Rescattering Effects in Quarkonium Production

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We study etac and J/psi 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 etac and of the cross section for the J/psi. The J/psi 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/psi and of the Upsilon 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/psi production.