Analysis of $J/\psi(\eta_c)\to \gamma ~+$ invisible decays in the   standard model
  Motivated by the experimental study of $J/\psi\to\gamma ~+$ invisible decay by the CLEO Collaboration, we analyze the process $J/\psi\to \gamma \nu\bar{\nu}$, as the standard model background for this invisible decay, at the lowest order. Our investigation shows that Br($J/\psi\to \gamma \nu\bar{\nu}$) is far below the present experimental upper limit on the branching ratio of $J/\psi\to\gamma ~+$ invisible, which indicates that some interesting room for new physics in this channel might be expected. We also present a similar analysis of the $\eta_c\to \gamma \nu\bar{\nu}$ decay. 
Dao-Neng Gao
hep-ph hep-ex