Rescattering Effects in Quarkonium Production

We study ηc and J/ψ hadroproduction induced by multiple scattering off fixed centres in the target. We determine the minimum number of hard scatterings required and show that additional soft scatterings may be factorized, at the level of the production amplitude for the ηc and of the cross section for the J/ψ. The J/ψ provides an interesting example of soft rescattering effects occurring inside a hard vertex. We also explain the qualitative difference between the transverse momentum broadening of the J/ψ and of the Υ observed in collisions on nuclei. We point out that rescattering from spectators produced by beam and target parton evolution may have important effects in J/ψ production. Work supported in part by the EU/TMR contract ERB FMRX-CT96-0008.