Measurements of Hyperon Semileptonic Decays at the {CERN} Super Proton Synchrotron. 3. The $\Sigma^- \to n e^-$ Anti-neutrino Decay Mode

The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays ofΣ−,Ξ− andΛ. A magnetic channel selects 100 GeV/c negatively charged particles produced in the forward direction by interaction of the 200 GeV/c SPS proton beam on a BeO target. TheΣ− andΞ− hyperons are concurrently identified in a DISC Cerenkov counter, and their decay products are analysed by a magnetic spectrometer. Electron-hadron discrimination is achieved by the combined use of lead-glass and lead/scintillator counters, transition radiation detectors, and a Cerenkov counter. In this article we report results on the\(\Sigma ^ -\to \Lambda e^ -\bar v\) decay mode. Measurements of the Λ polarization and of the centre-of-mass distributions (baryon kinetic energy, electron-neutrino correlation, and Dalitz plot distributions) yield the vector to axialvector form factor ratiof1/g1=+0.034±0.080, in agreement with the value expected from the conserved vector current hypothesis (f1/g1=0). TheΣ−→Λe−v→ branching ratio measurement gives a value of (5.41±0.30)×10−5. The effects of radiative corrections are not included in these results. They are discussed in the text. Results on the otherΞ−,Σ−, andΛ semileptonic decays are reported in separate articles.