Thesis defense of Nils Julius Abicht

This thesis presents studies of single top-quark production in association with a photon (tqgamma), using 140 fb^-1 of proton-proton collision data at a center-of-mass energy of 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider during Run 2. With a predicted cross-section of the order of one picobarn, this process is among the rarest top-quark processes in the Standard Model and provides a direct probe of the electroweak coupling between the top quark and the photon. The inclusive tqgamma cross-section has previously been measured by the ATLAS collaboration and was found to be larger than the theoretical prediction, motivating further investigation. Essential components of the previous ATLAS measurement are revisited under updated reconstruction and software conditions. A dedicated signal modeling method is employed, combining next-to-leading-order Monte Carlo samples to consistently describe photon radiation in both the production and decay of the top quark. Background contributions from processes with misidentified or non-prompt photons are estimated using data-driven techniques. A Neural Network is trained to select a high-purity signal-enriched sample, enabling the study of the kinematic properties of the process. The resulting distributions are found to be in good agreement with theoretical predictions within the statistical uncertainties. The work presented in this thesis provides the foundation for a future differential cross-section measurement of single top-quark production in association with a photon.