Two Lectures on the Phase Diagram of QCD

TL;DR

This paper explores the QCD phase diagram at finite temperature and density, using large N_c limits and string models to describe thermodynamics and phase transitions, including a novel Quarkyonic phase.

Contribution

It introduces a string-based model for QCD thermodynamics with no free parameters and discusses the existence of multiple phases, including a new Quarkyonic phase, at different densities.

Findings

String model describes QCD thermodynamics below 160 MeV

At least three phases exist at zero baryon density with different N_c dependencies

A distinct Quarkyonic phase appears at high density with unique chiral properties

Abstract

The phase diagram of QCD at finite temperature and density is discussed. Large numbers of quark colors, $N_{\rm c} >> 1$, is used to explain generic features of the phase diagram. For temperatures below $ T \le 160$~MeV at zero baryon number density, the three dimensional string model is shown to describe the thermodynamics of QCD, and as well, the integrated spectrum of non-Goldstone mesons and glueballs. The lowest mass state in the spectrum of the open and closed string is treated separately due to the tachyon problem of string theory. This is with no undetermined free parameters. It is argued that there are at least three phases at zero baryon number density characterized by the $N_{\rm c}$ dependence of extensive thermodynamic quantities. It is also argued that the intermediate phase has restored chiral symmetry. At high baryon number density and low temperature, again there are three phases. A Quarkyonic phase, with energy density of order $N_{\rm c}$, is distinguished from its counterpart at low baryon density and temperature by its chiral properties.