Measurement of the decay Bs0→Ds-π+ and evidence for Bs0→Ds K± in e+e- annihilation at s 10.87GeV

We have studied Bs0→Ds-π+ and Bs0→Ds K± decays using 23.6fb-1 of data collected at the Υ(5S) resonance with the Belle detector at the KEKB e+e- collider. This highly pure Bs0→Ds-π+ sample is used to measure the branching fraction, B(Bs0→Ds-π+)=[3.67-0.33+0.35(stat)-0. 42+0.43(syst)±0.49(fs)]×10-3 (fs=NBs()B̄s()/ Nbb̄) and the fractions of Bs0 event types at the Υ(5S) energy, in particular NBsB̄s/NBs()B̄s()=(90.1-4.0+3. 8±0.2)%. We also determine the masses M(Bs0)=(5364.4±1.3±0. 7)MeV/c2 and M(Bs*)=(5416.4±0.4±0.5)MeV/c2. In addition, we observe Bs0→Ds K± decays with a significance of 3.5σ and measure B(Bs0→Ds K±)=[2.4-1.0+1.2(stat)±0.3(syst)±0. 3(fs)]×10-4. © 2009 The American Physical Society.