Observation of the Production of aWBoson in Association with a Single Charm Quark

The first observation of the production of a $W$ boson with a single charm quark ($c$) jet in $p\overline{p}$ collisions at $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ is reported. The analysis uses data corresponding to $4.3\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, recorded with the CDF II detector at the Fermilab Tevatron. Charm quark candidates are selected through the identification of an electron or muon from charm-hadron semileptonic decay within a hadronic jet, and a $Wc$ signal is observed with a significance of 5.7 standard deviations. The production cross section ${\ensuremath{\sigma}}_{Wc}({p}_{Tc}g20\text{ }\text{ }\mathrm{GeV}/c,|{\ensuremath{\eta}}_{c}|l1.5)\ifmmode\times\else\texttimes\fi{}B(W\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu})$ is measured to be ${13.6}_{\ensuremath{-}3.1}^{+3.4}\text{ }\text{ }\mathrm{pb}$ and is in agreement with theoretical expectations. From this result the magnitude of the quark-mixing matrix element ${V}_{cs}$ is derived, $|{V}_{cs}|=1.08\ifmmode\pm\else\textpm\fi{}0.16$ along with a lower limit of $|{V}_{cs}|g0.71$ at the 95% confidence level, assuming that the $Wc$ production through $c$ to $s$ quark coupling is dominant.