Measurement of the Branching Ratio and Asymmetry of the Decay $\mathit{\Xi }°\to \mathit{\Sigma }°\mathit{\gamma }$

We have studied the rare weak radiative hyperon decay $\Xi °\to \Sigma °\gamma$ in the KTeV experiment at Fermilab. We have identified 4045 signal events over a background of 804 events. The dominant $\Xi °\to \Lambda \pi °$ decay, which was used for normalization, is the only important background source. An analysis of the acceptance of both modes yields a branching ratio of $B(\Xi °\to \Sigma °\gamma )/B(\Xi °\to \Lambda \pi °)=(3.34\pm 0.05\pm 0.09)\times {10}^{-3\mathrm{}}$. By analyzing the final state decay distributions, we have also determined that the $\Sigma °$ emission asymmetry parameter for this decay is ${\alpha }_{\Xi \Sigma }=-0.63\mathrm{}\pm 0.09$.