Measurement of the production of a W boson in association with a charm quark in pp collisions at sqrt(s) $=$ 7 TeV with the ATLAS detector

The production of a $W$ boson in association with a single charm quark is studied using 4.6 fb$^{\mathrm{-1}}$ of \textit{pp} collision data at sqrt(s) $=$ 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a $W$ boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the $W$-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be .96 $^{\mathrm{+0.26}}_{\mathrm{\thinspace -0.30}}$ at $Q^{2\thinspace }=$ 1.9 \textit{GeV }$^{\mathrm{2}}$, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio $\sigma $(W$^{\mathrm{+}}+$c-bar) / $\sigma $(W$^{\mathrm{-}}+$c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the $s-$s-bar quark asymmetry.