Thermalization of a color glass condensate and review of the 'bottom-up' scenario

摘要

The thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within microscopical parton cascade BAMPS. Our main focus lies on the detailed comparison of thermalization, observed in BAMPS with that suggested in the "bottom-up" scenario. We demonstrate that the tremendous production of soft gluons via gg -> ggg, which is shown in the "bottom-up" picture as the dominant process during the early pre-equilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction ggg -> gg hinders the absolute particle multiplication. Moreover, contrary to the "bottorn-up" scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration in BAMPS calculations is of order alpha(2)(S)(ln alpha(S))(-2)Q(S)(-1). After this time the gluon system exhibits nearly hydrodynamical behavior. The shear viscosity to entropy density ratio has a weak dependence on Q(S) and lies close to the lower bound of the AdS/CFT conjecture. (c) 2008 Elsevier B.V. All rights reserved.