E-catalog        

Библиогр.: 74 назв.

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¯tγ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79 26 and 120 39 non-t¯tγ background events, respectively. The production of t¯tγ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The t¯tγ 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 σfid t¯tγ × BR ¼ 63 8ðstatÞþ17 −13 ðsystÞ 1ðlumiÞ fb per lepton flavor, in good agreement with the leadingorder theoretical calculation normalized to the next-to-leading-order theoretical prediction of 48 10 fb.