Time calibration of the High Granularity Timing Detector and measurement of top quarks spin correlation with the ATLAS detector.

This thesis consists of two main parts. The first part corresponds to the timing calibration of all channels of the High Granularity Timing Detector (HGTD), a crucial new detector for the HL-LHC phase-II upgrade. HGTD is designed to enhance the capabilities of all-Silicon ITK by adding timing measurement to charged-particle trajectories as well as spatial information. The timing calibration procedure is proposed to calibrate the distribution of time hit, thits, measured by each HGTD sensor. This procedure is intended to reduce the intrinsic time differences between the sensors to maintain a good temporal resolution of 30ps-50ps. The second part is on the measurements of top quark spin correlation observables in tt ̄ events which represents a unique possibility to test the standard model (SM) predictions and probe the new physics effects. Potential deviations from the SM expectations are parametrised within the framework of the Effective Field Theory (EFT). Furthermore, we present the first proposal for detecting entanglement in a pair of quarks , as well as the violation of Bell inequalities (BIs) at LHC.