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

Abstract

A search is performed for top-quark pairs (t[bar over t]) produced together with a photon (γ) with transverse energy greater than 20 GeV using a sample of t[bar over 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[superscript −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[bar over 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[bar over t]γ background events, respectively. The production of t[bar over t]γ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The t[bar over 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 ATLAS acceptance. The measured value is σ[fid over t[bar over t]γ] × BR = 63 ± 8(stat)[+17 over −13](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.