Observation ofB+→ΛΛ¯K+

We report the first observation of the charmless hyperonic $B$ decay, $B^{+}\to \Lambda \overline{\Lambda }{K}^{+}$, using a $140{\mathrm{f}\mathrm{b}}^{-1}$ data sample recorded at the ${\rm Y}(4\mathrm{S})$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The measured branching fraction is $\mathcal{B}(B^{+}\to \Lambda \overline{\Lambda }{K}^{+})=({2.91}_{-0.70}^{+0.90}\pm 0.38)\times {10}^{-6}$. We also perform a search for the related decay mode $B^{+}\to \Lambda \overline{\Lambda }{\pi }^{+}$, but do not find a significant signal. We set a 90% confidence-level upper limit of $\mathcal{B}(B^{+}\to \Lambda \overline{\Lambda }{\pi }^{+})<2.8\times {10}^{-6}$.