The Skinny on Bulk Viscosity and Cavitation in Heavy Ion Collisions

TL;DR

This paper investigates how bulk viscosity and cavitation influence the evolution of quark-gluon plasma in both small and large heavy ion collision systems, using hydrodynamic simulations and comparing different pre-hydrodynamic models.

Contribution

It extends the study of QGP properties to smaller collision systems and analyzes the effects of bulk viscosity and cavitation using multiple hydrodynamic frameworks.

Findings

Bulk viscosity significantly affects QGP evolution in small systems.

Cavitation can occur in the hydrodynamic expansion of QGP.

Different pre-hydrodynamic models yield varying insights into early-stage physics.

Abstract

Relativistic heavy ion collisions generate nuclear-sized droplets of quark-gluon plasma (QGP) that exhibit nearly inviscid hydrodynamic expansion. Smaller collision systems such as p+Au, d+Au, and $^{3}$He+Au at the Relativistic Heavy Ion Collider, as well as p+Pb and high-multiplicity p+p at the Large Hadron Collider may create even smaller droplets of QGP. If so, the standard time evolution paradigm of heavy ion collisions may be extended to these smaller systems. These small systems present a unique opportunity to examine pre-hydrodynamic physics and extract properties of the QGP, such as the bulk viscosity, where the short lifetimes of the small droplets makes them more sensitive to these contributions. Here we focus on the influence of bulk viscosity, its temperature dependence, and cavitation effects on the dynamics in small and large systems using the publicly available hydrodynamic codes SONIC and MUSIC. We also compare pre-hydrodynamic physics in different frameworks including AdS/CFT strong coupling, IP-GLASMA weak coupling, and free streaming or no coupling.