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

The production of a $W$ boson in association with a single charm quark is studied using 140 ${\mathrm{f}\mathrm{b}}^{-1}$ of $\sqrt{s}=13\mathrm{T}\mathrm{e}\mathrm{V}$ proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by a charmed hadron, reconstructed with a secondary-vertex fit. The $W$ boson is reconstructed from an electron/muon decay and the missing transverse momentum. The mesons reconstructed are $D^{\pm }\to K^{\mp }{\pi }^{\pm }{\pi }^{\pm }$ and $D^{\ast \pm }\to D^0{\pi }^{\pm }\to (K^{\mp }{\pi }^{\pm }){\pi }^{\pm }$, where $p_{\text{T}}(e,\mu )>30\mathrm{G}\mathrm{e}\mathrm{V}$, $\mathrm{\mid }\eta (e,\mu )\mathrm{\mid }<2.5$, $p_{\text{T}}(D)>8\mathrm{G}\mathrm{e}\mathrm{V}$, and $\mathrm{\mid }\eta (D)\mathrm{\mid }<2.2$. 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 meson, are extracted from the data using a profile likelihood fit. The measured fiducial cross-sections are ${\sigma }_{\mathrm{f}\mathrm{i}\mathrm{d}}^{\mathrm{O}\mathrm{S}-\mathrm{S}\mathrm{S}}(W^{-}+D^{+})=50.2\pm 0.2(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}){}_{-2.3}^{+2.4}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{b}$, ${\sigma }_{\mathrm{f}\mathrm{i}\mathrm{d}}^{\mathrm{O}\mathrm{S}-\mathrm{S}\mathrm{S}}(W^{+}+D^{-})=48.5\pm 0.2(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}){}_{-2.2}^{+2.3}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{b}$, ${\sigma }_{\mathrm{f}\mathrm{i}\mathrm{d}}^{\mathrm{O}\mathrm{S}-\mathrm{S}\mathrm{S}}(W^{-}+D^{\ast +})=51.1\pm 0.4(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}){}_{-1.8}^{+1.9}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{b}$, and ${\sigma }_{\mathrm{f}\mathrm{i}\mathrm{d}}^{\mathrm{O}\mathrm{S}-\mathrm{S}\mathrm{S}}(W^{+}+D^{\ast -})=50.0\pm 0.4(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}){}_{-1.8}^{+1.9}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{b}$. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. The ratio of charm to anti-charm production cross-sections is studied to probe the $s$-$\bar{s}$ quark asymmetry and is found to be $R_c^{\pm }=0.971\pm 0.006(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.011(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$.