Physics of hot and dense QCD

TL;DR

This paper reviews the theoretical understanding of QCD at finite temperature and density, discussing phase transitions, hadron gas properties, and the limitations of perturbation theory at collider energies.

Contribution

It provides a concise overview of the current theoretical insights into QCD phase behavior at finite temperature and density, highlighting the potential absence of a temperature-driven phase transition.

Findings

Phase transition at finite temperature is likely absent.

Finite density effects can induce a strong phase transition.

Perturbation theory is not yet reliable at RHIC energies.

Abstract

We give a brief review of modern theoretical understanding of the physics of QCD at finite temperature and density. We concentrate in particular on the properties of hadron gas at relatively small temperatures and discuss in details the physics of the phase transition. We argue that the phase transition in temperature is probably absent, but it can appear with a vengeance when finite density effects are taken into account. We notice also that the temperatures one can expect to reach at the heavy ion collider at RHIC are not yet high enough for the perturbation theory in the QCD coupling constant to work well.