Measurements of $t\bar{t}$ in association with charm quarks at 13 TeV with the ATLAS experiment

This talk presents the ATLAS Collaboration’s first measurement of the inclusive cross-section for top-quark pair production in association with charm quarks. Using the full Run~2 proton–proton collision data sample at \sqrt{s} = 13\,\text{TeV} s = 13 TeV , collected with the ATLAS experiment at the LHC between 2015 and 2018, the measurement selects t\bar{t} t t ‾ events with one or two charged leptons and at least one additional jet in the final state. A custom flavour-tagging algorithm is employed to simultaneously identify b b -jets and c c -jets. The fiducial cross-sections for t\bar{t}+{≥}2c t t ‾ + ≥ 2 c and t\bar{t}+1c t t ‾ + 1 c production are found to largely agree with predictions from various t\bar{t} t t ‾ simulations, though all underpredict the observed values.