Observation of the Very Rare Σ +

The first observation of the Σ + → p μ + 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 + → p μ + 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.