Early time dynamics far from equilibrium via holography

TL;DR

This paper explores the early-time behavior of holographic QGP in heavy ion collisions, proposing new definitions for shear viscosity and speed of sound far from equilibrium, and analyzing their evolution and attractors.

Contribution

It introduces a far-from-equilibrium shear viscosity and speed of sound, and studies their evolution and attractors in holographic QGP, including effects of magnetic fields and chemical potentials.

Findings

Shear viscosity ratio decreases to 60% then overshoots to 110% of equilibrium value.

Energy-momentum tensor exhibits hydrodynamic attractor behavior.

Chiral magnetic effect studied far from equilibrium.

Abstract

We investigate the early time dynamics of heavy ion collisions studying the time evolution of the energy-momentum tensor as well as energy-momentum correlations within a uniformly thermalizing holographic QGP. From these quantities, we suggest a far-from equilibrium definition of shear viscosity, which is a crucial property of QCD matter as it significantly determines the generation of elliptic flow already at early times. During an exemplary initial heating phase of the holographic QGP the shear viscosity of entropy density ratio decreases down to 60%, followed by an overshoot to 110% of the near-equilibrium value, $\eta/s=1/(4\pi)$. Implications for the QCD QGP are discussed. Subsequently, we consider a holographic QGP which is Bjorken-expanding. Its energy-momentum tensor components have a known hydrodynamic attractor to which all time evolutions collapse independent of the initial conditions. Based on this, we propose a definition for a far from equilibrium speed of sound, and analytically compute its hydrodynamic attractor. Subjecting this Bjorken-expanding plasma to an external magnetic field and an axial chemical potential, we study the chiral magnetic effect far from equilibrium.