Study of the Suppressed Decays B^- to [K^+π^-]_D K^- and B^- to [K^+π^-]_D π^-

We report a study of the suppressed decays $B^- \to [K^+\pi^-]_D K^-$ and $B^- \to [K^+\pi^-]_D \pi^-$, where $[K^+\pi^-]_D$ indicates that the $K^+\pi^-$ pair originates from a neutral $D$ meson. These decay modes are sensitive to the Unitarity Triangle angle $\phi_3$. We use a data sample containing 275 million $B\bar{B}$ pairs recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric $e^+e^-$ storage ring. The signal for $B^- \to [K^+\pi^-]_D K^-$ is not statistically significant, and we set a limit $r_B < 0.27$ at 90% confidence level, where $r_B$ is the magnitude of the ratio of amplitudes $| A(B^- \to \bar{D}^0K^-) / A(B^- \to D^0K^-) |$. We observe a signal with $6.4\sigma$ statistical significance in the related mode, $B^- \to [K^+\pi^-]_D \pi^-$.