Asymmetry Parameter and Branching Ratio ofΣ+→pγ

An experiment to study the decay ${\Sigma }^{+}\to p\gamma$ was performed in the Berkeley 25-in. hydrogen bubble chamber. An analysis was made of 48 000 events of the type $K^{-}p\to {\Sigma }^{+}{\pi }^{-}$, ${\Sigma }^{+}\to p+\mathrm{neutral}$ with $K^{-}$ momenta near 400 MeV/c. The $\Sigma \text{'}\mathrm{s}$ produced in this momentum region are polarized because of the interference of the ${Y_0}^{*} \mathrm{}\left(1520\right)\mathrm{}$ amplitude with the background amplitudes. We have measured the proton asymmetry parameter $\alpha$ for 61 ${\Sigma }^{+}\to p\gamma$ events with an average polarization of 0.4. We found $\alpha =-{{1.03}_{-0.42\mathrm{}}}^{+0.52\mathrm{}}$. $\mathrm{SU}\left(3\right)\mathrm{}$ predicts a value $\alpha =0\mathrm{}$. A more restricted sample of events was used to determine the ${\Sigma }^{+}\to p\gamma$ branching ratio. From 31 ${\Sigma }^{+}\to p\gamma$ events and 11 670 ${\Sigma }^{+}\to p{\pi }^0$ events, we found $\frac{({\Sigma }^{+}\to p\gamma )}{({\Sigma }^{+}\to p{\pi }^0)}=(2.76\mathrm{}\pm 0.51)\times {10}^{-3\mathrm{}}$. The result is in agreement with the previous measurements.