Exclusive J/ψ productions at e+e- colliders

Exclusive quarkonium pair production in electron-positron collisions is studied in non-relativistic QCD. The obtained cross section for J/ψ ηc production in the leading order is confronted against the recent measurements by the Belle Collaboration at KEKB. It is shown that a large renormalization K-factor is necessary to explain the experimental data. We point out that the JPC=0-+ nature of the hadronic systems that are assigned to be ηc should be tested by the triple angular distributions in terms of the scattering angle, and, polar and azimuthal angles of J/ψ into leptons. We further study J/ψ J/ψ and Υ Υ productions at LEP energies. Although the axial-vector couplings of the Z-boson to charm and bottom quarks allow production of such pairs when one of them is polarized transversally and the other longitudinally, we find that the integrated luminosity at Z pole accumulated by LEP is not large enough to observe the exclusive pair production of quarkonium