Study of the suppressed B meson decay B-→DK-, D→K+π-

We report a study of the suppressed B meson decay B-→DK- followed by D→K+π-, where D indicates a D0 or D0 state. The two decay paths interfere and provide information on the CP-violating angle φ3. We use a data sample containing 657x106 BB pairs recorded at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- storage ring. We do not find significant evidence for the mode B-→DK-, D→K+π-, and set an upper limit of rB<0.19, where rB is the magnitude of the ratio of amplitudes |A(B-→D0K-)/A(B-→D0K-)|. The decay B-→Dπ-, D→K+π- is also analyzed as a reference, for which we observe a signal with 6.6σ significance, and measure the charge asymmetry ADπ to be -0.02-0.16+0.15(stat)±0.04(syst). In addition, the ratio B(B-→D0K-)/B(B-→D0π-) is measured to be [6.77±0.23(stat)±0.30(syst)]x10-2.