First measurement of the (Ξ−→∑−γ)/(Ξ−→Λπ−) branching ratio

In an experiment performed at the CERN SPS charged hyperon beam, we have observed the radiative decay Ξ−→∑−γ. From a sample of 11 candidates, containing an estimated background of 1.6 events, the branching ratio is found to be: $$\Gamma (\Xi ^ - \to \Sigma ^ - \gamma )/(\Xi ^ - \to \Lambda \pi ^ - ) = (2.3 \pm 1.0) \times 10^{ - 4} .$$