Alignment of the ATLAS Inner Detector and probing the top quark polarization in single top t-channel production.

This thesis presents on the one hand performance work involved with the alignment of ATLAS Inner Detector during early Run 2. On the other hand, this thesis presents an analysis to probe the Wtb vertex using polarized single top quark events produced in the t-channel, recorded by the ATLAS detector at a center of mass energy of 13 TeV, using 80.5 fb⁻¹ of proton-proton collisions data collected during Run 2. The work in this thesis has contributed to provide high quality of recorded data of the ATLAS experiment, especially during early Run 2. The tracker of the Inner Detector must be correctly calibrated in order to provide good quality tracks to the reconstruction algorithms used in the ATLAS experiment. The basics of the alignment algorithm, as well as the work developed to provide good quality alignment corrections is presented in this thesis. The alignment algorithm is able to determine the real position of the ID sensors and also to follow the eventual changes that happen during data taking is crucial. However, systematic uncertainties invisible in the alignment algorithm, known as weak modes, need to be included to have an accurate description of the detector. Chapter 3 describes briefly the basics of the ID alignment procedure and the strategy that was followed during the Run 2. The analysis performed to probe the Wtb vertex is presented in Chapter 4. Top quark events produced singly with one charged lepton, two jets (one of them b-tagged) and missing transverse momentum are used to probe the Wtb vertex in this analysis. The analysis consists of a measurement of angular observables asymmetries related with the top quark polarization. The observables that have been used are sensitive to possible anomalous couplings in the Wtb vertex. The methodology is based on angular distributions that are unfolded to particle level in a fiducial region and the forward-backward asymmetries are extracted. This approach has been followed by the ATLAS collaboration extensively; being more precise thanks to a reduction of the signal modelling related uncertainties. In addition, it was previously studied that the sensitivity to the W tb anomalous couplings was retained for various forward backward asymmetries, defined at particle level within the detector acceptance. The dataset and the Monte Carlo samples used as well as the methodology followed to perform the measurement and the estimation of the uncertainties are explained in Chapter 4 too. Finally, the conclusion of the thesis dissertation is presented in Chapter 5.