Differentiating Top-Quark - Photon Coupling: Inclusive and Differential Cross-Section Measurements of Top-Quark Pair Production in Association with a Photon in the Single Lepton Channel in pp Collisions Data at $\sqrt{s}$ = 8TeV with the ATLAS Experiment

This thesis presents measurements of inclusive and differential production cross sections of top- anti-top-quark pairs in association with an additional prompt photon ($\sigma_{t\bar{t}\gamma}$) in the single lepton final state at the LHC analysing data recorded by the ATLAS detector. The data analysed corresponds to 20.2 fb$^{−1}$ of integrated luminosity recorded during 2012 at a center-of-mass energy of $\sqrt{s}$ = 8 TeV. The cross.section is measured with a template fit method. Background processes are estimated with data driven and Monte Carlo based methods. The differential cross section is measured as a function of transverse momentum and pseudo-rapidity of the photon. A total of 1256 and 1816 candidate $t\bar{t}\gamma$ events are observed in the electron and muon channel, respectively. Both inclusive and differential measurements are performed in a fiducial phase space within the detector acceptance. The differential measurements are performed as a function of transverse momentum and pseudo-rapidity of the photon. The inclusive $t\bar{t}\gamma$ production cross section times the branching ratio (BR) is measured to be $\sigma_{t\bar{t}\gamma}$ $\times$ BR = 71.4 $\pm$ 13.0 fb in the electron channel. The measured cross section in the muon channel reads $\sigma_{t\bar{t}\gamma}$ $\times$ BR = 70.0 $\pm$ 9.8 fb. The results are in agreement within the uncertainties with the corresponding standard model predictions at next-to-leading order.