Observation of $B\to D^{(\ast )}{K}^{-}{K}_S^0$ decays using the 2019-2022 Belle II data sample

We present a measurement of the branching fractions of four $B^{0,-}\to D^{(\ast )+,0}{K}^{-}{K}_S^0$ decay modes. The measurement is based on data from SuperKEKB electron-positron collisions at the $\mathrm{{\rm Y}}(4S)$ resonance collected with the Belle II detector and corresponding to an integrated luminosity of $362{\text{fb}}^{-1}$. The event yields are extracted from fits to the distributions of the difference between expected and observed $B$ meson energy to separate signal and background, and are efficiency-corrected as a function of the invariant mass of the $K^{-}{K}_S^0$ system. We find the branching fractions to be: $$\text{B}(B^{-}\to D^0{K}^{-}{K}_S^0)=(1.89\pm 0.16\pm 0.10)\times {10}^{-4},$$ $$\text{B}(\bar{B}{}^0\to D^{+}{K}^{-}{K}_S^0)=(0.85\pm 0.11\pm 0.05)\times {10}^{-4},$$ $$\text{B}(B^{-}\to D^{\ast 0}{K}^{-}{K}_S^0)=(1.57\pm 0.27\pm 0.12)\times {10}^{-4},$$ $$\text{B}(\bar{B}{}^0\to D^{\ast +}{K}^{-}{K}_S^0)=(0.96\pm 0.18\pm 0.06)\times {10}^{-4},$$ where the first uncertainty is statistical and the second systematic. These results include the first observation of $\bar{B}{}^0\to D^{+}{K}^{-}{K}_S^0$, $B^{-}\to D^{\ast 0}{K}^{-}{K}_S^0$, and $\bar{B}{}^0\to D^{\ast +}{K}^{-}{K}_S^0$ decays and a significant improvement in the precision of $\text{B}(B^{-}\to D^0{K}^{-}{K}_S^0)$ compared to previous measurements.