Measurement of the charge asymmetry in top-quark pair production in association with a photon with the ATLAS experiment

A measurement of the charge asymmetry in top-quark pair ( 𝑡 ¯ 𝑡 ) production in association with a photon is presented. The measurement is performed in the single-lepton 𝑡 ¯ 𝑡 decay channel using proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 2015–2018, corresponding to an integrated luminosity of 139 fb − 1 . The charge asymmetry is obtained from the distribution of the diﬀerence of the absolute rapidities of the top quark and antiquark using a proﬁle likelihood unfolding approach. It is measured to be 𝐴 C = − 0 . 003 ± 0 . 029 in agreement with the Standard Model expectation.