Phase diagram of QCD matter in the temperature–baryon density plane. Baryons are hadrons containing three valence quarks; the most common are protons and neutrons, shown at the lower left. Colored spheres indicate individual quarks, which are not bound together in the quark-gluon plasma. RHIC (blue ovals) and LHC (green oval) explore matter with almost equal numbers of quarks and antiquarks. At lower beam energies, RHIC produces matter with a surplus of quarks, corresponding to high net baryon density. There may be a critical point (yellow circle) in the phase diagram, at the end of a line indicating a first-order phase transition.