Viscous QCD matter in a hybrid hydrodynamic+Boltzmann approach

TL;DR

This paper introduces a hybrid hydrodynamic and Boltzmann approach to model viscous QCD matter in heavy-ion collisions, highlighting the importance of combining macroscopic and microscopic methods for accurate description of the late-stage hadronic phase.

Contribution

It presents a novel hybrid model that integrates viscous hydrodynamics with Boltzmann transport to better simulate the entire evolution of QCD matter in collisions.

Findings

Hybrid approach improves modeling of late dilute phase.

Extracted shear viscosity depends on prior hydrodynamic history.

Pure hydrodynamics cannot fully describe hadron resonance gas dynamics.

Abstract

A hybrid transport approach for the bulk evolution of viscous QCD matter produced in ultra-relativistic heavy-ion collisions is presented. The expansion of the dense deconfined phase of the reaction is modeled with viscous hydrodynamics while the dilute late hadron gas stage is described microscopically by the Boltzmann equation. The advantages of such a hybrid approach lie in the improved capability of handling large dissipative corrections in the late dilute phase of the reaction, including a realistic treatment of the non-equilibrium hadronic chemistry and kinetic freeze-out. By varying the switching temperature at which the hydrodynamic output is converted to particles for further propagation with the Boltzmann cascade we test the ability of the macroscopic hydrodynamic approach to emulate the microscopic evolution during the hadronic stage and extract the temperature dependence of the effective shear viscosity of the hadron resonance gas produced in the collision. We find that the extracted values depend on the prior hydrodynamic history and hence do not represent fundamental transport properties of the hadron resonance gas. We conclude that viscous fluid dynamics does not provide a faithful description of hadron resonance gas dynamics with predictive power, and that both components of the hybrid approach are needed for a quantitative description of the fireball expansion and its freeze-out.