Studies in the BNL 21-GeV/cnegative-hyperon beam. II. Leptonic decays ofΞandΣ

The decays ${\Sigma }^{-}\to \Lambda e^{-}\nu$ and ${\Xi }^{-}\to \Lambda e^{-}\nu$ are studied in a counter—spark-chamber experiment using unpolarized hyperons from the BNL hyperon beam. A threshold Čerenkov counter identifies the electron. From a sample of 119 reconstructed ${\Sigma }^{-}\to \Lambda e^{-}\nu$ events (including ∼ 5 background events), we find $\frac{({\Sigma }^{-}\to \Lambda e^{-}\nu )}{({\Sigma }^{-}\to n\pi )}=(0.63\mathrm{}\pm 0.10)\times {10}^{-4\mathrm{}}$ and $\frac{g_V}{g_A}=-0.32\mathrm{}\pm 0.30$, $g_{\mathrm{WM}}=-0.6\mathrm{}\pm 3.6$ (with sign convention of the Particle Data Group tables). We find $\mathrm{Im}\left(\frac{g_V}{g_A}\right)=0.2\mathrm{}\pm 0.7$, consistent with time-reversal invariance. From a sample of 14 ${\Xi }^{-}\to \Lambda e^{-}\nu$ events (including ≈ 3.4 background events) we find $\frac{({\Xi }^{-}\to \Lambda e^{-}\nu )}{({\Xi }^{-}\to \Lambda \pi )}=(0.30\mathrm{}\pm 0.14)\times {10}^{-3\mathrm{}}$. We also searched for ${\Xi }^{-}\to \Lambda \mu \nu$ and ${\Xi }^{-}\to {\Sigma }^{0}e\nu$ and set upper limits on these decays from our data.