Observation of the very rare Σ⁺ → 𝑝⁢𝜇⁺𝜇^- decay

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.