Measurement of the top quark pair production cross-section involving a Tau & Muon Studies with TileCal in ATLAS

Abstract

This thesis has been developing in the framework of the ATLAS experiment, one of the main general-purpose detectors of the Large Hadron Collider (LHC) located at CERN. This work has two differentiated parts related to hardware and physics studies. It starts with the basic description of the physics that involves LHC studies and focus in the top quark. The pair production of the top quark is one of the dominant processes in the proton-proton interactions, after QCD jets and the production of W and Z bosons. Also, the tau particle is described as one of the main ingredients of the physics analysis. After this, the fundamental parts of the LHC accelerator and the ATLAS detector are revealed. The following is dedicated to TileCal sub-detector, describing mainly the electronic and the data quality assessment performed during commissioning. This is completed with the description of the TileMUID algorithm, based on TileCal and implemented in the second muon level trigger of ATLAS. These studies have contributed to the commissioning of TileCal using cosmic muons taken during the previous period of collisions, showing the correct operation of the detector. The main part of this work is the physics analysis, with research concerning the top quark physics. It starts with the selection of common objects for the whole analysis and the description of the samples used. This is followed by tau identification studies, with the calculation of tau-misidentification probabilities using γ +jets samples. The purpose of this study is to choose the best tau identification algorithm for the measurement of the cross-section. In the next section it is calculated the scale factors of Z background, in order to obtain a better agreement between data and Monte Carlo (MC). These numbers are necessary for the measurement of the cross-section. The description of the event selections and the cutflow study is also shown, with the goal of studying the background and to maximize the number of events in the signal against the background. Finally, it is presented the measurement of the top quark pair production cross-section in the lepton + tau channel, applying the fitting method to 4,73 fb-1 samples. This method uses the TauID technique to separate the process from tau leptons (signal) from those with jets that can be misidentified as taus (fake taus), applying a χ2 fit to signal and background samples, allowing us to obtain the number of events from the samples and calculate the cross-section. The study of the main systematic uncertainties is also presented, in order to measure more precisely the cross-section in the lepton + tau channel. The result of the measurement agrees with the Standard Model predictions and the previous measurements in the same channel.