Far-from-equilibrium slow modes and momentum anisotropy in expanding plasma

TL;DR

This paper investigates the far-from-equilibrium evolution of an expanding plasma in heavy-ion collisions, revealing the behavior of slow and fast modes, and extending the attractor paradigm to various components of the energy-momentum tensor.

Contribution

It introduces a new understanding of slow modes in far-from-equilibrium plasmas, including their infinite nature at early times and their relation to hydrodynamic degrees of freedom.

Findings

Slow modes are identified as key to early-time dynamics.

Momentum anisotropy decays slowly and can persist until relaxation.

Generalization of the attractor paradigm to energy-momentum tensor components.

Abstract

The momentum distribution of particle production in heavy-ion collisions encodes information about thermalization processes in the early-stage quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium evolution of an expanding plasma with an anisotropic momentum-space distribution. We identify slow and fast degrees of freedom in the far-from-equilibrium plasma from the evolution of moments of this distribution. At late times, the slow modes correspond to hydrodynamic degrees of freedom and are naturally gapped from the fast modes by the inverse of the relaxation time, $\tau_R^{-1}$. At early times, however, there are an infinite number of slow modes with a gap inversely proportional to time, $\tau^{-1}$. From the evolution of the slow modes we generalize the paradigm of the far-from-equilibrium attractor to vector and tensor components of the energy-momentum tensor, and even to higher moments of the distribution function that are not part of the hydrodynamic evolution. We predict that initial-state momentum anisotropy decays slowly in the far-from-equilibrium phase and may persist until the relaxation time.