Search for the double-charmonium state with η c J/ψ at Belle

Abstract We measure the cross section of e + e − → η c J/ψ at the Υ(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 fb −1. We also search for double charmonium production in e + e − → η c J/ψ via initial state radiation near the η c J/ψ threshold. No evident signal of the double charmonium state is found, but evidence for the e + e − → η c J/ψ process is found with a statistical significance greater than 3.3σ near the η c J/ψ threshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions