First observation of the isospin violating decay $J/\psi \to \Lambda {\bar{\Sigma }}^0+c.c.$

Abstract

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

Comments: 9 pages, 6 figures; This is a publication-version at Phys. Rev. D