QGP

Quark-Gluon Plasma (QGP) is a state of matter in quantum chromodynamics in which quarks and gluons are deconfined, not confined within hadrons. This occurs at extreme temperatures or densities, when color confinement is overcome as predicted by asymptotic freedom. In the early universe, microseconds after the Big Bang, matter is believed to have existed as QGP before cooling into hadrons. Today it is studied in relativistic heavy-ion collisions at RHIC and the LHC, where nuclei are briefly melted into a hot, dense medium with energy densities above the confinement threshold.

A defining feature is color deconfinement with approximate restoration of chiral symmetry at high temperature. The medium behaves as a strongly interacting fluid with very low viscosity, exhibiting collective flow. The equation of state and transport properties are explored with lattice QCD and relativistic hydrodynamics.

Experimental signatures include jet quenching, elliptic flow, strangeness enhancement, and enhanced photons and dileptons. Together with theory, these signatures support the creation of a deconfined, expanding medium in heavy-ion collisions. Lattice QCD indicates a crossover transition at zero baryon chemical potential around Tc ~ 150–160 MeV.

At finite baryon density, a critical point or first-order transition may occur; beam energy scans at RHIC and future facilities search this region. While QGP cannot be observed directly, its properties are inferred from multiparticle correlations and jet-medium interactions.

See also: Quark matter, Quantum chromodynamics, Heavy-ion collision.