First Observation of a Baryonic B0s Decay

We report the first observation of a baryonic B0s decay, B0s → p anti-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(B0s → p anti-Λ K−) + B(B0s → anti-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 B0 → p anti-Λ π− decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of b-quark hadronization probabilities f_s/f_d.