We report the first observation of the decay $B^{+}\to {\overline{\Xi }}_c^0{\Lambda }_c^{+}$ with a significance of $8.7\sigma$ and evidence for the decay $B^0\to {\overline{\Xi }}_c^{-}{\Lambda }_c^{+}$ with a significance of $3.8\sigma$. The product $\mathcal{B}(B^{+}\to {\overline{\Xi }}_c^0{\Lambda }_c^{+})\times \mathcal{B}({\overline{\Xi }}_c^0\to {\overline{\Xi }}^{+}{\pi }^{-})$ is measured to be $({4.8}_{-0.9}^{+1.0}\pm 1.1\pm 1.2)\times {10}^{-5}$, and $\mathcal{B}(B^0\to {\overline{\Xi }}_c^{-}{\Lambda }_c^{+})\times \mathcal{B}({\overline{\Xi }}_c^{-}\to {\overline{\Xi }}^{+}{\pi }^{-}{\pi }^{-})$ is measured to be $({9.3}_{-2.8}^{+3.7}\pm 1.9\pm 2.4)\times {10}^{-5}$. The errors are statistical, systematic and the error of the ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ branching fraction, respectively. The decay $B^{+}\to {\overline{\Xi }}_c^0{\Lambda }_c^{+}$ is the first example of a two-body exclusive $B^{+}$ decay into two charmed baryons. The data used for this analysis was accumulated at the ${\rm Y}(4S)$ resonance, using the Belle detector at the $e^{+}{e}^{-}$ asymmetric-energy collider KEKB. The integrated luminosity of the data sample is equal to $357{\mathrm{fb}}^{-1}$, corresponding to $386\times {10}^6$ $B\overline{B}$ pairs.

• Received 27 October 2005

DOI:https://doi.org/10.1103/PhysRevD.74.111105