| Summary: | The first observation of the production of a W boson with a single charm quark (c) jet in pp̅ collisions at √s=1.96 TeV is reported. The analysis uses data corresponding to 4.3 fb[superscript -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 σ[subscript Wc](p[subscript Tc]>20 GeV/c,|η[subscript c]|<1.5)×B(W→ℓν) is measured to be 13.6[subscript -3.1][superscript +3.4] pb and is in agreement with theoretical expectations. From this result the magnitude of the quark-mixing matrix element V[subscript cs] is derived, |V[subscript cs]|=1.08±0.16 along with a lower limit of |V[subscript cs]|>0.71 at the 95% confidence level, assuming that the Wc production through c to s quark coupling is dominant.
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