Observation of the Decay Ξb− → p K− K−

Decays of the Ξb− and Ωb− baryons to the charmless final states p h− h'−, where h(') denotes a kaon or pion, are searched for with the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies √s = 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb^−1. The decay Ξb− → p K− K− is observed with a significance of 8.7 standard deviations, and evidence at the level of 3.4 standard deviations is found for the Ξb− → p K− π− decay. Results are reported, relative to the B− → K+ K− K− normalization channel, for the products of branching fractions and b-hadron production fractions. The branching fractions of Ξb− → p K− π− and Ξb− → p π− π− relative to Ξb− → p K− K− decays are also measured.