Degeneracy patterns of chiral companions at finite temperature

TL;DR

This paper reviews how chiral symmetry restoration at finite temperature leads to degeneracy patterns among chiral partners in various effective QCD models, highlighting different mechanisms and the case of multiple partners.

Contribution

It provides a comprehensive comparison of degeneracy patterns in different effective theories and discusses the novel scenario involving more than two chiral partners.

Findings

Chiral partners become degenerate above the transition temperature.

Different models predict distinct degeneracy patterns.

Multiple chiral partners can be involved in symmetry restoration.

Abstract

Chiral symmetry represents a fundamental concept lying at the core of particle and nuclear physics. Its spontaneous breaking in vacuum can be exploited to distinguish chiral hadronic partners, whose masses differ. In fact, the features of this breaking serve as guiding principles for the construction of effective approaches of QCD at low energies, e.g., the chiral perturbation theory, the linear sigma model, the (Polyakov)--Nambu--Jona-Lasinio model, etc. At high temperatures/densities chiral symmetry can be restored bringing the chiral partners to be nearly degenerated in mass. At vanishing baryochemical potential, such restoration follows a smooth transition, and the chiral companions reach this degeneration above the transition temperature. In this work I review how different realizations of chiral partner degeneracy arise in different effective theories/models of QCD. I distinguish the cases where the chiral states are either fundamental degrees of freedom or (dynamically-generated) composed states. In particular, I discuss the intriguing case in which chiral symmetry restoration involves more than two chiral partners, recently addressed in the literature.