Self-Screened Parton Cascades

The high density of scattered partons predicted in nuclear collisions at very high energy makes color screening effects significant. We explain how these screening mechanisms may suppress nonperturbative, soft QCD processes, permitting a consistent calculation of quark-gluon plasma formation within the framework of perturbative QCD. We present results of a model calculation of these effects including predictions for the initial thermalized state for heavy nuclei colliding at RHIC and LHC. Most recent theoretical predictions for the initial conditions at which a thermalized quark-gluon plasma will be produced at heavy ion colliders are based on the concept of perturbative partonic cascades. The parton cascade model [1] starts from a relativistic transport equation of the form p ∂ ∂xμ Fi(x, p) = Ci(x, p|Fk) i = q, g, (1) where Fi(x, p) denote the phase space distributions of quarks and gluons. The collision terms Ci are obtained in the framework of perturbative QCD from elementary 2 → 2 scattering amplitudes allowing for additional initialand final state radiation due to scale evolution of the perturbative quanta. To regulate infrared divergences, the parton cascade model requires a momentum cut-off for the 2 → 2 scattering amplitudes (usually p T = 1.5 − 2 GeV/c) and a virtuality cut-off for time-like branchings (μ0 = 0.5− 1 GeV/c).