Observation of charmless baryonic decays B₍⁰_s) → pph⁺h⁰⁻

Decays of B⁰ and B_s⁰ mesons to the charmless baryonic final states pph⁺h⁰⁻, where h and h⁰ 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 B_s⁰ decays are observed for the first time. The decays B_s⁰ → ppK⁺K⁻, B_s⁰ → ppK^±π^∓, B⁰ → ppK^±π^∓ and B⁰ → ppπ⁺π⁻ are observed with a significance greater than 5 standard deviations; evidence at 4.1 standard deviations is found for the B⁰ → ppK⁺K⁻ decay and an upper limit is set on the branching fraction for B_s⁰ → ppπ⁺π⁻. Branching fractions in the kinematic region m(pp) < 2850 MeV/c² are measured relative to the B⁰ → J/ψ(→ pp)K^∗(892)⁰ channel.