Quantum Phase Transitions in Dense QCD

TL;DR

This paper reviews the complex phase structure of QCD at finite temperature and density, focusing on quantum phase transitions like chiral symmetry breaking, deconfinement, and color superconductivity, with implications for heavy-ion collisions and neutron stars.

Contribution

It provides a comprehensive summary of the current understanding of QCD phase transitions, emphasizing symmetry aspects and potential connections to ultracold atom physics.

Findings

Different QCD phases characterized by distinct condensates

Quantum phase transitions linked to symmetry changes

Relevance to heavy-ion collisions and astrophysical objects

Abstract

Quantum chromodynamics (QCD) at finite temperature, $T$, and quark chemical potential, $\mu$, has a rich phase structure: at low $T$ and low $\mu$, the Nambu-Goldstone (NG) phase with nearly massless pions is realized by the dynamical breaking of chiral symmetry through condensation of quark-anti-quark pairs, while, at low $T$ and high $\mu$, a Fermi liquid of deconfined quarks is expected to appear as a consequence of asymptotic freedom. Furthermore, in such a cold quark matter, condensation of quark-quark pairs leads to the color superconductivity (CSC). At high $T$ for arbitrary $\mu$, all the condensates melt away and a quark-gluon plasma (QGP) is realized. The experimental exploration of thermal phase transition from the NG phase to QGP is being actively pursued in ultrarelativistic heavy ion collisions at RHIC (Relativistic Heavy Ion Collider), and will be continued in the future at LHC (Large Hadron Collider). The quantum phase transition from the NG phase to the CSC at low $T$ is also relevant to heavy-ion collisions at moderate energies, and is of interest in the interiors of neutron stars and possible quark stars. In this Chapter, after a brief introduction to the basic properties of QCD, the current status of the QCD phase structure and associated quantum phase transitions will be summarized with particular emphasis on the symmetry realization of each phase. Possible connection between the physics of QCD and that of ultracold atoms is also discussed.