Precision Measurement of the Decay ${\mathrm{\Sigma }}^{+}\to p\gamma$ in the Process $J\mathrm{/}\psi \to {\mathrm{\Sigma }}^{+}{\bar{\mathrm{\Sigma }}}^{-}$

Using $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events collected with the BESIII detector, the radiative hyperon decay $\mathrm{\Sigma }\to p\gamma$ is studied at an electron-positron collider experiment for the first time. The absolute branching fraction is measured to be $(0.996\pm {0.021}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.018}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$, which is lower than its world average value by $4.2$ standard deviations. Its decay asymmetry parameter is determined to be $-0.652\pm {0.056}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.020}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}$. The branching fraction and decay asymmetry parameter are the most precise to date, and the accuracies are improved by $78\mathrm{\%}$ and $34\mathrm{\%}$, respectively.