Mesure de la section efficace de production du single top en voie-t en utilisant des arbres de decision avec ATLAS a $\sqrt{s}$=7 TeV

This thesis presents a measurement of the cross-section of t-channel single top-quark production using 1.04 fb-1 data collected by the ATLAS detector at the LHC with proton-proton collision at center-of-mass sqrt(s) = 7 TeV. Selected events contain one lepton, missing transverse energy, and two or three jets, one of them b-tagged. The background model consists of multijets, W+jets and top-quark pair events, with smaller contributions from Z+jets and diboson events. By using a selection based on the distribution of a multivariate discriminant constructed with the boosted decision trees, the cross section of t-channel single top-quark production is measured, 97.3+30.7_30.2 pb, which is in good agreement with the prediction of the Standard Model. Assuming that the top-quark-related CKM matrix elements obey the relation |Vtb| >>|Vts|, |Vtd|, the coupling strength at the W-t-b vertex is extracted from the measured cross section, |Vtb| = 1.23+0.20-0.19. If it is assumed that |Vtb| <= 1 a lower limit of |Vtb| > 0:61 is obtained at the 95% con dence level.