Constraints on color dipole-nucleon cross section from diffractive heavy quarkonium production

We study the hard color dipole-nucleon cross section within perturbative QCD and discuss its relation to observables in diffractive leptoproduction of heavy quarkonium. The dipole cross section calculated with the unintegrated gluon density of the nucleon substantially differs from the well-known perturbative form $\sigma_{dip} \sim b^2$ for $b > 0.3$fm, where $b$ is the transverse separation of the dipole. We show the measured ratio of $\psi '$ to $J / \psi$ photoproduction cross sections constrains the dipole cross section at intermediate $b$, and in fact excludes the simple $\sigma_{dip} \sim b^2$ behavior. We also calculate the $t$-slopes of the diffractive $J / \psi, \psi '$ productions. We emphasize the difference of $t$-slopes, $B_{J/\psi} - B_{\psi '}$, is dominated by the dipole-nucleon dynamics. This difference is found to be about $0.3 {GeV}^{-2}$ with our dipole cross section.