Observation of rare decay modes of theΞhyperons

In an experiment based on the production of $8150 {\Xi }^{-1\mathrm{}}$ and $2975 {\Xi }^0$ hyperons, we have detected one example each of the decays ${\Xi }^{-}\to \Lambda {\mu }^{-}\overline{\nu }$ and ${\Xi }^0\to \Lambda \gamma$. The branching ratios of these hitherto unobserved decay modes are $\frac{\Gamma ({\Xi }^{-}\to \Lambda {\mu }^{-}\overline{\nu })}{\Gamma ({\Xi }^{-}\to \Lambda {\pi }^{-})}=(3\mathrm{}\pm 3)\times {10}^{-4\mathrm{}}$ and $\frac{\Gamma ({\Xi }^0\to \Lambda \gamma )}{\Gamma ({\Xi }^0\to \Lambda {\pi }^0)}=(5\mathrm{}\pm 5)\times {10}^{-3\mathrm{}}$. One event of the decay ${\Xi }^{-}\to \Lambda e^{-}\overline{\nu }$ has also been observed. There is no evidence for $\Delta S=2\mathrm{}$ decays; upper limits on these and other rare decay modes are presented.