Search for the double-charmonium state with ${\eta }_{c}J/\psi$ at Belle

Abstract

We measure the cross section of $e^{+}{e}^{-}\to {\eta }_{c}J/\psi$ at the ${\rm Y}(nS)(n=1$ -- $5)$ on-resonance and 10.52 GeV off-resonance energy points using the full data sample collected by the Belle detector with an integrated luminosity of $955\mathrm{f}{\mathrm{b}}^{-1}$. We also search for double charmonium production in $e^{+}{e}^{-}\to {\eta }_{c}J/\psi$ via initial state radiation near the ${\eta }_{c}J/\psi$ threshold. No evident signal of the double charmonium state is found, but evidence for the $e^{+}{e}^{-}\to {\eta }_{c}J/\psi$ process is found with a statistical significance greater than $3.3\sigma$ near the ${\eta }_{c}J/\psi$ threshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions.