Thermalization of a color glass condensate and review of the "Bottom-Up" scenario

TL;DR

This paper investigates the thermalization process of a color glass condensate in heavy ion collisions using BAMPS, contrasting it with the 'Bottom-Up' scenario, and finds that soft gluons thermalize simultaneously, with implications for hydrodynamics.

Contribution

It provides a detailed microscopical analysis of thermalization, challenging the dominant soft gluon production mechanism proposed in the 'Bottom-Up' scenario.

Findings

Soft and hard gluons thermalize simultaneously.

Back reaction suppresses soft gluon overproduction.

Thermalization time scale matches theoretical estimates.

Abstract

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 \BUp scenario. We demonstrate that the tremendous production of soft gluons via $gg \to ggg$, which is shown in the \BUp picture as the dominant process during the early preequilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction $ggg\to gg$ hinders the absolute particle multiplication. Moreover, different from the \BUp scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration obtained from BAMPS calculations is of order $\as^{-2} (\ln \as)^{-2} Q_s^{-1}$. After this time the gluon system exhibits nearly hydrodynamical behavior. The shear viscosity to entropy density ratio has weak dependence on $Q_s$ and lies close to the lower bound from the AdS/CFT conjecture.