Observation of B- -> J/psi Lambda pbar and searches for B- -> J/psi Sigma0 pbar and B0 -> J/psi p pbar Decays

We report the observation of B- -> J/psi Lambda pbar and searches for B- -> J/psi Sigma0 pbar and B0 -> J/psi p pbar decays, using a sample of 275 million BBbar pairs collected with the Belle detector at the Upsilon(4S) resonance. We observe a signal of 17.2 +/- 4.1 events with a significance of 11.1sigma and obtain a branching fraction of B(B- -> J/psi Lambda pbar) = [11.6 +/- 2.8 (stat.) +1.8 -2.3(sys.)] x 10^-6. No signal is found for either of the two decay modes, B- -> J/psi Sigma0 pbar and B0 -> J/psi p pbar, and upper limits for the branching fractions are determined to be B(B- -> J/psi Sigma0 pbar) < 1.1 x 10^{-5} and B(B0 -> J/psi p pbar) < 8.3 x 10^{-7} at 90% confidence level.