From top antitop production cross section measurements to the top-quark pole mass extraction and the MicroMegas upgrade project for the ATLAS experiment

This thesis is focused on two main arguments. The MicroMegas chambers in the context of the ATLAS muon spectrometer upgrade and the measurements of the total and differential cross sections of top-quark pairs production. The MicroMegas chambers will replace the current tracking chambers in the forward inner region of the ATLAS muon spectrometer. In this work, the MicroMegas chambers construction, and validation of the SM1 module type, in which the Italian collaboration has been deeply involved since the beginning of the project, are presented, together with the first studies on the high voltage stability of the chambers. Besides, the studies performed on the first non-prototype module during a test beam and the final validation of the chambers at CERN before their installation on the New Small Wheel are described. In this thesis the measurement of the $t\bar{t}$ production cross section is performed in the dileptonic channel, using proton-proton collision data collected by the ATLAS experiment at a centre-of-mass energy of 13 TeV. The total inclusive cross section is measured to be $830.3 \pm 1.2\ (stat) \pm 11.2\ (syst) \pm 15.9\ (lumi)\ pb$. The total cross section is measured in the full phase space, and in a fiducial phase space defined following the detector acceptance. In addition, the absolute and normalised single and double differential cross section as a function of several lepton kinematic variables are presented. The measurement of the top-quark pole mass is performed using two different methods: from the $t\bar{t}$ inclusive cross section measurement, and from normalised differential cross section measurements. The top pole mass is measured to be $m_{top} = 173.2^{+2.9}_{-2.6}\ GeV$ using the absolute total cross section while $m_{top} = 168.32 \pm 0.04\ (stat) \pm 1.19\ (syst)\ GeV$ using the fit template method using the normalized differential cross sections.