First Observation of the Decays ${\mathit{B}}^0\to {\mathit{D}}^{*-}\mathit{p}\overline{p}{\mathit{\pi }}^{+}$ and ${\mathit{B}}^0\to {\mathit{D}}^{*-}\mathit{p}\overline{n}$

We report the first observation of exclusive decays of the type $B\to D^{*}N\overline{N}X$, where $N$ is a nucleon. Using a sample of $9.7\times {10}^{6}B\overline{B}$ pairs collected with the CLEO detector operating at the Cornell Electron Storage Ring, we measure the branching fractions $B(B^0\to D^{*-}p\overline{p}{\pi }^{+})=({6.5}_{-1.2\mathrm{}}^{+1.3\mathrm{}}\pm 1.0)\times {10}^{-4\mathrm{}}$ and $B(B^0\to D^{*-}p\overline{n})=({14.5}_{-3.0\mathrm{}}^{+3.4\mathrm{}}\pm 2.7)\times {10}^{-4\mathrm{}}$. Antineutrons are identified by their annihilation in the CsI electromagnetic calorimeter.