Measurement of differential cross-sections of the $t\bar{t}\gamma$ process with the ATLAS detector

This thesis presents a measurement of the differential cross-sections of the associated production of a top quark pair and a photon ($t\bar{t}\gamma$) in proton-proton collisions at a center-of-mass energy of 13 TeV. The data were collected with the ATLAS detector at the Large Hadron Collider during Run 2 operation, corresponding to an integrated luminosity of 140.1 $\text{fb}^{-1}$. The analysis is performed in the single-lepton and dilepton top quark pair decay channels, focusing on events where the photon is radiated from the initial-state parton or one of the top quarks. Additionally, cross-sections are measured for the inclusive $t\bar{t}\gamma$ process, including events where the photon is radiated from the decay products of the top quarks. The measurement is performed at particle level within a fiducial phase space volume designed to closely mimic the detector acceptance. Detector effects are corrected using a profile likelihood unfolding method. The absolute and normalized differential cross-sections are measured as functions of various observables characterizing the photon, lepton, and jet kinematics as well as variables related to the angular separation between these objects. The results are compared to state-of-the-art Monte Carlo simulations, finding good overall agreement.