Differential cross-section measurements of the production of a top-antitop quark pair in association with a Z boson with the ATLAS detector

This thesis describes differential cross-section measurements of the production of a top-antitop quark pair in association with a $Z$-boson ($t\bar{t}Z$) in proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider (LHC) between 2015 and 2018 with a centre-of-mass-energy of $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of $140$ fb$^{-1}$. The analysis targets three- and four-lepton final states and aims to measure the differential cross-sections as precisely as possible. A profile likelihood unfolding method is used to extract differential cross-section measurements in a simultaneous fit in multiple orthogonal analysis regions. The differential cross-section measurements are presented as a function of a number of variables which probe the kinematics of the $t\bar{t}Z$ system. Both absolute and normalised differential cross-section measurements are performed at particle- and parton- level and are compared with theoretical predictions. Good agreement is seen between the data and predictions.