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

Abstract The weak radiative decay Ξ 0 → 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 ( Ξ 0 → 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 α Ξ Λ e e = − 0.8 ± 0.2 , consistent with that of Ξ 0 → Λ γ . The charge conjugate reaction Ξ 0 ¯ → Λ ¯ e + e − has also been observed.