Exploring the quantum chromodynamics landscape with high-energy nuclear collisions

A quantum chromodynamics (QCD) phase diagram is usually plotted as the temperature ( T ) versus the chemical potential associated with the conserved baryon number (μ B ). Two fundamental properties of QCD, related to confinement and chiral symmetry, allow for two corresponding phase transitions when T and μ B are varied. Theoretically, the phase diagram is explored through non-perturbative QCD calculations on a lattice. The energy scale for the phase diagram (Λ QCD ∼200 MeV) is such that it can be explored experimentally by colliding nuclei at varying beam energies in the laboratory. In this paper, we review some aspects of the QCD phase structure as explored through experimental studies using high-energy nuclear collisions. Specifically, we discuss three observations related to the formation of a strongly coupled plasma of quarks and gluons in the collisions, the experimental search for the QCD critical point on the phase diagram and the freeze-out properties of the hadronic phase.