Measurement of top-quark pair production in association with charm quarks in protonproton 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
𝑡̄𝑡 + ≥2𝑐 and 𝑡̄𝑡 + 1𝑐 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 𝑡̄𝑡 and 𝑡̄𝑡 + 𝑏̄𝑏 production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various 𝑡̄𝑡 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 𝑡̄𝑡 decay products and the jet multiplicity, the cross-section ratios of 𝑡̄𝑡 + ≥2𝑐 and 𝑡̄𝑡 + 1𝑐 to total 𝑡̄𝑡 + jets production are determined to be (1.23 ± 0.25)% and (8.8 ± 1.3)%.