Effects of Bulk Viscosity at Freezeout

TL;DR

This paper studies how bulk viscosity affects particle spectra and flow in heavy ion collisions, highlighting its significant impact despite being relatively small, and develops a consistent method to incorporate these effects.

Contribution

It introduces a consistent procedure to determine phase space distribution corrections due to bulk viscosity in multi-component systems and assesses their impact on observables.

Findings

Bulk viscosity has a noticeable effect on particle spectra.

Bulk viscosity influences elliptic flow coefficients.

The developed method ensures consistent microscopic-macroscopic variable matching.

Abstract

We investigate particle spectra and elliptic flow coefficients in relativistic heavy ion collisions by taking into account the distortion of phase space distributions due to bulk viscosity at freezeout. We first calculate the distortion of phase space distributions in a multi-component system within the Grad's fourteen moment method. We find some subtle issues when one matches macroscopic variables with microscopic momentum distributions in a multi-component system and develop a consistent procedure to uniquely determine the corrections in the phase space distributions. Next, we calculate particle spectra by using the Cooper-Frye formula to see the effect of the bulk viscosity. In spite of the relative smallness of the bulk viscosity, we find that it is likely to have a visible effect in particle spectra and elliptic flow coefficients. This indicates the importance of bulk viscosity together with shear viscosity if one wants to constrain the transport coefficients with better accuracy from experimental data.