Measurement of the production of a W boson in association with a charmed hadron in pp collisions at √s = 13 TeV with the ATLAS detector

The production of a W boson in association with a single charm quark is studied using 140 fb^-1 of √s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by the presence of a charmed hadron reconstructed with a secondary-vertex fit. The W boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The charmed mesons reconstructed are D^± → K^∓π^±π^± and D^∗± → D⁰π^± → (K^∓π^±)π^± and the charge conjugate decays in the fiducial regions where p_T​(e,μ) > 30 GeV, ∣η(e,μ)∣ < 2.5∣η(e,μ)∣ < 2.5, p_T(D) > 8 GeV, and ∣η(D)∣ < 2.2. The integrated and normalized differential cross sections as a function of the pseudorapidity of the lepton from the W boson decay, and of the transverse momentum of the charmed hadron, are extracted from the data using a profile likelihood fit. The measured total fiducial cross sections are σ_fid^OS−SS​(W⁻ + D⁺) = 50.2 ± 0.2(stat.)_−2.3^+2.4​(syst.) pb, σ_fid^OS−SS(W⁺ + D⁻) = 48.5 ± 0.2 (stat.)_−2.2^+2.3 (syst.) pb, σ_fid^OS−SS(W⁻ + D^∗+) = 51.1 ± 0.4 (stat.)_−1.8^+1.9 (syst.) pb, and σ_fid^OS−SS(W⁺ + D^∗−) = 50.0 ± 0.4 (stat.)_−1.8^+1.9(syst.) pb. . Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anticharm production cross sections is studied to probe the s - s ¯ quark asymmetry. The ratio is found to be R_c^± = 0.971 ± 0.006 ( stat ) ± 0.011 ( syst ) . The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric s - s ¯ sea, indicating that any s - s ¯ asymmetry in the Bjorken- x region relevant for this measurement is small.