Measurement of the $\Xi^{0} \to \Lambda\gamma$ decay asymmetry and branching fraction

In data taken with the NA48 experiment at the CERN SPS in 1999, 730 candidates of the weak radiative hyperon decay $\Xi^{0} \to \Lambda\gamma$ have been found with an estimated background of $58 \pm 8$ events. From these events the $\Xi^{0} \to \Lambda\gamma$ decay asymmetry has been determined to $\alpha(\Xi^{0} \to \Lambda\gamma) = -0.78 \pm 0.18_{stat} \pm 0.06_{syst}$, which is the first evidence of a decay asymmetry in $\Xi^{0} \to \Lambda\gamma$. The branching fraction of the decay has been measured to be $Br(\Xi^{0} \to \Lambda\gamma) = (1.16 \pm 0.05_{stat} \pm 0.06_{syst}) \times 10^{-3}$.