Abstract
A search is performed for top-quark pairs ($t\bar{t}$) produced together with a photon ($\gamma$) with transverse energy greater than 20 GeV using a sample of $t\bar{t}$ 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 $t\bar{t}\gamma$ candidate events are observed in the electron and muon channels, to be compared to the expectation of $79\pm 26$ and $120\pm 39$ non-$t\bar{t}\gamma$ background events respectively. The production of $t\bar{t}\gamma$ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The $t\bar{t}\gamma$ 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 $\sigma_{t\bar{t}\gamma}^{fid} = 63 \pm 8 (stat.) ^{+17}_{-13} (syst.) \pm 1 (lumi.)$ per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of $48\pm 10$ fb.
Highlights
Due to its large mass, the top quark is speculated to play a special role in electroweak symmetry breaking (EWSB).New physics connected with EWSB can manifest itself in top-quark observables
The track-isolation distribution of the photon candidates is used to discriminate between signal photons and neutral hadron decays to final states with photons and hadrons misidentified as photons
Background from Wγ þ jets production is estimated by extrapolating the number of Wγ þ jets candidate events in data from a control region (CR) to the ttγ signal region (SR) using Wγ þ jets Monte Carlo simulation [60]
Summary
Due to its large mass, the top quark is speculated to play a special role in electroweak symmetry breaking (EWSB). A measured yield of top-quark pair production in association with a photon (ttγ) can constrain models of new physics, for example those with composite top quarks [1], or with excited top-quark production, followed by the radiative decay tà → tγ. Observation of top-quark pair production in association with a photon in at a center-of-mass energy porfotponffisffi–1⁄4pro7toTneV(ppis) collisions presented using the full 2011 ATLAS data sample, which corresponds to an integrated luminosity of 4.59 fb−1. This analysis is performed on tt candidate events in the lepton plus jets final state.
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