Measurement of the branching fraction and decay asymmetry of $\mathrm{\Lambda }\to n\gamma$

The radiative hyperon decay $\mathrm{\Lambda }\to n\gamma$ is studied using $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events collected with the BESIII detector operating at BEPCII. The absolute branching fraction of the decay $\mathrm{\Lambda }\to n\gamma$ is determined with a significance of 5.6$\sigma$ to be $[0.832\pm 0.038(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.054(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})]\times {10}^{-3}$, which lies significantly below the current PDG value. By analyzing the joint angular distribution of the decay products, the first determination of the decay asymmetry ${\alpha }_{\gamma }$ is reported with a value of $-0.16\pm 0.10(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.05(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$.