Observation of B+→Λc+Λc-K+ and B0→Λc+Λc-K0 decays

We report the first measurements of the doubly charmed baryonic B decays B ->Lambda(+)(c)Lambda K--(c). The B+->Lambda(+)(c)Lambda K--(c)+ decay is observed with a branching fraction of (6.5(-0.9)(+1.0)+/- 1.1 +/- 3.4)x10(-4) and a statistical significance of 15.4 sigma. The B-0 ->Lambda(+)(c)Lambda K--(c)0 decay is observed with a branching fraction of (7.9(-2.3)(+2.9)+/- 1.2 +/- 4.1)x10(-4) and a statistical significance of 6.6 sigma. The branching fraction errors are statistical, systematic, and the error resulting from the uncertainty of the Lambda(+)(c)-> pK(-)pi(+) decay branching fraction. The analysis is based on 357 fb(-1) of data accumulated at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider.