We report the observation of $B^{-}\to J/\psi \Lambda \overline{p}$ and searches for $B^{-}\to J/\psi {\Sigma }^0\overline{p}$ and $B^0\to J/\psi p\overline{p}$ decays, using a sample of $275\times {10}^6$ $B\overline{B}$ pairs collected with the Belle detector at the ${\rm Y}(4S)$ resonance. We observe a signal of $17.2\pm 4.1$ events with a significance of $11.1\sigma$ and obtain a branching fraction of $\mathcal{B}(B^{-}\to J/\psi \Lambda \overline{p})=11.6\pm 2.8(\mathrm{stat}{)}_{-2.3}^{+1.8}(\mathrm{sys})\times {10}^{-6}$. No signal is found for either of the two decay modes, $B^{-}\to J/\psi {\Sigma }^0\overline{p}$ and $B^0\to J/\psi p\overline{p}$, and upper limits for the branching fractions are determined to be $\mathcal{B}(B^{-}\to J/\psi {\Sigma }^0\overline{p})<1.1\times {10}^{-5}$ and $\mathcal{B}(B^0\to J/\psi p\overline{p})<8.3\times {10}^{-7}$ at 90% confidence level.

• Received 4 August 2005

DOI:https://doi.org/10.1103/PhysRevD.72.051105