Bulk pressure in fluid-dynamical simulations of Pb-Pb and p-Pb collisions at the LHC energies

TL;DR

This paper investigates the impact of bulk viscosity in fluid dynamical simulations of Pb-Pb and p-Pb collisions at LHC energies, revealing that bulk pressure can dominate and challenge the validity of current models.

Contribution

It provides a detailed analysis of bulk viscosity parametrizations and their significant effects on the pressure dynamics in high-energy heavy-ion collisions.

Findings

Bulk viscosity effects can surpass shear viscosity at the particlization hypersurface.

Bulk pressure can nearly cancel the equilibrium pressure in large space-time regions.

This dominance of bulk pressure questions the applicability of fluid dynamics at LHC energies.

Abstract

State-of-the-art fluid dynamical simulations of relativistic heavy-ion collisions employ initial state models which result in a rather strong radial flow. In order to fit the experimental observables, a non-negligible bulk viscosity of the QGP and/or hadronic matter is required. We examine modern parametrizations of the bulk viscosity to entropy density ratio $\zeta/s$ used in recent fluid dynamical simulations, and explore the relative magnitude of the associated bulk viscous corrections in space-time, for Pb-Pb and p-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV LHC energy with a state-of-the-art initial state provided by TRENTo model. It turns out that, at a typical particlization hypersurface, the effect of bulk viscosity out-competes the one of shear viscosity, and in a significantly large part of the space-time volume, the bulk pressure strongly counteracts the equilibrium pressure, thus the effective pressure approaches zero. The latter potentially challenges the applicability of fluid-dynamical modelling of heavy ion-ion and proton-ion collisions at the LHC energies.