Establishing the Range of Applicability of Hydrodynamics in High-Energy Collisions

TL;DR

This paper investigates when relativistic viscous hydrodynamics accurately describes high-energy collision dynamics by comparing it with microscopic kinetic simulations, emphasizing the importance of the inverse Reynolds number and pre-equilibrium stages.

Contribution

The study quantifies the conditions under which hydrodynamics is valid in high-energy collisions, highlighting the role of the inverse Reynolds number and pre-equilibrium effects.

Findings

Hydrodynamics accurately models collective flow when inverse Reynolds number is low.

Properly accounting for pre-equilibrium stages extends hydrodynamics applicability.

Implications for small-system collisions like proton-proton and proton-nucleus are discussed.

Abstract

We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description in the conformal relaxation time approximation, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds number is sufficiently small and the early pre-equilibrium stage is properly accounted for. We further discuss the implications of our findings for the (in)applicability of hydrodynamics in proton-proton, proton-nucleus and light nucleus collisions.