Measurements of hyperon semileptonic decays at the CERN Super Proton Synchrotron

The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays of Σ−, Ξ−, and Λ. In this article we report results on the\(\sum \to ne\bar v\) decay mode. A magnetic channel selected 100 GeV/c (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\(\sum \to ne\bar v\) branching ratio measurement gave a value of (0.96±0.05) ×10−3. 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 |g1/f1|=0.34 ±0.05. From the study of the electron momentum spectrum, the negative sign ofg1/f1, which is expected within the Cabibbo theory framework, is favoured by at least 2.6 standard deviations. The effects ofq2-dependence off1 andg1 and of radiative corrections, the measurement of the weak magnetism form factorf2, and the effect of possible second-class current terms are discussed. Results on the other Ξ−, Σ−, and Λ semileptonic decays are reported in separate articles.