Observation of $B^{0} rightarrow pbar{Lambda} D^{(*)-}$

We report the first observation of the decays $B^0 rightarrow pbar{Lambda} D^{(*)-}$. The data sample of $711$ fb$^{-1}$ used in this analysis corresponds to $772$ million $Bbar{B}$ pairs, collected at the $Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We observe $19.8sigma$ and $10.8sigma$ excesses of events for the two decay modes and measure the branching fractions of $B^0 rightarrow pbar{Lambda} D^{-}$ and $B^0 rightarrow pbar{Lambda} D^{*-}$ to be $(25.1pm2.6pm3.5)times10^{-6}$ and $(33.6pm6.3pm4.4)times10^{-6}$, 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 ($pbar{Lambda}$) system is observed, consistent with that observed in similar $B$ decays.