Observation of top-quark pair production in association with a photon and measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>t</mml:mi><mml:mover accent="true"><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mrow><mml:mo accent="true" stretchy="false">¯</mml:mo></mml:mrow></mml:mover><mml:mi>γ</mml:mi></mml:mrow></mml:math>production cross section in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>p</mml:mi></mml:math>collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:mi>s</mml:mi></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>7</mml:mn></mml:mrow></mml:math>TeV using the ATLAS detector

A search is performed for top-quark pairs ($t\overline{t}$) produced together with a photon ($\ensuremath{\gamma}$) with transverse energy greater than 20 GeV using a sample of $t\overline{t}$ candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The data set used corresponds to an integrated luminosity of $4.59\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. In total, 140 and 222 $t\overline{t}\ensuremath{\gamma}$ candidate events are observed in the electron and muon channels, to be compared to the expectation of $79\ifmmode\pm\else\textpm\fi{}26$ and $120\ifmmode\pm\else\textpm\fi{}39$ non-$t\overline{t}\ensuremath{\gamma}$ background events, respectively. The production of $t\overline{t}\ensuremath{\gamma}$ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The $t\overline{t}\ensuremath{\gamma}$ production cross section times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the ATLAS acceptance. The measured value is ${\ensuremath{\sigma}}_{t\overline{t}\ensuremath{\gamma}}^{\mathrm{fid}}\ifmmode\times\else\texttimes\fi{}\mathrm{BR}=63\ifmmode\pm\else\textpm\fi{}8{(\text{stat})}_{\ensuremath{-}13}^{+17}(\text{syst})\ifmmode\pm\else\textpm\fi{}1(\text{lumi})\text{ }\text{ }\mathrm{fb}$ per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of $48\ifmmode\pm\else\textpm\fi{}10\text{ }\text{ }\mathrm{fb}$.

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