First Observation of a Baryonic B 0 s Decay

decay, B 0 s → p ¯ Λ K − , using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3.0 fb− 1. The branching fraction is measured to be B (B 0 s → p ¯ Λ K − ) + B ( B 0 s → ¯ p Λ K + ) = [ 5.46 ± 0.61 ± 0.57 ± 0.50 (B) ± 0.32 ( f s / f d)] × 10 – 6 , where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the B 0 → p ¯ Λ π − decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of b -quark hadronization probabilities f s / fd.