Measurement of the relative branching fraction Γ(D0→Kμν)/Γ(D0→μX)

The fraction f of ${\mathit{D}}^0$ semimuonic decays which occur through the Kμν mode has been measured in a hybrid emulsion spectrometer. Analysis of 124 semimuonic ${\mathit{D}}^0$-decay candidates gives f=0.32±0.05 (stat) ±0.05 (syst). From this measurement and existing data on the ${\mathit{D}}^0$ semileptonic branching ratio and lifetime, we obtain the branching ratio R(${\mathit{D}}^0$→${\mathit{K}}^{\mathrm{-}}$ ${\mathrm{\mu }}^{+}$ν)=(2.4±0.4±0.5)% and partial decay rate Γ(${\mathit{D}}^0$→${\mathit{K}}^{\mathrm{-}}$ ${\mathrm{\mu }}^{+}$ν)=(5.6±0.9±1.2)×${10}^{10}$ ${\mathrm{s}}^{\mathrm{-}1}$.