Mean field theory of effective spin models as a baryon fugacity expansion

TL;DR

This paper develops a mean field theory for effective spin models with chemical potential, revealing how baryon fugacity expansion can describe phase transitions and corrections in lattice gauge theories.

Contribution

It introduces a mean field approach that incorporates the chemical potential via baryon fugacity, providing a new way to analyze phase diagrams in SU(N) models.

Findings

Numerical solution of SU(3) mean field equations maps the phase diagram.

First corrections to mean field results significantly affect the transition endpoint.

The approach offers insights into deriving effective spin models from full QCD.

Abstract

The free energy of effective spin or "Polyakov line" models with a chemical potential, based on the U(N) group, does not depend on the chemical potential. In a mean field-inspired expansion, we show how the condition of unit determinant, taking U(N) to SU(N), reintroduces the chemical potential, and allows us to express the free energy, as a function of mean field variational parameters, in terms of an expansion in the baryon (rather than the quark) fugacity at each lattice site. We solve the SU(3) mean field equations numerically to determine the phase diagram and compute observables. We also calculate the first corrections to the leading order mean field results, and find that these can significantly shift the endpoint of a line of first order transitions. The problem of deriving an effective spin model from full QCD is discussed.