Do nuclear collisions create a locally equilibrated quark-gluon plasma?

TL;DR

This paper argues that the success of hydrodynamics in describing high-energy nuclear collision experiments does not necessarily imply local thermal equilibrium or isotropy, and predicts its breakdown at high momenta.

Contribution

It challenges the common assumption that hydrodynamics requires local equilibrium, providing arguments and predictions about its limitations in nuclear collision analysis.

Findings

Hydrodynamics can accurately describe experimental results without local equilibrium.

Hydrodynamics is predicted to break down at momenta around seven times the temperature.

Smallest QCD liquid drop size is estimated at 0.15 fm.

Abstract

Experimental results on azimuthal correlations in high energy nuclear collisions (nucleus-nucleus, proton-nucleus and proton-proton) seem to be well described by viscous hydrodynamics. It is often argued that this agreement implies either local thermal equilibrium or at least local isotropy. In this note, I present arguments why this is not the case. Neither local near-equilibrium nor near-isotropy are required in order for hydrodynamics to offer a successful and accurate description of experimental results. However, I predict the breakdown of hydrodynamics at momenta of order seven times the temperature, corresponding to a smallest possible QCD liquid drop size of 0.15 fm.