Bulk viscosity, particle spectra and flow in heavy-ion collisions

TL;DR

This paper investigates how bulk viscosity influences particle spectra and flow in heavy-ion collisions, using kinetic models and hydrodynamic simulations to estimate the viscosity's effects and bounds.

Contribution

It provides a detailed calculation of dissipative corrections to particle distributions due to bulk viscosity in QCD and implements these in hydrodynamic models for RHIC collisions.

Findings

Bulk viscosity significantly affects pT spectra and elliptic flow.

Viscous corrections are dominated by the distribution function correction df.

Estimated upper bound for bulk viscosity to entropy ratio is 0.05 near freeze-out.

Abstract

We study the effects of bulk viscosity on pT spectra and elliptic flow in heavy ion collisions. For this purpose we compute the dissipative correction df to the single particle distribution functions in leading-log QCD, and in several simplified models. We consider, in particular, the relaxation time approximation and a kinetic model for the hadron resonance gas. We implement these distribution functions in a hydrodynamic simulation of Au + Au collisions at RHIC. We find significant corrections due to bulk viscosity in hadron pT spectra and the differential elliptic flow parameter v2(pT). These corrections are dominated by viscous corrections to the distribution function. We find that the relation between df and the bulk viscosity is different in the quark gluon plasma and hadronic phases. Reliable bounds on the bulk viscosity require accurate calculations of df in a hadronic resonance gas. Based on v2 spectra at RHIC we conservatively estimate zeta/s <= 0.05 near freeze-out. We also find that effects of the bulk viscosity on the pT integrated v2 are small.