Measurement of B⁰ → D^- _s K⁰ _S π⁺ and B⁺ → D^- _sK⁺K⁺ branching fractions

We report a measurement of the B⁰ and B⁺ meson decays to the D^- _sK⁰ _Sπ⁺ and D^- _sK⁺K⁺ final states, respectively, using 657 × 10⁶BB¯ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e⁺ collider. Using the D^- _s → φ π^-, K^∗(892)⁰K^- and K⁰ _SK^- decay modes for the D_s reconstruction, we measure the following branching fractions: B(B⁰ → D^- _sK⁰ _sπ⁺) = [0.47 ± 0.06(stat) ± 0.05(syst)] × 10^-4 and B(B⁺ → D^- _sK⁺K⁺) = [0.93 ± 0.22(stat) ± 0.10(syst)] × 10^-5. We find the ratio of the branching fraction of B⁺ → D^- _sK⁺K⁺ to that of the analogous Cabibbo-favored B⁺ → D^- _sK⁺π⁺ decay to be R_B = 0.054 ± 0.013(stat) ± 0.006(syst), which is consistent with the naive factorization model. We also observe a deviation of the D_sK invariant-mass distribution from the three-body phase-space model for both studied decays.