Inclusive and differential measurements of the tt̄γ cross section and the tt̄γ /tt̄ cross section ratio in proton-proton collisions at √s=13 TeV

Inclusive and differential cross section measurements of top quark pair (tt̄) production in association with a photon (γ) are performed as a function of lepton, photon, top quark, and tt̄ kinematic observables, using data from proton-proton collisions at √s = 13 TeV, corresponding to an integrated luminosity of 138 fb⁻¹, collected at the CERN LHC with the CMS detector. Events containing two leptons (electrons or muons) and a photon in the final state are considered. The fiducial cross section of γ is measured to be 137 ± 8 fb, in a phase space including events with a high momentum, isolated photon. The fiducial cross section of tt̄γ is also measured to be 56 ± 5 fb when considering only events where the photon is emitted in the production part of the process. Both measurements are in agreement with the theoretical predictions, of 126 ± 19 fb and 57 ± 5 fb, respectively. Differential measurements are performed at the particle and parton levels. Additionally, inclusive and differential ratios between the cross sections of tt̄γ and tt̄ production are measured. The inclusive ratio is found to be 0.0133 ± 0.0005, in agreement with the standard model prediction of 0.0127 ± 0.0008. The top quark charge asymmetry in tt̄γ production is also measured to be −0.012 ± 0.042, compatible with both the standard model prediction and with no asymmetry.