Bottom-up thermalization in heavy ion collisions

TL;DR

The paper addresses how thermalization occurs in heavy ion collisions within perturbative QCD, assuming an initial gluon distribution set by the saturation scale $Q_s$ with $Q_s \gg Λ_{QCD}$. It proposes a bottom-up thermalization mechanism in which soft gluons first form a thermal bath and then extract energy from hard gluons through inelastic cascades, culminating in full thermalization at $τ \sim α^{-13/5} Q_s^{-1}$ and a maximal temperature $T_{\max} \sim α^{2/5} Q_s$. The approach combines a boost-invariant, one-dimensional expansion with Boltzmann kinetics including elastic and inelastic (2→3/3→2) processes and LPM suppression to describe energy transfer from the hard to soft sectors. The findings demonstrate that, at sufficiently high energies, the soft sector thermalizes rapidly and acts as an energy sink for hard gluons, supporting fast thermalization scenarios relevant for RHIC and LHC. This framework provides parametric scaling laws for the evolution of the soft bath temperature and clarifies the role of Debye screening and soft-gluon production in driving equilibration.

Abstract

We describe how thermalization occurs in heavy ion collisions in the framework of perturbative QCD. When the saturation scale $Q_s$ is large compared to $Λ_{QCD}$, thermalization takes place during a time of order $α^{-13/5}Q_s^{-1}$ and the maximal temperature achieved is $α^{2/5}Q_s$.

Figures (1)

• Figure 1: The diagrammatic representation of $C_{\rm prod}$