Measurements of hyperon semileptonic decays at the CERN super proton synchrotron. Pt. 3

The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays of Σ-, Ψ-, and Λ. In this article we report results on the Σ->neanti ν (130 GeV/c) negatively charged particles produced in the forward direction by interaction of the 210 GeV/c (240 GeV/c) SPS proton beam on a BeO target. The Σ- hyperons were identified in a DISC Cerenkov counter, and the negatively charged decay particle was analysed by a magnetic spectrometer. The neutron direction was obtained from the Σ- decay vertex and from the impact point measured in a neutron detector. Electron-hadron discrimination was achieved by the combined use of lead-glass and transition radiation detectors. The Σ->neanti ν branching ratio measurement gave a value of (0.96+-0.05)x10-3. Measurements of the centre-of-mass distributions (baryon kinetic energy, electron-antineutrino correlation, and Dalitz plot distributions) yield the axial-vector to vector form factor ratio vertical strokeg1/f1vertical stroke=0.34+-0.05. From the study of the electron momentum spectrum, the negative sign of g1/f1, which is expected within the Cabibbo theory framework, is favoured by at least 2.6 standard deviations. The effects of q2-dependence of f1 and g1 and of radiative corrections, the measurement of the weak magnetism form factor f2, and the effect of possible second-class current terms are discussed. Results on the other Ψ-, Σ-, and Λ semileptonic decays are reported in separate articles. (orig.)