Measurement of the inclusive $t\bar{t}\gamma$ cross section at $\sqrt{s} = 7$ TeV with the ATLAS detector

A first measurement of the $t\bar{t}\gamma$ cross section in $pp$ collisions at the LHC using 1.04 fb$^{-1}$ of data taken with the ATLAS detector is presented. A total of 122 candidate events were identified in the single electron and single muon channels. The contributions from background processes with prompt photons, and with electrons or hadrons misidentified as photons were estimated reducing the dependence on simulations by the use of data-driven techniques. The resulting cross section times branching ratio into the single lepton and dilepton decay channels for photons with $p_T > 8$ GeV reads \begin{equation*} \sigma_{t\bar{t}\gamma} \cdot \mathrm{BR} = 1.9 \pm 0.5 \, \mathrm{(stat.)} \pm 0.8 \, \mathrm{(syst.)} \pm 0.1 \, \mathrm{(lumi.)} \,\textrm{pb} \, , \end{equation*} which is consistent with the Standard Model expectation from theoretical calculations. The significance of the $t\bar{t}\gamma$ signal was estimated to $2.5 \, \sigma$.