First observation of the isospin violating decay $J/\psi\rightarrow \mathit{\Lambda\overline{\Sigma}}^0 + \text{c.c.}$

Abstract

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.