Observation of the baryonic B decay B[over ̄]^{0}→Λ_{c}^{+}Λ[over ̄]K^{-}

We report the observation of the baryonic B decay B̅ 0→Λc+Λ̅ K- with a significance larger than 7 standard deviations based on 471×106 BB̅ pairs collected with the BABAR detector at the PEP-II storage ring at SLAC. We measure the branching fraction for the decay B̅ 0→Λc+Λ̅ K- to be (3.8±0.8stat±0.2sys±1.0Λc+)×10-5. The uncertainties are statistical, systematic, and due to the uncertainty in the Λc+ branching fraction. We find that the Λc+K- invariant-mass distribution shows an enhancement above 3.5 GeV/c2.