Observation of Top-Quark Pair Production in Association with a Photon and Measurement of the tt̅γ Production Cross Section in pp Collisions at √s=7 TeV Using the ATLAS Detector

A search is performed for top-quark pairs tt̅ produced together with a photon (γ) with transverse energy greater than 20 GeV using a sample of tt̅ 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 tt̅γ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79 ± 26 and 120 ± 39 non-tt̅γ background events, respectively. The production of tt̅γ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The tt̅γ 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 {(tt̅γ)^(fid)} x BR = 63 ± 8(stat){(-13)^(+17) (syst) ± 1(lumi) fb 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.