QCD Under Extreme Conditions

The physics of strong interactions is described by Quantum Chromodynamics (QCD). At high energy, large temperatures and very large densities, QCD is a weakly interacting theory of quarks and gluons. At low energies, temperatures and densities, QCD enters a strongly-correlated regime, that is governed by the two phenomena of dynamical chiral symmetry breaking and confinement. Moreover, the dynamical degrees of freedom change from quarks and gluons to hadronic degrees of freedom, allowing for gas, liquid and potentially inhomogeneous phases. This regime as well as the dynamical change from the weakly interacting regime to the strongly-correlated one can only be assessed by putting QCD under extreme conditions such as in a heavy-ion collision.   

In the past decades, our understanding of QCD under extreme conditions has skyrocketed, mainly due to heavy-ion experiments and the accompanying theoretical advances. Astrophysical observations of neutron stars and mergers have added new insights, with much more data being collected in the coming years. Accordingly, QCD under extreme conditions remains a vibrant and challenging field, rich with unresolved phenomena and potential new phases.

In this talk, I will review our current understanding of QCD under extreme conditions, focusing on the hunt for the critical end point and and possible new phases of matter at high densities. I will also touch upon the challenge of reconstructing the equilibrium and non-equilibrium dynamics of heavy-ion collisions from the heavy-ion data. The talk is closed with a personal perspective on outstanding challenges and chances for QCD under extreme conditions in view of new experiments and theoretical advances.