Measurement of the production of a $W$ boson in association with a charmed hadron in $pp$ collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

The production of a $W$ boson in association with a single charm quark is studied using $140\text{}\text{}{\mathrm{fb}}^{-1}$ of $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ 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 ${D}^{+}\to {K}^{-}{\pi }^{+}{\pi }^{+}$ and ${D}^{*+}\to {D}^{0}{\pi }^{+}\to \left({K}^{-}{\pi }^{+}\right){\pi }^{+}$ and the charge conjugate decays in the fiducial regions where ${p}_{T}\left(e,\mu \right)>30\text{}\text{}\mathrm{GeV}$, $|\eta \left(e,\mu \right)|<2.5$, ${p}_{T}\left({D}^{\left(*\right)}\right)>8\text{}\text{}\mathrm{GeV}$, and $|\eta \left({D}^{\left(*\right)}\right)|<2.2$. 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 ${\sigma }_{\mathrm{fid}}^{\mathrm{OS}–\mathrm{SS}}\left({W}^{-}+{D}^{+}\right)=50.2±0.2{\left(\mathrm{stat}\right)}_{-2.3}^{+2.4}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{pb}$, ${\sigma }_{\mathrm{fid}}^{\mathrm{OS}–\mathrm{SS}}\left({W}^{+}+{D}^{-}\right)=48.5±0.2{\left(\mathrm{stat}\right)}_{-2.2}^{+2.3}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{pb}$, ${\sigma }_{\mathrm{fid}}^{\mathrm{OS}–\mathrm{SS}}\left({W}^{-}+{D}^{*+}\right)=51.1±0.4{\left(\mathrm{stat}\right)}_{-1.8}^{+1.9}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{pb}$, and ${\sigma }_{\mathrm{fid}}^{\mathrm{OS}–\mathrm{SS}}\left({W}^{+}+{D}^{*-}\right)=50.0±0.4{\left(\mathrm{stat}\right)}_{-1.8}^{+1.9}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{pb}$. 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 $s\text{−}\overline{s}$ quark asymmetry. The ratio is found to be ${R}_{c}^{±}=0.971±0.006\left(\mathrm{stat}\right)±0.011\left(\mathrm{syst}\right)$. The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric $s\text{−}\overline{s}$ sea, indicating that any $s\text{−}\overline{s}$ asymmetry in the Bjorken-$x$ region relevant for this measurement is small.