Observation of Charmless Baryonic Decays B0(s) → p anti-p h+ h'−

Decays of B0 and B0s mesons to the charmless baryonic final states p anti-p h+ h'−, where h and h′ each denote a kaon or a pion, are searched for using the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb^−1. Four-body charmless baryonic B0s decays are observed for the first time. The decays B0s → p anti-p K+ K−, B0s → p anti-p K± π∓, B0 → p anti-p K± π∓ and B0 → p anti-p π+ π− are observed with a significance greater than 5 standard deviations; evidence at 4.1 standard deviations is found for the B0 → p anti-p K+ K− decay and an upper limit is set on the branching fraction for B0s → p anti-p π+ π−. Branching fractions in the kinematic region m(p anti-p) < 2850 MeV/c^2 are measured relative to the B0 → J/ψ (→ p anti-p) K∗(892)0 channel.