Observation of the very rare ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ decay

The first observation of the ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of $5.4{\mathrm{f}\mathrm{b}}^{-1}$, collected with the LHCb detector at a centre-of-mass energy of 13~TeV. A yield of $237\pm 16$ ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ decays is obtained, where the uncertainty is statistical only. A branching fraction of $(1.08\pm 0.17)\times {10}^{-8}$ 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.