Review of the "Bottom-Up" scenario

TL;DR

This paper compares the thermalization process of a color glass condensate in heavy ion collisions as modeled by BAMPS with the traditional Bottom-Up scenario, revealing key differences in gluon production and thermalization timing.

Contribution

It provides a detailed comparison showing that soft gluon production is suppressed and soft and hard gluons thermalize simultaneously, challenging the predictions of the Bottom-Up scenario.

Findings

Soft gluon production via $gg o ggg$ is hindered by back reactions.

Soft and hard gluons thermalize simultaneously.

Thermalization time scale is of order $ ext{ extasciitilde}rac{1}{ar{ ext{a}}_s^2 ( ext{ln}ar{ ext{a}}_s)^2 Q_s}$.

Abstract

Thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within 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 $gg \to ggg$, which is shown in the Bottom-Up 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, contrary to the Bottom-Up scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration in BAMPS calculations is of order $\as^{-2} (\ln \as)^{-2} Q_s^{-1}$. After this time the gluon system exhibits nearly hydrodynamic 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.