Measurements of J/ψ and ψ(2S) decays into ΛΛπ0 and ΛΛη

Using 58 × 10 6 J/ψ and 14 × 10 6 ψ(2S) events collected by the BESII detector at the BEPC, branching fractions or upper limits for the decays J/ψ and ψ(2S) → ΛΛπ 0 and ΛΛη are measured. For the isospin violating decays, the upper limits are determined to be B(J/ψ → ΛΛπ 0 ) < 6.4 X 10 -5 and B[ψ(2S) → ΛΛπ 0 ] < 4.9 × 10 -5 at the 90% confidence level. The isospin conserving process J/ψ → ΛΛη is observed for the first time, and its branching fraction is measured to be B(J/ψ → ΛΛη) = (2.62 ± 0.60 ± 0.44) X 10 -4 , where the first error is statistical and the second one is systematic. No ΛΛη signal is observed in ψ(2S) decays, and B[ψ(2S) → ΛΛη] < 1.2 X 10 -4 is set at the 90% confidence level. Branching fractions of J/ψ decays into Σ + π - Λ and Σ - π + Λ are also reported, and the sum of these branching fractions is determined to be B(J/ψ → Σ + π - Λ + c.c.) = (1.52 ± 0.08 ± 0.16) × 10 -3 .