Hydrodynamic flow in heavy-ion collisions with large hadronic viscosity

TL;DR

This study uses viscous hydrodynamics to analyze how large hadronic shear viscosity affects particle spectra and elliptic flow in heavy-ion collisions, highlighting the importance of the relaxation time parameter.

Contribution

It investigates the impact of temperature-dependent hadronic shear viscosity and relaxation time on flow observables using advanced hydrodynamic simulations.

Findings

Hadron spectra are insensitive to increased eta/s in the hadronic stage.

Elliptic flow v_2(p_T) is significantly suppressed by large hadronic viscosity.

The suppression of v_2 can be reduced if kappa(T) increases as temperature decreases.

Abstract

Using the (2+1)-dimensional viscous hydrodynamic code VISH2+1 with a temperature dependent specific shear viscosity (eta/s)(T), we present a detailed study of the influence of a large hadronic shear viscosity and its corresponding relaxation time tau_pi on the transverse momentum spectra and elliptic flow of hadrons produced in 200 A GeV Au+Au collisions. Although theory, in principle, predicts a well-defined relation T*tau_pi = kappa(T)*(eta/s)(T), the precise form of kappa(T) for the matter created in relativistic heavy-ion collisions is not known. For the popular choice kappa=3 the hadron spectra are found to be insensitive to a significant rise of eta/s in the hadronic stage, whereas their differential elliptic flow v_2(p_T) is strongly suppressed by large hadronic viscosity. The large viscous effects on v_2 are strongly reduced if (as theoretically expected) kappa(T) is allowed to grow with decreasing temperature in the hadronic stage. This implies that, until reliable calculations of kappa(T) become available, an extraction of the hadronic shear viscosity from a comparison between VISH2+1 and a microscopic hadron cascade or experimental data requires a simultaneous fit of (eta/s)(T) and kappa(T).