The Top Quark and its Electroweak Couplings

The top quark was not discovered until 1995. Hence, many of its properties could not be measured at a high precision or even not be investigated at all so far. Since the top quark is so heavy, it might play a special role in electroweak symmetry breaking. Especially its coupling to the electroweak (EW) gauge bosons could reveal hints to New Physics, if there were deviations from the predictions by the Standard Model. The total cross section of the top quark pair production process with the additional emission of a photon has been determined with the ATLAS experiment as a first measurement of the couplings of EW gauge bosons to the top quark, since this process provides the largest cross section compared to other EW couplings. The ttγ cross section measurement has been performed in the semi-leptonic (e/μ+jets) decay channel using a template fit method with 1.04 ifb of ATLAS data collected at a center-of-mass energy of 7 TeV in 2011. Various background sources have been investigated, using data-driven methods whenever possible; the impact of systematic uncertainties has been evaluated using a statistical ensemble of pseudo experiments.