First observation and branching fraction and decay parameter measurements of the weak radiative decay $\Xi^0 \to \Lambda e^{+}e^{-}$

The weak radiative decay $\Xi^0 \rightarrow \Lambda e^{+} e^{-}$ has been detected for the first time. We find 412 candidates in the signal region, with an estimated background of $15 \pm 5$ events. We determine the branching fraction $\cal{B}(\Xi^0 \rightarrow \Lambda e^{+} e^{-}) = [7.6 \pm 0.4_{stat} \pm 0.4_{syst} \pm 0.2_{norm}] \times 10^{-6}$, consistent with an internal bremsstrahlung process, and the decay asymmetry parameter $\alpha_{\Xi \Lambda ee} = -0.8 \pm 0.2$, consistent with that of $\Xi^{0} \rightarrow \Lambda \gamma$. The charge conjugate reaction $\overline{\Xi^{0}} \rightarrow \overline{\Lambda} e^{+}e^{-}$ has also been observed.