A quark–gluon plasma (QGP) or quark soup is a state of matter in quantum chromodynamics (QCD) which exists at extremely high temperature and/or density. This state is thought to consist of asymptotically free strong-interacting quarks and gluons, which are ordinarily confined by color confinement inside atomic nuclei or other hadrons. This is in analogy with the conventional plasma where nuclei and electrons, confined inside atoms by electrostatic forces at ambient conditions, can move freely. Experiments to create artificial quark matter started at CERN in 1986/87, resulting in first claims that were published in 1991. It took several years before the idea became accepted in the community of particle and nuclear physicists. Formation of a new state of matter in Pb–Pb collisions was officially announced at CERN in view of the convincing experimental results presented by the CERN SPS WA97 experiment in 1999, and later elaborated by Brookhaven National Laboratory's Relativistic Heavy Ion Collider. Quark matter can only be produced in minute quantities and is unstable and impossible to contain, and will radioactively decay within a fraction of a second into stable particles through hadronization; the produced hadrons or their decay products and gamma rays can then be detected. In the quark matter phase diagram, QGP is placed in the high-temperature, high-density regime, whereas ordinary matter is a cold and rarefied mixture of nuclei and vacuum, and the hypothetical quark stars would consist of relatively cold, but dense quark matter. It is believed that up to a few microseconds (10 to 10 seconds) after the Big Bang, known as the quark epoch, the Universe was in a quark–gluon plasma state.
The strength of the color force means that unlike the gas-like plasma, quark–gluon plasma behaves as a near-ideal Fermi liquid, although research on flow characteristics is ongoing. Liquid or even near-perfect liquid flow with almost no frictional resistance or viscosity was claimed by research teams at RHIC and LHC's Compact Muon Solenoid detector. QGP differs from a "free" collision event by several features; for example, its particle content is indicative of a temporary chemical equilibrium producing an excess of middle-energy strange quarks vs. a nonequilibrium distribution mixing light and heavy quarks ("strangeness production"), and it does not allow particle jets to pass through ("jet quenching").
Experiments at CERNs Super Proton Synchrotron (SPS) begun experiments to create QGP in the 1980s and 1990s: the results led CERN to announce evidence for a "new state of matter" in 2000. Scientists at Brookhaven National Laboratorys Relativistic Heavy Ion Collider announced they had created quark–gluon plasma by colliding gold ions at nearly the speed of light, reaching temperatures of 4 trillion degrees Celsius. Current experiments (2017) at the Brookhaven National Laboratorys Relativistic Heavy Ion Collider (RHIC) on Long Island (New York, USA) and at CERNs recent Large Hadron Collider near Geneva (Switzerland) are continuing this effort, by colliding relativistically accelerated gold and other ion species (at RHIC) or lead (at LHC) with each other or with protons. Three experiments running on CERN's Large Hadron Collider (LHC), on the spectrometers ALICE, ATLAS and CMS, have continued studying the properties of QGP. CERN temporarily ceased colliding protons, and began colliding lead ions for the ALICE experiment in 2011, in order to create a QGP. A new record breaking temperature was set by ALICE: A Large Ion Collider Experiment at CERN in August 2012 in the ranges of 5.5 trillion () kelvin as claimed in their Nature PR.
The formation of a quark–gluon plasma occurs as a result of a strong interaction between the partons (quarks, gluons) that make up the nucleons of the colliding heavy nuclei called heavy ions. Therefore, experiments are referred to as relativistic heavy ion collision experiments. Theoretical and experimental works show that the formation of a quark–gluon plasma occurs at the temperature of T ≈ 150–160 MeV, the Hagedorn temperature, and an energy density of ≈ 0.4–1 GeV / fm. While at first a phase transition was expected, present day theoretical interpretations propose a phase transformation similar to the process of ionisation of normal matter into ionic and electron plasma.