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A predicted state of matter containing deconfined quarks and gluons. According to the theory of strong interactions, called quantum chromodynamics, hadrons such as mesons and nucleons (the generic name for protons and neutrons) are bound states of more fundamental objects called quarks. The quarks are confined within the individual hadrons by the exchange of particles called gluons. However, calculations indicate that at sufficiently high temperatures or densities, hadronic matter should evolve into a new phase of matter containing deconfined quarks and gluons, called a quark-gluon plasma or quark matter. Such a state of matter is thought to have existed briefly in the period about 1–10 microseconds after the big bang, and might also exist inside the cores of dense neutron stars. See Hadron, Quantum chromodynamics
The study of such a new state of matter requires a means for producing it under controlled laboratory conditions. Experimentally the transition from the hadronic to the quark-gluon phase requires collisions of beams of heavy ions such as nuclei of gold or uranium (although lighter nuclei can be used) with other heavy nuclei at high enough energies to produce the necessary extreme conditions of heat and compression. Quantum chromodynamics calculations using the lattice gauge model indicate that energy densities of at least 1–2 GeV/fm3 (1 femtometer = 10-15 m), about 10 times that found in ordinary nuclear matter, must be produced in the collision for plasma formation to occur. See Nuclear reaction, Relativistic heavy-ion collisions
Accelerator experiments using beams of nuclei with energies of 10–200 GeV/nucleon bombarding stationary nuclear targets have found interesting phenomena such as nuclear stopping. In such cases, the colliding nucleons of the target and projectile are observed to pile up on each other, achieving large nuclear matter densities (two to four times normal nuclear density, or higher) corresponding to energy densities near the threshold for quark matter production. Other results of these experiments suggest that conditions favorable to thermal and chemical equilibrium may be present in some of these collisions. Such experiments can provide critical tests of the theory of the strong interaction and illuminate the earliest moments of the universe. See Elementary particle, Gluons, Quarks