How dense does parton matter get in Pb + Pb Collisions at the CERN SPS?

We examine the qualitative features of parton production through materialization in heavy-ion collisions within perturbative QCD, and estimate the magnitude of the resulting parton density created during the early stage of the collisions. The implications for ``anomalous'' $J/ψ$ suppression observed in Pb+Pb collisions at the CERN SPS are discussed. We argue that the A-dependence of absorption of $J/ψ$ by (partonic) comovers is steeper than assumed in most phenomenological models, because the absorption process is dominated by quasi-perturbative QCD interactions. Our argument is supported by results recently obtained in the framework of the parton cascade model. We predict significant ``anomalous'' suppression for Pb+Pb collisions at the CERN-SPS, but not for S+U collisions.

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