QCD matter in extreme environments

Abstract

We review various theoretical approaches to the states of quantum chromodynamics (QCD) matter out of quarks and gluons in extreme environments such as the high-temperature states at zero and finite baryon density and the dimensionally reduced state under an intense magnetic field. The topics at high temperature include the Polyakov loop and the ’t Hooft loop in the perturbative regime, the Polyakov loop behaviour and the phase transition in some of the non-perturbative methods; the strong-coupling expansion, the large-Nc limit and the holographic QCD models. These analyses are extended to hot and dense matter with a finite baryon chemical potential. We point out that the difficulty in the finite-density problem bears similarity to that under a strong magnetic field. We make a brief summary of results related to the topological contents probed by the magnetic field and the chiral magnetic effect. We also address the close connection to the (1+1)-dimensional system.