Possible violation of the δ S =δ Q selection rule in leptonic decays of σ + -hyperons

We have found a possible example of the rare decayσ+→nΜ+Ν, which violatesδS=δQ. The positive decay track of theσ+ comes to rest in the hydrogen bubble chamber and decays into ane+. This track has all the characteristics of a stoppingΜ+. The decay neutron fortuitously scatters twice, producing two recoil protons. The only other possible interpretation of the event isσ+→nγ(π+→Μ+Ν), where theπ+→Μ+Ν decay produces no deflection (θ<0.1 rad) and no significant change in curvature. Using thep-wave radiative decay predictions ofBarshay et al. we calculate that the integrated branching ratio for such “accidental” events isγ(σ+→nγ(π+→Μ stop + Ν))/γ(σ+→nπ+)=1.6×10−6.