The top quark, being the heaviest of the six quarks in the Standard Model
of Particle Physics, had not been 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 (EWSB). 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 (tTy) 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 tTy cross section measurement has been performed in the semi-leptonic
(e+jets and µ+jets) decay channel using a template fit method with 1.04
ifb of ATLAS data collected at a center-of-mass energy of sqrt(s) = 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 3000
pseudo experiments.