Exclusive double-charmonium production from e ⁺e ^- annihilation into two virtual photons

We calculate the contributions from QED processes involving two virtual photons to the cross sections for e ⁺e ^- annihilation into two charmonium states with the same C parity. Genetically, the cross sections are three orders of magnitude smaller than those for charmonia with opposite C parity because they are suppressed by a factor of α ²/ α _s ². However, if both charmonia have quantum numbers J ^pc = 1 ^-, then there is a contribution to the cross section that involves the fragmentation of each photon into a charmonium. The fragmentation contribution is enhanced by powers of E _beam/m _c, the ratio of the beam energy to the charm-quark mass, and this enhancement can compensate for the suppression factor that is associated with the coupling constants. In particular, the predicted cross section for J/ψ+J/ψ at the B factories is larger than that for J/ψ + η _c.