Observation of Exclusive $\mathit{B}$ Decays to Final States Containing a Charmed Baryon

Using data collected in the region of the $\Upsilon \left(4S\right)$ resonance with the CLEO-II detector, we report on the first observation of exclusive decays of the $B$ meson to final states with a charmed baryon. We have measured the branching fractions $\mathcal{B}(B^{-}\to {\Lambda }_c^{+}\overline{p}{\pi }^{-})=({0.62}_{-0.20\mathrm{}}^{+0.23\mathrm{}}\pm 0.11\pm 0.10)\times {10}^{-3\mathrm{}}$ and $\mathcal{B}({\overline{B}}^0\to {\Lambda }_c^{+}\overline{p}{\pi }^{+}{\pi }^{-})=({1.33}_{-0.42\mathrm{}}^{+0.46\mathrm{}}\pm 0.31\pm 0.21)\times {10}^{-3\mathrm{}}$, where the first error is statistical, the second is systematic, and the third is due to uncertainty in the ${\Lambda }_c^{+}$ branching fractions. In addition, we report upper limits for final states of the form $\overline{B}\to {\Lambda }_c^{+}\overline{p}\left(n\pi \right)$ and ${\Lambda }_c^{+}\overline{p}\left(n\pi \right){\pi }^0$, where $\left(n\pi \right)$ denotes up to four charged pions.