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

Abstract

The first observation of the ${\Sigma }^{+}\to p{\mu }^{+}{\mu }^{-}$ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$, collected with the LHCb detector at a centre-of-mass energy of 13~TeV. A yield of $237\pm 16$ ${\Sigma }^{+}\to p{\mu }^{+}{\mu }^{-}$ 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.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/3835/ (LHCb public pages)

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