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 decay σ →nΜ Ν, which violates δS=δQ. The positive decay track of the σ comes to rest in the hydrogen bubble chamber and decays into an e. This track has all the characteristics of a stopping Μ. The decay neutron fortuitously scatters twice, producing two recoil protons. The only other possible interpretation of the event is σ→ nγ(π →Μ Ν), where the π →Μ Ν decay produces no deflection ( θ<0.1 rad) and no significant change in curvature. Using the p-wave radiative decay predictions of Barshay et al. we calculate that the integrated branching ratio for such 'accidental' events is γ(σ →nγ(π →Μ Ν))/γ(σ →nπ)=1.6×10. Most of the contribution to this 'accidental' branching ratio comes from radiative decays where the π mesons have ranges less than 1 mm ( p<20 MeV/ c). If one excludes those Μ's with ranges less than 1.2 cm the above 'accidental' branching ratio becomes 5.5×10. With this figure we estimate that we should have seen 6.5×10 events of this type thusfar in our experiment. The neutron momentum does not help in deciding between the two hypotheses. We therefore assign a confidence level of 7% for the radiative hypothesis. For the leptonic hypothesis we obtain an estimate of the branching ratio, γ(σ →nΜ Ν)/γ(σ →nπ)=5×10. If one further accepts the σ →nπ Ν event reported by Barbaro-Galtieri et al. and the σ →ne Ν event reported by Nauenberg et al., one obtains the δS=−δQ leptonic branching ratio [ γ(σ →nΜ Ν)+γ(σ →ne Ν)]/γ(σ →nπ)=(4±3)×10. [ABSTRACT FROM AUTHOR]