First Observation of a Baryonic Bs0 Decay

We report the first observation of a baryonic Bs0 decay, Bs0→pΛK-, 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(Bs0→pΛK-)+B(Bs0→pΛK+)=[5.46±0.61±0.57±0.50(B)±0.32(fs/fd)]×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→pΛπ- decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of b-quark hadronization probabilities fs/fd.