Measurement of the t-channel single top-quark production using boosted decision trees in ATLAS experiment at √(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 √(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 multi-jets, W+jets and top-quark pair events, with smaller contributions from Z+jets and di-boson 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: σt = (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 Wtb 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% confidence level. (author)