Identified particles from viscous hydrodynamics

TL;DR

This paper investigates how viscous hydrodynamics models particle observables, showing that using transport theory-based phase space corrections improves the understanding of particle flow and equilibrium states.

Contribution

It introduces a novel approach to phase space corrections derived from linearized covariant transport theory, contrasting with the traditional Grad ansatz.

Findings

Protons are closer to equilibrium than pions in viscous hydrodynamics.

Linear response effectively describes shear stress sharing but not phase space corrections.

Grad's quadratic ansatz better captures the momentum dependence of phase space corrections.

Abstract

Identified particle observables from viscous hydrodynamics are sensitive to the fluid-to-particle conversion. Instead of the commonly assumed "democratic" Grad ansatz for phase space corrections $\delta f$, we utilize corrections calculated from linearized covariant transport theory. Estimates based on a pion-proton system with binary collisions indicate that protons are much closer to equilibrium than pions, significantly affecting the dissipative reduction of differential elliptic flow in Au+Au at RHIC. In addition, we test linear response against fully nonlinear transport for a two-component massless system in a Bjorken scenario. Strikingly, we find that, while linear response accounts well for the dynamical sharing of shear stress, the momentum dependence of phase space corrections is best described by Grad's quadratic ansatz, and not the linear response solution.