Measurement of the Branching Fractions of B → D(*)K− K_(S)^(∗)0 and B → D(*) D_s^- Decays at Belle II

We present measurements of the branching fractions of eight B^0 → D(*)+K− K_(S)^(∗)0 , B− → D(*)0K− K_(S)^(∗)0 decay channels. The results are based on data from SuperKEKB electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector, corresponding to an integrated luminosity of 362 fb−1. The event yields are extracted from fits to the distributions of the difference between expected and observed B meson energy, and are efficiency-corrected as a function of m(K− K_(S)^(∗)0 ) and m(D(*) K_(S)^(∗)0 ) in order to avoid dependence on the decay model. These results include the first observation of B^0 → D+K− K_S^0 , B− → D*0K− K_S^0 , and B^0 → D*+K− K_S^0 decays and a significant improvement in the precision of the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of the K− K_(S)^(∗)0 systems are compatible with quasi-two-body decays via a resonant transition with spin-parity JP = 1− for the K− K_S^0 systems and JP = 1+ for the K−K*0 systems. We also present measurements of the branching fractions of four B^0 → D(*)+ D_s^- , B− → D(*)0 D_s^- decay channels with a precision compatible to the current world averages.