Evidence for the suppressed decay B^- -> DK^-, D -> K^+pi^-pi^0

We report a study of the suppressed decay B^- -> DK^-, D -> K^+pi^-pi^0, where D denotes either a D^0 or a bar{D}^0 meson. The decay is sensitive to the CP-violating parameter phi_3. Using a data sample of 772 x 10^6 Bbar{B} pairs collected at the Upsilon(4S) resonance with the Belle detector, we measure the ratio of branching fractions of the above suppressed decay to the favored decay B^- -> DK^-, D -> K^-pi^+pi^0. Our result is R_{DK} = [1.98 +/- 0.62(stat.) +/- 0.24(syst.)] x 10^{-2}, which indicates the first evidence of the signal for this suppressed decay with a significance of 3.2 standard deviations. We measure the direct CP asymmetry between the suppressed B^- and B^+ decays to be A_{DK} = 0.41 +/- 0.30 (stat.) +/- 0.05 (syst.). We also report measurements for the analogous quantities R_{Dpi} and A_{Dpi} for the decay B^- -> Dpi^-, D -> K^+pi^-pi^0.