Search for $\eta$ and $\eta'$ Invisible Decays in $J/\psi\to\phi\eta$   and $\phi\eta'$
  Using a sample of $(225.3\pm 2.8)\times 10^{6}$ $J/\psi$ decays collected with the BESIII detector at BEPCII, searches for invisible decays of $\eta$ and $\eta^\prime$ in $J/\psi\to\phi\eta$ and $\phi\eta^\prime$ are performed. Decays of $\phi \to K^{+}K^{-}$ are used to tag the $\eta$ and $\eta^\prime$ decays. No signals above background are found for the invisible decays, and upper limits at the 90% confidence level are determined to be $2.58\times10^{-4}$ for the ratio $\frac{\mathcal{B}(\eta\to\rm{invisible})}{\mathcal{B}(\eta\to\gamma\gamma)}$ and $2.39\times10^{-2}$ for $\frac{\mathcal{B}(\eta^\prime\to\rm{invisible})}{\mathcal{B}(\eta^\prime \to\gamma\gamma)}$. 
M. Ablikim, M. N. Achasov, O. Albayrak, D. J. Ambrose, F. F. An, Q.   An, J. Z. Bai, Y. Ban, J. Becker, J. V. Bennett, M. Be...
hep-ex hep-ph