Quarkonium production through hard comover scattering

We propose a qualitatively new mechanism for quarkonium production, motivated by the global features of the experimental data and by the successes or failures of existing models. In QCD, heavy quarks are created in conjunction with a bremsstrahlung color field emitted by the colliding partons. We study the effects of perturbative gluon exchange between the quark pair and a comoving color field. Such scattering can flip the spin and color of the quarks to create a nonvanishing overlap with the wave function of physical quarkonium. Several observed features that are difficult to understand in current models find simple explanations. Transverse gluon exchange produces unpolarized $J/\psi$’s, the ${\chi }_{c1\mathrm{}}$ and ${\chi }_{c2\mathrm{}}$ states are produced at similar rates, and the anomalous dependence of the $J/\psi$ cross section on the nuclear target size can be qualitatively understood.