Observation of $B^0\to p\bar{\mathrm{\Lambda }}{D}^{(\ast )-}$

We report the first observation of the decays $B^0 \rightarrow p\bar{\Lambda} D^{()-}$\mathrm{.}Thedatasampleof$711$fb$^{-1}$ used in this analysis corresponds to $772$ million $B\bar{B}$ pairs, collected at the $\mathrm{{\rm Y}}(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We observe $19.8\sigma$ and $10.8\sigma$ excesses of events for the two decay modes and measure the branching fractions of $B^0 \rightarrow p\bar{\Lambda} D^{-}$and$B^0 \rightarrow p\bar{\Lambda} D^{-}$tobe$(25.1\pm2.6\pm3.5)\times10^{-6}$and$(33.6\pm6.3\pm4.4)\times10^{-6}$, respectively, where the first uncertainties are statistical and the second are systematic. These results are not compatible with the predictions based on the generalized factorization approach. In addition, a threshold enhancement in the di-baryon ($p\bar{\mathrm{\Lambda }}$) system is observed, consistent with that observed in similar $B$ decays.