Measurements of hyperon semileptonic decays at the CERN Super Proton Synchrotron

Abstract

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 Čerenkov 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 Čerenkov 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 ratiof ₁/g ₁=+0.034±0.080, in agreement with the value expected from the conserved vector current hypothesis (f ₁/g ₁=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.