System size dependence of pre-equilibrium and applicability of hydrodynamics in heavy-ion collisions

TL;DR

This study investigates the applicability of relativistic viscous hydrodynamics in high-energy heavy-ion collisions, showing it works well for large systems like lead-lead but not for smaller systems such as proton-proton collisions.

Contribution

The paper quantifies the system size and energy dependence of hydrodynamics validity using microscopic simulations, clarifying its limits in small collision systems.

Findings

Hydrodynamics accurately describes collective flow in large lead-lead collisions at LHC energies.

Hydrodynamics is not suitable for proton-proton or proton-lead collisions due to inadequate collective flow development.

The validity of hydrodynamics depends on the inverse Reynolds number and proper modeling of pre-equilibrium stages.

Abstract

We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description, 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 $\mathrm{Re}^{-1}$ is sufficiently small and the early pre-equilibrium stage is properly accounted for. By determining the breakdown of hydrodynamics as a function of system size and energy, we find that it is quantitatively accurate in central lead-lead collisions at LHC energies, but should not be used in typical proton-lead or proton-proton collisions, where the development of collective flow can not accurately be described within hydrodynamics.