Measurement of top-quark pair production in association with charm quarks in proton-proton collisions at SQUARE ROOT𝑠� = 13 TeV with the ATLAS detector

Abstract

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 byrequiring one or two charged leptons (electrons and muons), two 𝑏-tagged jets, and at least one additional jetin the final state. A custom flavor-tagging algorithm is employed for the simultaneous identficationof 𝑏-jetsand 𝑐-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for𝑡̄𝑡 + >=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 measurementsare primarily limited by uncertainties in the modeling of inclusive 𝑡̄𝑡 and 𝑡̄𝑡 + 𝑏̄𝑏production, in the calibrationof the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various 𝑡̄𝑡 simulations arelargely consistent with the measured cross-section values, though all underpredict the observed values by 0.5to 2.0 standard deviations. In a phase-space volume without requirements on the 𝑡̄𝑡 decay products and the jetmultiplicity, 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)%.