Evidence and cross section measurement of the process $pp\rightarrow t\bar{t}\gamma$ at a center-of-mass energy of 7 TeV with the ATLAS experiment

The cross section measurement of top quark pair production with an additional prompt photon in the final state ($t\bar{t}\gamma$) is presented. The total 2011 dataset of 4.7\,fb$^{-1}$ has been analyzed in this thesis, recorded by the ATLAS detector in proton-proton collisions at the LHC with a center-of-mass energy of $\sqrt{s}=7\,$TeV. \\ A event selection has been developed for the single electron and single muon channels, following closely the selection used in \ttbar\ cross section measurements published by the \\ ATLAS collaboration, with the addition of photon selection cuts, identifying a total of 414 \ttg\ event candidates in the dataset. Various background contributions have been estimated, which can be categorized in three classes according to the true type of the selected photon. Besides true prompt photons, electrons and hadrons from jet fragmentation can be misidentified as a photon. A signifcant difference has been found in the photon distribution shape of the track isolation variable \ptcone\ between misidentified hadrons and prompt photons. This has been exploited to distinguish these types of photons using a template fit to the \ptcone\ data distribution and consequently extract the final \ttg\ cross section. Background processes including a prompt photon or misidentified electron have been estimated separately, which include data-driven methods for contributions from $W$ boson production with additional jets and QCD multijet events. The remaining background yields from $Z$ boson production with additional jets as well as single top, diboson and top quark pair production have been determined from Monte Carlo simulation. These separate estimations have been then included in the template fit. \\ The final result of the \ttg cross section in the non full hadronic decay channels with a transverse mometum of the prompt photon above 20\,GeV and a pseudorapidity of $|\eta|<2.37$ as well as the cuts used in the signal generation reads: \begin{equation*} \sigma_{t\bar{t}\gamma}\times\text{BR}=\left[1.11\pm0.15(\text{stat.})^{+0.25}_{-0.21}(\text{syst.})\pm0.02(\text{lumi.})\right]\,\text{pb}\,. \end{equation*} This results in an evidence of the \ttg\ process with a significance of 4.96\,$\sigma$ with respect to the background only hypothesis. Further, this measurement is in very good agreement with the Standard Model expectation of $\left(\sigma_{t\bar{t}\gamma}\times\text{BR}\right)_{\text{theo.}}=\left[1.0\pm0.2\right]\,\text{pb}$.