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

Aad, G. and Abbott, B. and Abdallah, J. and Khalek, S. A. and Abdinov, O. and Krasznahorkay, Attila and Nagai, Yoshikazu and Pásztor, Gabriella and Tóth, József (2015) Observation of top-quark pair production in association with a photon and measurement of the t¯tγ production cross section in pp collisions at √s = 7 TeV using the ATLAS detector. PHYSICAL REVIEW D, 91 (7). ISSN 2470-0010

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Abstract

A search is performed for top-quark pairs (t¯t) produced together with a photon (γ) with trans- verse 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 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 t¯tγ candidate events are observed in the electron and muon channels, to be compared to the ex- pectation 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 hypoth- esis. 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 = 63 ± 8(stat.) +17 −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.

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