Measurement of differential di-photon plus jet cross sections using the ATLAS detector

A good understanding of the strong interaction is crucial for every experiment at a hadron collider. Events with photons provide a direct access to study processes of the strong interaction. This thesis presents the measurement of differential cross sections of events with two photons as a function of the number of hadronic jets. Photons are required to be isolated from hadronic activity in the event. Collision data is used corresponding to an integrated luminosity of 20.3/fb at a proton-proton collision energy of √s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider (LHC) in 2012. The dominant background contribution comes from hadronic jets which are identified as photons, occurring mainly when a π^{0} carries most of the jet energy. Two methods to remove this contribution are presented. A smaller background contribution arising from electrons misidentified as photons is removed as well. The measured event yields are corrected for detector inefficiencies and resolution effects. An unfolding method based on singular value decomposition of the resolution matrix is extended to two-dimensional distributions and applied to the reconstructed distributions. All systematic uncertainties are evaluated, the dominant uncertainties arise from the jet background subtraction and the uncertainty in the jet energy scale. 16 observables are studied to explore the photon and jet kinematics. Double differential cross sections are shown as function of those observables and the number of jets, both inclusively (≥ n jets) and exclusively (= n jets). The results are compared to theoretical predictions from Sherpa, Pythia and GoSam.