W polarization and $t\bar{t}$ spin correlation study in $t\bar{t}$ semi-lepton decay at ATLAS

The particle physics came out and was developed to explore the composition of the universe. The Standard Model (SM) of particle physics, developed in the early 1970s, provides a comprehensive and systematic understanding, validated in numerous aspects by the experiments. Although the Standard Model has been a great success, it is not the final theory and the predicted Higgs boson has not been found before LHC. The Large Hadron Collider (LHC), which is the largest high energy collider up to now, is constructed to test the SM, searching for the Higgs boson and new physics beyond SM. Four main detectors, ATLAS, CMS, ALICE and LHCb are designed to record and study collision events produced by the LHC for their specific scientific program. The LHC is also a top quark factory, with an expected 8 million top quark pairs produced per year at the nominal designed collision energy and luminosity. The top quark is the only observed particle whose mass extends to the electroweak scale, which provide the opportunity to understand the SM and search for new physics with the top quark as a probe. In this thesis, the measurements of the helicity fractions of the W boson produced in top quark decay and the ttbar spin correlation strength of the produced top quark pair are presented. The SM allows the calculation of these quantities at the next-leading order. Any deviation of the measurement from the prediction may reveal the existence of new physics. The analysis is based on the 4.6fb-1 data collected in 2011 by the ATLAS experiment from proton-proton collisions at a 7TeV center of mass energy. It looks at the semi-leptonic top decay channel as signal where one W coming from the top quark decays further decays into leptons and the other W from the second top quark decays into two light quarks. To achieve the measurements, the event is fully reconstructed and the angles between the secondary particles are calculated. The distributions of the measured variables, at reconstruction level, are unfolded to the parton level with an iterative unfolding technique to allow further comparison with theoretical predictions. Both of the two measurements have a good agreement with the SM predictions within the statistical and systematic uncertainties. In the secondary particles from the top quark pair decay, the leptons are the most precisely measured and the best objects for the event triggering. In this thesis, the study of the electron reconstruction and trigger performances, critical to any ATLAS analysis, is presented. Those results integrated to the ATLAS egamma working group performance results were provided to all the ATLAS community. My work on the electron identification efficiency study at ATLAS, which is used to improve the agreement of the electron performance between real data and simulated Monte Carlo is presented in the first part of this thesis.