Measurement of top-quark pair production in association with charm quarks in proton-proton collisions at √s = 13 TeV with the ATLAS detector

Inclusive cross-sections for top-quark pair production in association with charm quarks are measured with proton–proton collision data at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb⁻¹, collected with the ATLAS experiment at the LHC between 2015 and 2018. The measurements are performed by requiring one or two charged leptons (electrons and muons), two b-tagged jets, and at least one additional jet in the final state. A custom flavor-tagging algorithm is employed for the simultaneous identification of b-jets and c-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for tt^- + 2c and tt^- +  1c production are measured to be 1.28^+0.27_-0.24 pb and 6.4^+1.0_-0.09 pb, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive tt^-and tt^- + bb^- production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various tt^- simulations are largely consistent with the measured cross-section values, though all underpredict the observed values by 0.5 to 2.0 standard deviations. In a phase-space volume without requirements on the tt^- decay products and the jet multiplicity, the cross-section ratios of tt^- + ≥ 2c and tt^- + 1c to total tt^- + jets production are determined to be (1.23 ± 0.25) % and (8.8 ± 1.3)%.