Search

The radiativeS+ ? np+? decay mode was analysed in the 81 cm SACLAY hydrogen bubble chamber exposed to a stoppingK- beam at the CERN PS. The pion spectrum was measured up to momenta of 150 MeV/c in theS rest frame.

A sample of 1.2× 106Λ-hyperons was analyzed in order to detect the rare decay mode: Λ →pμ−¯v. The Λ-hyperons were produced by stoppingK−-mesons in the 81 cm Saclay hydrogen bubble chamber at the CERN PS. We obtained for the branching ratio: $$\frac{{\Gamma {\text{(}}\Lambda \to p\mu ^ - \bar \nu )}}{{\Gamma {\text{(}}\Lambda \to {\text{all)}}}} = (1.4 \pm 0.5) \times 10^{ - 4} ,$$ based on 20 events, of which 6 have to be attributed to the background from the reaction $$\Lambda \to p\pi ^ - , \pi ^ - \to \mu ^ - \bar v.$$ The background was determined by a Monte Carlo calculation.

In a stoppingK−exposure of the Saclay 81 cm hydrogen bubble chamber, we have obtained a sample of 352 kinematically determined Λ→pe−¯vdacays. From the electron-neutrino angular distribution of these events, we find the ratio of the axial vector to vector coupling constants ¦g1/f1¦=0.74−0.09+0.12. This result is in good agreement with Cabibbo's theory. The method of analysis is insensitive to the relative strength of weak magnetism.

The decay modesΣ±→nπ±γ, Σ+→pγ,Σ+→pe+e}- were studied in the 81 cm Saclay hydrogen bubble chamber. In the radiative decayΣ±→nπ±γ only low momentum pions which stop in the chamber were accepted.

The ratio of the axial-vector to vector coupling constant, ¦g1/f1¦, in theΣ−→ne− $$\bar v$$ decay was determined from the electron-neutrino angular distribution. In a sample of 454Σ−→ne− $$\bar v$$ decays a search was made for subsequent neutron proton scattering in order to detect the decay neutrons. TheΣhyperons were produced by stoppingK−-mesons in the 81 cm Saclay hydrogen bubble chamber. For the kinematical analysis 33Σ−→ne−νevents with associatednp→npinteractions were accepted. The value ¦g1/f1¦=0.37−0.19+0.26was obtained.

Approximately 106Σ±decays of the stopping K−film (80 cm Saclay HBC) have been measured with the Heidelberg KONRAD-system1. This system is capable of detecting about 75% of the Σ−→ ne−¯vdecays. In addition, about 106pictures have been scanned for Σ±leptonic decays with secondary particle momenta below 80 MeV/c. This covers about 15% of the lepton spectrum. The data yield the following branching ratios: (1) $$\frac{{\Gamma \left( {\sum ^ - \to ne^ - \bar v} \right)}}{{\Gamma \left( {\sum ^ - \to \pi ^ - n} \right)}} = \left( {1.02 \pm 0.08} \right) \times 10^{ - 3} ,$$ (2) $$\frac{{\Gamma \left( {\sum ^ - \to n\mu ^ - \bar v} \right)}}{{\Gamma \left( {\sum ^ - \to \pi ^ - n} \right)}} = \left( {0.43 \pm 0.06} \right) \times 10^{ - 3} .$$ Since only a part of the lepton spectra is detected, the branching ratios depend slightly on the form of the transition matrix elements. The values quoted above were derived taking the Cabibbo solution2g1/f1=−0.24 and the CVC prediction f2/f1=−1.143. The branching ratios are in good agreement withμ, euniversality.

In a sample of 106Σ+hyperons we have searched for the decaysσ+→ ne+νandσ+→nμ+ν, which would violate theΔQ=ΔSrule. NoΣ+→ne+νdecay was found, whereas 313σ−→ne−νdecays were identified in the same film. With 90% confidence we obtain $$\frac{{\Gamma (\sum ^ + \to ne^ + v)}}{{\Gamma (\sum ^ - \to ne^ - \bar v)}} \leqq 3.10^{ - 2} .$$ Having found two candidates for the decayσ+→nμ+νand 104Σ−→nμ−νevents we obtain for the muonic mode $$\frac{{\Gamma (\sum ^ + \to ne^ + v)}}{{\Gamma (\sum ^ - \to ne^ - \bar v)}} \leqq 3.10^{ - 2} .$$ The probability that theμ+-events are simulated byσ+→nπ+γwith subsequent undetectedπ+→μ+νdecay is less than 4%.

From about 6×106stoppingK−-meson reactions in the Saclay 80 cm hydrogen bubble chamber 70Σ−→Λe−¯ν and 11Σ+→Λe+ν decays have been identified. The ratio of the vector to axial vector coupling constant has been determined,f1/g1=−0.22±0.28, consistent with the predictionf1=0 of the CVC-theory. The branching ratios for the two decay modes are,Γ(Σ−→Λe−ν)/Γ(Σ−→ all modes)=(0.69±0.12). 10−4,Γ(Σ+→Λe+ν)/Γ(Σ+→ all modes)=(0.29±0.10). 10−4. The ratio, Γ(Σ+→Λe+)/Γ(Σ−→Λe−v)=0.69±0.18, agrees with the phase space ratio, 0.61, therefore a theory that includes no second class currents is consistent with the data.

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+Νdecay produces no deflection (θ<0.1 rad) and no significant change in curvature. Using thep-wave radiative decay predictions ofBarshayet al. we calculate that the integrated branching ratio for such “accidental” events isγ(σ+→nγ(π+→Μstop+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 6.5×10−2events 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+Νevent reported byBarbaro-Galtieriet al. and theσ+→ne+Νevent reported byNauenberget al., one obtains theδS=−δQleptonic branching ratio [γ(σ+→nΜ+Ν)+γ(σ+→ne+Ν)]/γ(σ+→nπ+)=(4±3)×10−5.

In a bubble chamber experiment about 2×106S±-decays have been measured to separateS±?ne±¯? events from the two-body modes. NoS+?ne+? event was found whereas 607S-?ne-¯? decays could be identified. The data yield for the ?Q=-?S decay an upper limit: $$\frac{{\Gamma \left( {\sum {^ + } \to ne^ + v} \right)}}{{\Gamma \left( {\sum {^ - } \to ne^ - v} \right)}}< 1.9 x 10^{ - 2} (90\% confidence level)$$ and the branching ratio: $$\frac{{\Gamma \left( {\sum {^ - } \to ne^ - v} \right)}}{{\Gamma \left( {\sum {^ - } \to n\pi ^ - } \right)}} = (1.09 \pm 0.06) x 10^{ - 3} .$$