Measurement of the Absolute Branching Fraction and Decay Asymmetry of $\Lambda \to n\gamma $

The radiative hyperon decay $\Lambda \to n\gamma $ is studied using $\left(10087±44\right)×{10}^{6}\text{}\text{}J/\psi $ events collected with the BESIII detector operating at BEPCII. The absolute branching fraction of the decay $\Lambda \to n\gamma $ is determined to be $\left(0.832±0.03{8}_{\mathrm{stat}}±0.05{4}_{\mathrm{syst}}\right)×{10}^{-3}$, which is a factor of 2.1 lower and 5.6 standard deviations different than the previous measurement. 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 $-0.16±0.1{0}_{\mathrm{stat}}±0.0{5}_{\mathrm{syst}}$.