Cabibbo-suppressed decays $B \to D^{ (*)} K^-$ using a 10.4 fb$^{ -1}$ data sample accumulated at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ storage ring. The high-momentum particle identification system of Belle is used to isolate signals for $B\to D^0 K^-$, $D^+K^-$, $D^{ *0}K^-$ and $D^{ *+}K^-$ from the $B\to D^{ (*)}\pi^-$ decay processes which have much larger branching fractions. We report ratios of Cabibbo-suppressed to Cabibbo-favored branching fractions of: ${ \cal B}(B^- \to D^0 K^-)/{ \cal B}(B^- \to D^0\pi^-) = 0.079\pm0.009\pm0.006$; ${ \cal B}(\bar{ B^0} \to D^+ K^-)/{ \cal B}(\bar{ B^0} \to D^+\pi^-) = 0.068\pm0.015\pm0.007$; ${ \cal B}(B^-\to D^{ *0}K^-)/{ \cal B}(B^-\to D^{ *0}\pi^-) = 0.078 \pm 0.019 \pm 0.009$; and ${ \cal B}(\bar{ B}^0\to D^{ *+}K^-)/{ \cal B}(\bar{ B}^0\to D^{ *+}\pi^-)= 0.074 \pm 0.015 \pm 0.006$. The first error is statistical and the second is systematic. These are the first reported observations of the $B\to D^+K^-$, $D^{ *0}K^-$ and $D^{ *+}K^-$ decay processes.
Categories: hep-ex