transverse energy and at least 4 jets. The QCD multijet background is measured with the data driven “Matrix Method”. Three data driven methods are applied to measure the W+jets background, the charge asymmetry method, the Berends Scaling method and the method combining both. Other backgrounds are estimated from MC. 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 top pair cross section is measured to be 148.1±34.9 pb. It is consistent with the Standard Model prediction at NNLO calculated by Hathor tool. It also agrees with the measurements from other methods. With more collision data in 2011, one can expect a more precise measurement on top pair cross section.