Transition Radiation Tracker calibration, searches beyond the Standard Model and multiparticle correlations in ATLAS

Abstract: This thesis contains two different aspects of my research work towards physics in proton-proton collisions in the ATLAS experiment at the LHC. The first part is focused on the understanding and developing of a calibration system to obtain the best possible charged particle reconstruction in the Transition Radiation Tracker. The method explained in this thesis is the current calibration technique used in the TRT and it is applied to all the data collected by ATLAS. Thanks to the method developed, the detector design resolution is achieved, and even improved in the central region of the TRT. In the second part, three different analyses are presented. Due to my interest in tracking and thanks to the new energy range available at the LHC, the first analysis is the study of multiparticle correlations at 900 GeV and 7 TeV. This analysis is performed with the first ATLAS data collected during 2010. Two different aspects are studied: the high order moments and an attempt to measure the normalized factorial moments in $eta$ bins. The other two data analyses described in this thesis are focused on the discovery of physics Beyond Standard Model. The search of same-sign top quarks and b-like fourth generation quarks is one of them. For this analysis the probability to reconstruct a lepton with the wrong charge measurement is studied in detail. New data-driven methods are developed, of which a likelihood technique has shown a good performance, and it is adopted by other analysis in ATLAS. This search shows that data is in agreement with the Standard Model expectations. The last analysis is the search for Supersymmetry with two leptons in the final state and large missing transverse energy. A detailed description of the diboson production, which is one of the main backgrounds of this analysis, is presented. The final measurement shows no excess with respect to the Standard Model expectations.