Quark–gluon plasma (QGP) is a state of matter in which quarks, antiquarks, and gluons form an extended, interacting medium rather than remaining confined within individual hadrons. It occurs at extreme temperatures and energy densities and is described by quantum chromodynamics (QCD), the theory of the strong interaction. Experiments create short-lived droplets through high-energy nuclear collisions, enabling the study of matter resembling that present during the first microseconds after the Big Bang. Despite its name, the QGP produced in these collisions behaves predominantly as a strongly interacting fluid rather than a dilute gas. (home.cern)
Physical basis
In ordinary nuclear matter, color confinement restricts quarks and gluons to composite particles, including protons and neutrons. Their color charge is the charge associated with QCD, not a visual property. In QGP, color-carrying constituents participate in a medium extending beyond the dimensions of an individual hadron. Deconfinement does not mean that interactions disappear, nor that isolated quarks can be collected after the medium cools. (arxiv.org)
QCD exhibits asymptotic freedom: its interaction strength decreases at sufficiently high momentum scales. This helped motivate early expectations of a comparatively weakly interacting quark–gluon gas at very high temperatures. However, experimental plasma near accessible transition temperatures has substantial collective interactions. The distinction between deconfinement and weak coupling is therefore essential. (arxiv.org)
Unlike an ordinary plasma, whose characteristic constituents are electrically charged ions and electrons, QGP is defined by the behavior of strongly interacting constituents and color fields. The two uses of “plasma” share the idea of a medium containing mobile charge carriers, but involve different underlying interactions. (arxiv.org)
Temperature and the QCD transition
The transition to QGP is studied using lattice QCD, which evaluates the theory numerically on a discretized spacetime grid. With physical quark masses and approximately zero net baryon density, calculations find a smooth crossover rather than an abrupt first-order phase transition. A characteristic crossover temperature is around 155 MeV in thermal-energy units, corresponding to approximately 1.8 trillion kelvin. Because the transition is gradual, different observables need not identify precisely the same characteristic temperature. (indico.bnl.gov)
The QCD equation of state relates pressure, energy density, and other thermodynamic quantities. It supplies an important connection between microscopic calculations and the macroscopic expansion of collision-produced matter. At greater net baryon densities, the phase structure is less firmly established; searches investigate whether a first-order transition and an associated critical point exist. These possibilities are research questions rather than necessary features of every QGP system. (bnl.gov)
Production and experimental history
High-energy collisions of heavy atomic nuclei, especially gold or lead, deposit energy into a small region. The resulting matter expands, cools, and undergoes hadronization, forming the particles subsequently recorded by detectors. Thus, experiments infer plasma properties from collision products rather than observing a stable sample of QGP. Particle distributions, correlations, and energy losses provide complementary information about its evolution. (home.cern)
On February 10, 2000, CERN announced evidence for a new, deconfined state of matter from its Super Proton Synchrotron heavy-ion programme. The announcement drew on combined results from several experiments and described matter displaying characteristics expected of QGP. (home.cern)
At Brookhaven National Laboratory, the Relativistic Heavy Ion Collider (RHIC) extended these investigations. On April 18, 2005, its four experimental collaborations reported that the hot, dense matter produced in gold–gold collisions behaved more like a strongly interacting liquid than the weakly interacting gas many researchers had anticipated. (bnl.gov)
The Large Hadron Collider provides another experimental environment. Its ALICE experiment was specifically designed to investigate strongly interacting matter at extreme energy densities. ATLAS and CMS also study heavy-ion collisions, including the modification of energetic particle production by the medium. (home.cern)
Fluid behavior and experimental probes
A major signature is collective flow: the momenta of emitted particles reflect pressure-driven expansion and the geometry of the initial collision region. In noncentral collisions, the overlap region is anisotropic, producing characteristic directional differences in particle emission. Descriptions using relativistic hydrodynamics reproduce important features of these patterns and support the interpretation of QGP as a collectively evolving fluid. (bnl.gov)
Its viscosity is small relative to its entropy density. The expression “nearly perfect liquid” refers to limited dissipative effects, not strictly zero viscosity. Extracting transport coefficients requires modeling the initial conditions, expansion, and later hadronic stage; the resulting values are inferred rather than directly measured with a conventional viscometer. (bnl.gov)
Another major probe is jet quenching. Energetic quarks or gluons generated in an initial hard collision lose energy while traversing the medium, modifying the resulting particle jets. Researchers compare jet yields, internal structure, and momentum balance with reference collisions to investigate how energy and momentum are transferred to the plasma. (home.cern)
Heavy quarks provide additional probes. Their diffusion, energy loss, and eventual formation of heavy-flavor hadrons constrain microscopic interactions and transport properties. Heavy quark–antiquark bound states also respond to the surrounding medium. Interpreting these measurements requires accounting for both in-medium evolution and hadronization, so a consistent picture draws on several observables rather than a single exclusive signal. (arxiv.org)
Cosmological context
In cosmology, QGP describes the strongly interacting component of the early universe before cooling allowed hadrons to dominate. Laboratory collisions reproduce aspects of this hot QCD matter, but not the entire cosmological environment: collision systems are finite and rapidly expanding, whereas the primordial medium evolved within the expanding universe. Their connection rests on shared microscopic physics and thermodynamic properties, not identical size, lifetime, or composition. (home.cern)