First observation and branching fraction and decay parameter measurements of the weak radiative decay Ξ0→Λe+e−

The weak radiative decay Ξ0→Λe+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→Λe+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 αΞΛee=−0.8±0.2, consistent with that of Ξ0→Λγ. The charge conjugate reaction Ξ0¯→Λ¯e+e− has also been observed.