Observation of top-quark pair production in association with a photon and measurement of the tt ¯ γ production cross section in pp collisions at s √ =7 TeV using the ATLAS detector

A search is performed for top-quark pairs (tt ¯ ) produced together with a photon (γ ) with transverse energy greater than 20 GeV using a sample of tt ¯ candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The dataset used corresponds to an integrated luminosity of 4.59 fb ¯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 tt ¯ γ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79±26 and 120±39 non-tt ¯ γ background events respectively. The production of tt ¯ γ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The tt ¯ γ 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 σ fid tt ¯ γ =63±8(stat.) +17 −13 (syst.)±1(lumi.) per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of 48±10 fb.