A search is performed for top-quark pairs (t t) produced together with a photon (γ) with transverse energy greater than 20 GeV using a sample of t 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 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 t t γ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79±26 and 120±39 non-t t γ background events, respectively. The production of t t γ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The t t γ production cross section times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the …
