Measurements of hyperon semileptonic decays at the CERN Super Proton Synchrotron I The Sigma ^{-} to Lambda e^{-} nu decay mode

The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays of Sigma ^{-}, Xi ^{-} and Lambda . 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. The Sigma ^{-} and Xi ^{-} hyperons are concurrently identified in a DISC Cernekov 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. The authors report results on the Sigma ^{-} to Lambda e^{-} nu decay mode. Measurements of the GL polarization and of the centre-of-mass distributions (baryon kinetic energy, electron-neutrino correlation, and Dalitz plot distributions) yield the vector to axial-vector form factor ratio f/sub 1//g/sub 1/=+0.034+or-0.080, in agreement with the value expected from the conserved vector current hypothesis (f/sub 1 //g/sub 1/=0). The Sigma ^{-} to Lambda e^{-} nu to branching ratio measurement gives a value of (5.41+or-0.30)*10/sup -5/. The effects of radiative corrections are not included in these results. They are discussed in the text. Results on the other Xi ^{-}, Sigma ^{-}, and Lambda semileptonic decays are reported in separate articles.