Inclusive and differential measurements of the t t-bar gamma cross section and the t t-bar gamma / t t-bar cross section ratio in proton-proton collisions at sqrt(s) = 13 TeV

Inclusive and differential cross section measurements of top quark pair (t (t) over bar) production in association with a photon (gamma) are performed as a function of lepton, photon, top quark, and t (t) over bar kinematic observables, using data from proton-proton collisions at root 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 t (t) over bar gamma is measured to be 137 +/- 8 fb, in a phase space including events with a high momentum, isolated photon. The fiducial cross section of t (t) over bar gamma 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 t (t) over bar gamma and t (t) over bar 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 t (t) over bar gamma production is also measured to be -0.012 +/- 0.042, compatible with both the standard model prediction and with no asymmetry.