Mesure de la section efficace de production électrofaible de quark top dans le mode associé W+t dans le canal dilepton auprès de l'expérience ATLAS au LHC

This document presents the first attempts to measure the single-top production cross-section in the mode associating a W boson and a top quark, using 2.1/fb of pp-collisions data at 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. A first part of this PhD thesis presents a simple cut-and-count analysis based on the early data accumulated in 2010 at 7 TeV, which lead to early limits at 95% CL on the production cross-section. With higher statistics an approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions has then be performed to determine the cross-section. The expected sensitivity assuming the Standard Model production rate is 3.2σ. The corresponding measured cross-section is determined and found to be : σ(Wt) = 10.2 +/- 4 pb (observed) σ(Wt) = 14.4+/- 4.5 pb (expected), in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb| = 0.80+/-0.16 is derived assuming that the Wt production through |Vts| and |Vtd| is small.