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

Using $\left(10\text{}087±44\right)×{10}^{6}$ $J/\psi $ events collected with the BESIII detector, the radiative hyperon decay ${\Sigma }^{+}\to p\gamma $ is studied at an electron-positron collider experiment for the first time. The absolute branching fraction is measured to be $\left(0.996±0.02{1}_{\mathrm{stat}}±0.01{8}_{\mathrm{syst}}\right)×{10}^{-3}$, which is lower than its world average value by 4.2 standard deviations. Its decay asymmetry parameter is determined to be $-0.652±0.05{6}_{\mathrm{stat}}±0.02{0}_{\mathrm{syst}}$. The branching fraction and decay asymmetry parameter are the most precise to date, and the accuracies are improved by 78% and 34%, respectively.