Inclusive and differential measurements of the tt¯γ cross section and the tt¯γ/tt¯ cross section ratio in proton-proton collisions at s=13 TeV

Inclusive and differential cross section measurements of top quark pair (tt¯) production in association with a photon (γ) are performed as a function of lepton, photon, top quark, and tt¯ 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 tt¯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 tt¯γ 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 tt¯γ and tt¯ 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 tt¯γ production is also measured to be −0.012 ± 0.042, compatible with both the standard model prediction and with no asymmetry.