First Observation of the Rare Purely Baryonic Decay B0 → p anti-p

The first observation of the decay of a B0 meson to a purely baryonic final state, B0 → p anti-p, is reported. The proton-proton collision data sample used was collected with the LHCb experiment at center-of-mass energies of 7 and 8 TeV and corresponds to an integrated luminosity of 3.0 fb^−1. The branching fraction is determined to be B(B0 → p anti-p) = (1.25 ± 0.27 ± 0.18) × 10^−8, where the first uncertainty is statistical and the second systematic. The decay mode B0 → p anti-p is the rarest decay of the B0 meson observed to date. The decay B0s → p anti-p is also investigated. No signal is seen and the upper limit B(B0s → p anti-p) < 1.5 × 10^−8 at 90% confidence level is set on the branching fraction.