Observation of Cabibbo SuppressedB→D(*)K−Decays at Belle

We report observations of the Cabibbo suppressed decays $B\to D^{(*)}{K}^{-}$ using a $10.4{\mathrm{fb}}^{-1\mathrm{}}$ data sample accumulated at the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ storage ring. We find that the ratios of Cabibbo suppressed to Cabibbo favored branching fractions are $B(B^{-}\to D^0{K}^{-})/B(B^{-}\to D^0{\pi }^{-})=0.079\mathrm{}\pm 0.009\pm 0.006$, $B({\overline{B}}^0\to D^{+}{K}^{-})/B({\overline{B}}^0\to D^{+}{\pi }^{-})=0.068\mathrm{}\pm 0.015\pm 0.007$, $B(B^{-}\to D^{*0\mathrm{}}{K}^{-})/B(B^{-}\to D^{*0\mathrm{}}{\pi }^{-})=0.078\mathrm{}\pm 0.019\pm 0.009$, and $B({\overline{B}}^0\to D^{*+}{K}^{-})/B({\overline{B}}^0\to D^{*+}{\pi }^{-})=0.074\mathrm{}\pm 0.015\pm 0.006$. These are the first observations of the $B\to D^{+}{K}^{-}$, $D^{*0\mathrm{}}{K}^{-}$, and $D^{*+}{K}^{-}$ decay processes.