Validity of the color dipole approximation for diffractive production of heavy quarkonium - art. no. 031501

We study the diffractive photo- and leptoproductions of J/psi and psi' on t he proton, and examine the validity of the small-size color dipole approxim ation to the production of radially excited heavy quarkonium. The dipole mo del predicts a small ratio of psi' to J/psi photoproduction cross sections, which does not agree with experimental data. We show that this discrepancy originates from a large transverse size of psi' which makes the convergenc e of the transverse size expansion questionable, and the calculation withou t the dipole approximation turns out to be consistent with the data. Produc tions of Y'(2S,3S) are also discussed, and the dipole approximation is foun d to be reasonable for the Y family.