Collective dynamics in heavy and light-ion collisions -- I) Kinetic Theory vs. Hydrodynamics

TL;DR

This paper compares kinetic theory and hydrodynamics in modeling collective flow in high-energy nuclear collisions, assessing the accuracy of hydrodynamics and the role of non-equilibrium effects in small and large systems.

Contribution

It provides a systematic comparison between kinetic theory and viscous hydrodynamics for different collision systems at RHIC and LHC energies, highlighting the limits of hydrodynamic descriptions.

Findings

Hydrodynamics accurately describes collective flow in large systems.

Non-equilibrium effects are significant in small systems like OO collisions.

Hydrodynamic models have limitations in small system descriptions.

Abstract

High-energy nuclear collisions exhibit collective flow, which emerges as a dynamical response of the Quark-Gluon Plasma (QGP) to the initial state geometry of the collision. Collective flow in heavy-ion collisions is usually described within multi-stage evolution models, which employ a viscous relativistic hydrodynamic description of the space-time evolution of the QGP. By comparing event-by-event simulations in kinetic theory and viscous hydrodynamics in OO, AuAu and PbPb collisions at RHIC and LHC energies, we quantify to what extent a macroscopic hydrodynamic description can accurately describe the development of collective flow and to what extent collective flow in small systems, such as OO, is sensitive to the non-equilibrium evolution of the QGP beyond hydrodynamics.