Chiral Symmetry and Heavy-Ion Collisions

TL;DR

This paper reviews the QCD phase diagram related to chiral symmetry and deconfinement in hot and dense matter, highlighting areas of theoretical certainty and exploring alternative phase transition scenarios.

Contribution

It clarifies which regions of the QCD phase diagram are theoretically understood and discusses the possibility of non-standard phase transition scenarios.

Findings

Certain parts of the phase diagram are well-understood theoretically.

Alternative phase transition scenarios are logically possible.

The role of chiral symmetry restoration outside the critical point is uncertain.

Abstract

I revisit the phase structure of hot and dense matter out of quarks and gluons with some historical consideration on the color deconfinement and chiral phase transitions. My goal is to make clear which part of the QCD phase diagram is under theoretical control and which part is not. I demonstrate that an uncommon but logically possible scenario other than the standard phase diagram cannot be ruled out. My emphasis is on the concern that one should correctly understand what kind of phenomenon occurs associated with the phase boundary line on the diagram. It is not quite obvious, in particular, where chiral symmetry restoration plays a phenomenological role in the temperature and baryon density plane except at the QCD (chiral) critical point.