Progress of Theoretical Physics Supplement No. 153, 2004 139 QCD Phase Diagram and the Critical Point

The recent progress in understanding the QCD phase diagram and the physics of the QCD critical point is reviewed. §1. Preamble Quantum Chromodynamics is one of the most remarkable theories of Nature. Its mathematical foundations are concise, yet the phenomenology which the theory describes is broad and diverse. QCD phenomenology at finite temperature and baryon number density is one of the least explored regimes of the theory. There are several experimental windows into this regime. One is the physics associated with the interior of neutron stars. Another, which is the subject of the ongoing and planned experimental programs, is the physics of heavy ion collisions. This report is a review of recent developments in our understanding, mostly theoretical, but also experimental, of the phase diagram of QCD. There exist a number of excellent recent reviews1)–6) which discuss many of the questions addressed here as well as the related material not covered in this report. The report focuses on the physics of the critical point of QCD and its search. There are many open questions in this field, and some of the theoretical as well as experimental results and expectations discussed here might not hold under further scrutiny. Nevertheless it is hoped that this report will provide a useful contemporary guide to both theorists and experimentalists entering the field as well as a stimulating reading to the field’s experts. §2. What is the QCD critical point? 2.1. The phase diagram Figure 1 shows a sketch of the QCD phase diagram as it is perceived by a modern theorist. By a phase diagram we shall mean the information about the location of the phase boundaries (phase transitions) as well as the physics of the phases that these transitions delineate. The phase transitions are the thermodynamic singularities of the system. The system under consideration is a region (in theory, infinite) occupied by strongly interacting matter, described by QCD, in thermal and chemical equilibrium, characterized by the given values of temperature T and baryo