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

Inclusive cross-sections for top-quark pair production in association with charm quarks (c) 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 ttˉ +1c-jet production are measured to be 1.28+0.27-0.24 pb and 6.4+1.0-0.9 pb, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive ttbar and ttbar + bbbar production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various ttbar 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 ttbar + >= 2c and ttbar + 1c to total ttbar + jets production are determined to be (1.23+.0.25)% (8.8+-1.3)%