Bulk viscosity and the phase transition of the linear sigma model

TL;DR

This paper investigates the behavior of bulk viscosity near the phase transition in the linear sigma model using different theoretical approaches, revealing the presence or absence of a maximum at the critical temperature.

Contribution

It compares the large-N limit and CJT-Hartree approximation in analyzing bulk viscosity, highlighting their complementary insights and limitations.

Findings

Maximum bulk viscosity absent in large-N limit

Maximum appears in CJT-Hartree approximation

Both approaches provide complementary understanding of phase transition behavior

Abstract

In this work we deal with the critical behavior of the bulk viscosity in the linear sigma model (LSM) as an example of a system which can be treated by using different techniques. Starting from the Boltzmann-Uehling-Uhlenbeck equation we compute the bulk viscosity over entropy density of the LSM in the large-N limit. We search for a possible maximum of the bulk viscosity over entropy density at the critical temperature of the chiral phase transition. The information about this critical temperature, as well as the effective masses, is obtained from the effective potential. We find that the expected maximum (as a measure of the conformality loss) is absent in the large N in agreement with other models in the same limit. However, this maximum appears when, instead of the large-N limit, the Hartree approximation within the Cornwall-Jackiw-Tomboulis (CJT) formalism is used. Nevertheless, this last approach to the LSM does not give rise to the Goldstone theorem and also predicts a first order phase transition instead of the expected second order one. Therefore both, the large-N limit and the CJT-Hartree approximations, should be considered as complementary for the study of the critical behavior of the bulk viscosity in the LSM.