ABSTRACT
Quantum chromodynamics (QCD) at finite temperature, T , and quark chemical potential, μ, has a rich phase structure: at low T and low μ, the NambuGoldstone (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 μ, a Fermi liquid of deconfined quarks is expected to appear as a consequence of asymptotic freedom. Furthermore, in such cold quark matter, condensation of quark-quark pairs leads to color superconductivity (CSC). At high T for arbitrary μ, all the condensates melt away and a quark-gluon plasma (QGP) is realized. The experimental exploration of thermal phase transitions 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 at the LHC (Large Hadron Collider). The quantum phase transition (QPT) 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 QPTs will be summarized with particular emphasis on the symmetry realization of each phase. Possible connections between the physics of QCD and that of ultracold atoms are also discussed.
