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

Abstract

Using a sample of $(225.2±2.8)×10^6$ $J/\psi$ events collected with the BESIII detector, we present results of a study of $J/\psi→\gamma\mathit{\Lambda\overline{\Lambda}}$ and report the first observation of the isospin violating decay $J/\psi→\mathit{\Lambda\overline{\Sigma}}^0+\text{c. c.},$ in which $\mathit{\overline{\Sigma}}^0$ decays to $\gamma\overline{\Lambda}$. The measured branching fractions are $\mathscr{B}(J/\psi→\mathit{\overline{\Lambda}\Sigma}^0)=(1.46±0.11±0.12)×10^{−5}$ and $\mathscr{B}(J/\psi→\mathit{\Lambda\overline{\Sigma}}^0)=(1.37±0.12±0.11)×10^{−5}$. We search for $\mathit{\Lambda}(1520)→\gamma\mathit{\Lambda}$ decay, and find no evident signal, and an upper limit for the product branching fraction $\mathscr{B}(J/\psi→\mathit{\Lambda\overline{\Lambda}}(1520)+\text{c. c.})×\mathscr{B}(\mathit{\Lambda}(1520)→\mathit{\gamma\Lambda})<4.1×10^{−6}$ is set at the 90% confidence level. We also report the observation of $\eta_c→\mathit{\Lambda}\overline{\mathit{\Lambda}}$ in $J/\psi→\gamma\eta_c, \eta_c→\mathit{\Lambda\overline{\Lambda}}$ and measure the branching fraction $\mathscr{B}(\eta_c→\mathit{\Lambda\overline{\Lambda}})=(1.16±0.12(\text{stat})±0.19(\text{syst})±0.28(\text{PDG}))×10^{−3}$.