Evidence for the rare decay ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$

A search for the rare decay ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ is performed using $pp$ collision data recorded by the LHCb experiment at centre-of-mass energies $\sqrt{s}=7$ and $8$ TeV, corresponding to an integrated luminosity of $3 fb^{-1}$. An excess of events is observed with respect to the background expectation, with a signal significance of 4.1 standard deviations. No significant structure is observed in the dimuon invariant mass distribution, in contrast with a previous result from the HyperCP experiment. The measured ${\mathrm{\Sigma }}^{+}\to p{\mu }^{+}{\mu }^{-}$ branching fraction is $(2.2{}_{-1.3}^{+1.8})\times {10}^{-8}$, where statistical and systematic uncertainties are included, which is consistent with the Standard Model prediction.