%0 Journal Article %T Validity of the Color Dipole Approximation for Diffractive Production of Heavy Quarkonium %A K. Suzuki %A A. Hayashigaki %A K. Itakura %A J. Alam %A T. Hatsuda %J Physics %D 2000 %I arXiv %R 10.1103/PhysRevD.62.031501 %X We study the diffractive photo- and leptoproductions of $J / \psi$ and $\psi '$ on the proton, and examine the validity of the small-size color dipole approximation to the production of radially excited heavy quarkonium. The dipole model 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 convergence of the transverse size expansion questionable, and the calculation without the dipole approximation turns out to be consistent with the data. Productions of $\Upsilon '(2S,3S)$ are also discussed, and the dipole approximation is found to be reasonable for the $\Upsilon$-family. %U http://arxiv.org/abs/hep-ph/0005250v2