THERMAL AND CHEMICAL EQUILIBRATION IN RELATIVISTIC HEAVY-ION COLLISIONS

We investigate the thermalization and the chemical equilibration of a parton plasma created from Au+Au collision at LHC and RHIC energies st arting from the early moment when the particle momentum distributions in the central region become for the first time isotropic due to longi tudinal cooling. Using the relaxation time approximation for the colli sion terms in the Boltzmann equations for gluons and for quarks and th e real collision terms constructed from the simplest QCD interactions, we show that the collision times have the right behavior for equilibr ation. The magnitude of the quark (antiquark) collision time remains b igger than the gluon collision time throughout the lifetime of the pla sma so that gluons are equilibrating faster than quarks both chemicall y and kinetically. That is we have a two-stage equilibration scenario as has been pointed out already by Shuryak sometimes ago, Full kinetic equilibration is however slow and chemical equilibration cannot be co mpleted before the onset of the deconfinement phase transition assumed to be at T-c = 200 MeV. By comparing the collision entropy density ra tes of the different processes, we show explicitly that inelastic proc esses, and not elastic processes as is commonly assumed, are dominant in the equilibration of the plasma and that gluon branching leads the other processes in entropy generation, We also show that, within pertu rbative QCD, processes with higher power in alpha(s) need not be less important for the purpose of equilibration than those with lower power . The state of equilibration of the system has also a role to play. We compare our results with those of the parton cascade model.