The Phase Structure of QCD: Fluctuations and Correlations

The strong interaction, governed by quantum chromodynamics (QCD), shapes the structure of the visible Universe. At about 10 μs after the big bang, the quark–gluon plasma (QGP)—the primordial matter made up of quarks and gluons plus leptons, photons, and neutrinos—became cool enough to create, in a phase transition, the protons and neutrons of ordinary matter, along with other strongly interacting unstable hadrons. This new phase of matter was predicted within the framework of QCD and has been studied in accelerator laboratories worldwide for about 40 years to date. This review explores recent breakthroughs in the study of the QCD phase diagram. We highlight measurements of particle production and fluctuations and compare them with theoretical predictions. We summarize our current understanding of the QCD structure and outline future experimental opportunities with high-energy nuclear collisions at fixed-target and collider facilities worldwide.