Observation of the Very Rare Σ^{+}→pμ^{+}μ^{-} Decay

The first observation of the Σ^{+}→pμ^{+}μ^{-} decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of 5.4  fb^{-1}, collected with the LHCb detector at a center-of-mass energy of 13 TeV. A yield of 237±16  Σ^{+}→pμ^{+}μ^{-} decays is obtained, where the uncertainty is statistical only. A branching fraction of (1.08±0.17)×10^{-8} is measured, where the uncertainty includes statistical and systematic sources. No evidence of resonant structures is found in the dimuon invariant-mass distribution. All results are compatible with standard model expectations. This represents the rarest decay of a baryon ever observed.