Self-consistent conversion of a viscous fluid to particles

TL;DR

This paper derives species-dependent shear viscous phase space corrections from Boltzmann transport theory to improve the conversion of hydrodynamic fluids to particles in heavy-ion collision models, affecting particle flow observables.

Contribution

It provides a self-consistent method for species-dependent viscous corrections in the fluid-to-particle conversion process, enhancing the accuracy of hybrid heavy-ion collision simulations.

Findings

Proton elliptic flow exceeds pion flow at moderate high pT with the new corrections.

Species-dependent corrections differ significantly from the traditional democratic Grad ansatz.

Tabulated corrections facilitate implementation in hydrodynamic and hybrid models.

Abstract

Comparison of hydrodynamic and "hybrid" hydrodynamics+transport calculations to heavy-ion data inevitably requires the conversion of the fluid to particles. For dissipative fluids the conversion is ambiguous without additional theory input complementing hydrodynamics. We obtain self-consistent shear viscous phase space corrections from linearized Boltzmann transport theory for a gas of hadrons. These corrections depend on the particle species, and incorporating them in Cooper-Frye freezeout affects identified particle observables. For example, with additive quark model cross sections,proton elliptic flow is larger than pion elliptic flow at moderately high $p_T$ in $Au+Au$ collisions at RHIC. This is in contrast to Cooper-Frye freezeout with the commonly used "democratic Grad" ansatz that assumes no species dependence. Various analytic and numerical results are also presented for massless and massive two-component mixtures to aid the interpretation. Self-consistent viscous corrections for each species are tabulated in Appendix F for convenient inclusion in pure hydrodynamic and hybrid calculations.