UC Irvine

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 t t ¯ + ≥ 2 c and t t ¯ + 1 c production are measured to be 1.28 − 0.24 + 0.27 pb and 6.4 − 0.9 + 1.0 pb , respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive t t ¯ and t t ¯ + b b ¯ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various t t ¯ 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 t t ¯ decay products and the jet multiplicity, the cross-section ratios of t t ¯ + ≥ 2 c and t t ¯ + 1 c to total t t ¯ + jets production are determined to be ( 1.23 ± 0.25 ) % and ( 8.8 ± 1.3 ) % .