First observation of the isospin violating decay J/ψ→ΛΣ̄⁰+c.c.

Using a sample of (225.2±2.8)×10⁶ J/ψ events collected with the BESIII detector, we present results of a study of J/ψ→γΛΛ¯¯¯ and report the first observation of the isospin violating decay J/ψ→ΛΣ¯¯¯⁰+c.c., in which Σ¯¯¯⁰ decays to γΛ¯¯¯. The measured branching fractions are B(J/ψ→Λ¯¯¯Σ⁰)=(1.46±0.11±0.12)×10⁻⁵ and B(J/ψ→ΛΣ¯¯¯⁰)=(1.37±0.12±0.11)×10⁻⁵. We search for Λ(1520)→γΛ decay, and find no evident signal, and an upper limit for the product branching fraction B(J/ψ→ΛΛ¯¯¯(1520)+c.c.)×B(Λ(1520)→γΛ)<4.1×10⁻⁶ is set at the 90% confidence level. We also report the observation of ηc→ΛΛ¯¯¯ in J/ψ→γηc, ηc→ΛΛ¯¯¯ and measure the branching fraction B(ηc→ΛΛ¯¯¯)=(1.16±0.12(stat)±0.19(syst)±0.28(PDG))×10⁻³.