Second-order Hydrodynamics in Next-to-Leading-Order QCD

TL;DR

This paper calculates second-order hydrodynamic relaxation times in hot QCD at next-to-leading order, compares them with bounds valid for kinetic theory, and discusses deviations in strongly coupled theories with holographic duals.

Contribution

It provides the first next-to-leading order computation of relaxation times in hot QCD and analyzes their relation to kinetic theory bounds and holographic theories.

Findings

Computed relaxation times $ au_$ and $ au_j$ at NLO in QCD.

Found that ratios of second- to first-order coefficients are above kinetic theory bounds.

Holographic theories violate these bounds significantly.

Abstract

We compute the hydrodynamic relaxation times $\tau_\pi$ and $\tau_j$ for hot QCD at next-to-leading order in the coupling with kinetic theory. We show that certain dimensionless ratios of second-order to first-order transport coefficients obey bounds which apply whenever a kinetic theory description is possible; the computed values lie somewhat above these bounds. Strongly coupled theories with holographic duals strongly violate these bounds, highlighting their distance from a quasiparticle description.