Measurements of Branching Fractions for B0 --> Ds+pi- and B0-bar --> Ds+K-

We present improved measurements of the branching fractions for the decays B0 --> Ds+pi- and B0-bar --> Ds+K- using a data sample of 657x10^6 BB-bar events collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The results are BF(B0 --> Ds+pi-) = (1.99 +/- 0.26 +/- 0.18)x10^-5 and BF(B0-bar --> Ds+K-) = (1.91 +/- 0.24 +/- 0.17)x10^-5, where the uncertainties are statistical and systematic, respectively. Based on these results, we determine the ratio between amplitudes of the doubly Cabibbo suppressed decay B0 --> D+pi- and the Cabibbo favored decay B0 --> D-pi+, R_Dpi = [1.71 +/- 0.11(stat) +/- 0.09(syst) +/- 0.02(theo)]%, where the last term denotes the theory error.