Measurements of hyperon semileptonic decays at the CERN super proton synchrotron. Pt. 2

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. A magnetic channel selected 100 GeV/c negatively charged particles produced in the forward direction by interaction of a 210 GeV/c proton beam on a BeO target. The Σ- and Ψ- hyperons were concurrently identified in a DISC Cerenkov counter, and their decay products were analysed by a magnetic spectrometer. Electron-hadron discrimination was 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 Ψ->Λπ(Λ->peanti ν), Ψ->Λeanti ν(Λ->pπ), and Ψ->Σ0eanti ν(Σ0->Λγ) (Λ->pπ) decay modes. Samples of 7,111 Λ->peanti ν, 2,608 Ψ->Λeanti ν, and 154 Ψ->Σ0eanti ν were used in our analysis. The branching ratio measurements gave values of (8.57+-0.36)x10-4, (5.64+-0.31)x10-4, and (0.87+-0.17)x10-4 for Λ->peanti ν, and Ψ->Σ0eanti ν, respectively. Measurements of the Λ polarization and of the centre-of-mass distributions yield the axial vector to vector form factor ratio, g1/f1=+0.70+-0.03 for Λ->peanti ν, and g1/f1=+0.25+-0.05 for Ψ->Λeanti ν. The effects of q2-dependence of f1 and g1 and of radiative corrections, the measurement of the weak magnetism term f2, and the effect of possible second-class current terms are discussed. Results on the Σ->Λeanti ν and Σ->neanti ν decay modes are reported in separate articles. (orig.)