QuarkGluonPlasma

Quark-Gluon Plasma (QGP) is a state of matter in quantum chromodynamics in which quarks and gluons are deconfined and can move over distances larger than those of individual hadrons. It is expected to occur at extreme temperatures or baryon densities and is believed to have filled the early universe briefly after the Big Bang. Laboratory creation of QGP is pursued in ultrarelativistic heavy-ion collisions, where nuclei are accelerated to near-light speeds and collided.

In QGP, color charges are not bound into mesons or baryons, and chiral symmetry is approximately restored. The system is described by an equation of state that connects pressure, energy density, and temperature. Experimental and theoretical studies indicate that at low net baryon density the transition from hadronic matter to QGP is a crossover, with a critical temperature around 150–160 MeV. The QGP formed in collisions behaves as a strongly coupled fluid with very low viscosity relative to entropy density.

Observables in experiments include collective flow patterns, jet quenching, and enhanced production of strange hadrons. Elliptic flow measurements reveal rapid thermalization and hydrodynamic-like expansion. Jet quenching shows that high-energy partons lose energy while traversing the medium. Direct photons and heavy quarks provide information about the temperature and transport properties of the plasma.

Theoretical tools used to study QGP include lattice quantum chromodynamics, which yields equilibrium thermodynamics, and relativistic hydrodynamics, which models the expansion and cooling of the plasma. Perturbative and nonperturbative methods contribute to transport coefficients such as the jet quenching parameter and the shear viscosity to entropy density ratio. These studies inform our understanding of strong interactions under extreme conditions and the physics of the early universe.