SEARCHING FOR PHYSICS BEYOND THE STANDARD MODEL IN ATLAS WITH THE THIRD GENERATION QUARKS

ATLAS (A Toroidal LHC Apparatus) is one of the general-purpose detectors at the CERN's Large Hadron Collider (LHC), designed to study proton- proton collisions at a center of mass energy of 14 TeV. This PhD thesis starts with a performance measurement of the Inner Detector, followed by the measurement of an interesting Standard Model process on early data in which tracking and b-tagging play a fundamental role. Finally a search for new physics on the high integrated luminosity sample is presented. It includes a detailed measurement of the tracks impact parameter resolution as a function on the pseudorapidity η, the pT and the number of hits and shared hits in the silicon detectors of the tracks. The transverse impact parameter is measured to be ~10 μm for high pT tracks (pT>20 GeV) in a central η region with one hit in the innermost pixel layer (b-layer) and without shared hits with other tracks in the silicon detectors. Two measurements are then presented, that profit of this excellent resolution and the related b-tagging performance. The first measurement of the charge asymmetry in the production of top quark pairs in the semileptonic decay channel. A dataset corresponding to an integrated luminosity of 1.04 fb-1, obtained at a centre-of-mass energy of √s=7 TeV is used. The charge asymmetry is determined using the differential distribution of the reconstructed observable|yt|-|yT|, where yt and yT denote the top and antitop quark rapidities, respectively. The total charge asymmetry after unfolding is measured to be -0.019 ± 0.028 (stat.) ± 0.024 (syst.) in agreement with the Standard Model prediction of (0.60±0.03)%. Finally, the results of a search for direct pair production of supersymmetric partners of the top quark in 4.7 fb-1 and 13.0 fb-1 of integrated luminosity from proton-proton collisions respectively collected at 7 TeV in 2011 and 8 TeV in 2012 are reported. Scalar top quarks decaying into a b-quark and a superymmetric chargino are searched for in events with two leptons in the final state. This events have a final state topology similar that of top-antitop events and this analysis builds up on the experience that I gained with the top-antitop charge asymmetry measurement. No excess above the Standard Model expectation is observed. A scalar top with mass between 150 and 450 GeV is excluded at 95% CL for a chargino approximately degenerate with the scalar top and a massless ligthest neutralino.