First observation of the rare purely baryonic decay $B^0\to p\bar{p}$

The first observation of the decay of a $B^0$ meson to a purely baryonic final state, $B^0\to p\bar{p}$, 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 $$\mathcal{B}(B^0\to p\bar{p})=(1.25\pm 0.27\pm 0.18)\times {10}^{-8},$$ where the first uncertainty is statistical and the second systematic. The decay mode $B^0\to p\bar{p}$ is the rarest decay of the $B^0$ meson observed to date. The decay $B_s^0 \to p \bar{p}$isalsoinvestigated\mathrm{.}Nosignalisseenandtheupperlimit${\cal B}(B_s^0 \to p \bar{p}) < 1.5 \times 10^{-8}$ at 90% confidence level is set on the branching fraction.