Observation of Cabibbo Suppressed<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">B</mml:mi><mml:mi/><mml:mo>→</mml:mo><mml:mi/><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">D</mml:mi></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mi>*</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">K</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>Decays at Belle

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