First observation and branching fraction and decay parameter measurements of the weak radiation decay Xi0 ---> Lambda e+ e-

The weak radiative decay Ξ0 → Λ e+ e- has been detected for the first time. We find 412 candidates in the signal region, with an estimated background of 15 ± 5 events. We determine the branching fraction B (Ξ0 → Λ e+ e-) = [7.6 ± 0.4 (stat) ± 0.4 (syst) ± 0.2 (norm)] × 10-6, consistent with an internal bremsstrahlung process, and the decay asymmetry parameter αΞ Λ e e = - 0.8 ± 0.2, consistent with that of Ξ0 → Λ γ. The charge conjugate reaction over(Ξ0, -) → over(Λ, -) e+ e- has also been observed.