Possible violation of the δS=δQ selection rule in leptonic decays of σ+-hyperons
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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.
Most of the contribution to this “accidental” branching ratio comes from radiative decays where theπ+ mesons have ranges less than 1 mm (pπ<20 MeV/c). If one excludes thoseΜ's with ranges less than 1.2 cm the above “accidental” branching ratio becomes 5.5×10−7. With this figure we estimate that we should have seen 6.5×10−2 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+)=5×10−5.
If one further accepts theσ+→nπ+Ν event reported byBarbaro-Galtieri et al. and theσ+→ne+Ν event reported byNauenberg et al., one obtains theδS=−δQ leptonic branching ratio [γ(σ+→nΜ+Ν)+γ(σ+→ne+Ν)]/γ(σ+→nπ+)=(4±3)×10−5.
KeywordsHydrogen Elementary Particle Confidence Level Selection Rule Radiative Decay
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