Measurement of top-quark pair production in association with charm quarks in proton–proton collisions at 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 $t\overline{t}+\ge 2c$ and $t\overline{t}+1c$ production are measured to be ${1.28}_{-0.24}^{+0.27}\phantom{\rule{0.25em}{0ex}}\text{pb}$ and ${6.4}_{-0.9}^{+1.0}\phantom{\rule{0.25em}{0ex}}\text{pb}$, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive $t\overline{t}$ and $t\overline{t}+b\overline{b}$ production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various $t\overline{t}$ 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 $t\overline{t}$ decay products and the jet multiplicity, the cross-section ratios of $t\overline{t}+\ge 2c$ and $t\overline{t}+1c$ to total $t\overline{t}+\text{jets}$ production are determined to be $\left(1.23±0.25\right)\text{%}$ and $\left(8.8±1.3\right)\text{%}$.