Measurement of the hyperon weak radiative decay ${\mathrm{\Xi }}^0\to \gamma {\mathrm{\Sigma }}^0$ at BESIII

The hyperon weak radiative decay ${\mathrm{\Xi }}^0\to \gamma {\mathrm{\Sigma }}^0$ is measured via the process $J\mathrm{/}\psi \to {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0$. The absolute branching fraction of ${\mathrm{\Xi }}^0\to \gamma {\mathrm{\Sigma }}^0$ is determined to be $(3.69\pm {0.21}_{\text{stat}}\pm {0.12}_{\text{syst}})\times {10}^{-3}$, based on $(10.087\pm 0.044)\times {10}^9$ $J\mathrm{/}\psi$ events collected with the BESIII detector operating at the BEPCII collider. The decay asymmetry parameter is measured, with a complete angular analysis of the cascade decay chain, to be ${\alpha }_{\gamma }=-0.807\pm {0.095}_{\text{stat}}\pm {0.011}_{\text{syst}}$.