Thermodynamic properties and bulk viscosity near phase transition in the Z(2) and O(4) models

TL;DR

This paper explores thermodynamic and transport properties near phase transitions in Z(2) and O(4) models, revealing how bulk viscosity behavior correlates with different types of phase transitions and its potential as a critical endpoint indicator.

Contribution

It provides a detailed analysis of bulk viscosity and thermodynamic properties across various phase transition scenarios using the CJT formalism, highlighting their interrelations and implications for RHIC.

Findings

Bulk viscosity over entropy density peaks sharply at first-order transitions.

A cusp appears in bulk viscosity over entropy density at second-order transitions.

Crossover transitions show smooth or no peaks in bulk viscosity over entropy density.

Abstract

We investigate the thermodynamic properties including equation of state, the trace anomaly, the sound velocity and the specific heat, as well as transport properties like bulk viscosity in the Z(2) and O(4) models in the Hartree approximation of Cornwall-Jackiw-Tomboulis (CJT) formalism. We study these properties in different cases, e.g. first order phase transition, second order phase transition, crossover and the case without phase transition, and discuss the correlation between the bulk viscosity and the thermodynamic properties of the system. We find that the bulk viscosity over entropy density ratio exhibits an upward cusp at the second order phase transition, and a sharp peak at the 1st order phase transition. However, this peak becomes smooth or disappears in the case of crossover. This indicates that at RHIC, where there is no real phase transition and the system experiences a crossover, the bulk viscosity over entropy density might be small, and it will not affect too much on hadronization. We also suggest that the bulk viscosity over entropy density ratio is a better quantity than the shear viscosity over entropy density ratio to locate the critical endpoint.