Using $(10\:087\pm44)\times10^{ 6}$ $J/\psi$ events collected with the BESIII detector, the radiative hyperon decay $\Sigma\rightarrow p\gamma$ is studied at an electron-positron collider experiment for the first time. The absolute branching fraction is measured to be $\left(0.996 \pm 0.021_{ \rm stat.}\pm 0.018_{ \rm syst.}\right)\times 10^{ -3}$, which is lower than its world average value by $4.2$ standard deviations. Its decay asymmetry parameter is determined to be $-0.652 \pm 0.056_{ \rm stat.}\pm 0.020_{ \rm syst.}$. The branching fraction and decay asymmetry parameter are the most precise to date, and the accuracies are improved by $78\%$ and $34\%$, respectively.
Categories: hep-ex