Color superconductivity

Color superconductivity is a predicted phase of dense quark matter in which quarks form Cooper pairs. The pairing pattern modifies color gauge symmetry and gives color gauge fields effective masses.

BCS theory: A theory of superconductivity in which fermions form Cooper pairs near a Fermi surface. Quark pairing at high density extends the Cooper-pairing mechanism developed for electrons.
Quantum chromodynamics: The quantum field theory describing strong interactions among quarks and gluons. Its color gauge interaction supplies the forces and symmetries involved in quark pairing.
Neutron star: A compact stellar remnant composed mainly of densely packed neutrons and other matter. Its core may reach densities at which deconfined quark matter and pairing are possible.
QCD phase diagram: A map of the phases of quantum chromodynamics across temperature and baryon density. It places color superconductivity among the proposed phases at high density and relatively low temperature.
Nuclear superfluidity: A phase in which nuclear constituents, such as neutrons, form Cooper pairs and exhibit superfluid behavior. It involves paired hadrons rather than deconfined quarks and can occupy different density regimes.
Cooper pair: A correlated pair of fermions that can form when an attractive interaction acts near a Fermi surface. Attractive interactions bind quarks into the pairs responsible for color superconductivity.
Quark: An elementary fermion that carries color charge and participates in the strong interaction. Quarks are the fermions that pair in this phase.
Quark star: A hypothetical compact star made largely of deconfined quark matter. A quark-star interior could host color-superconducting phases.
Sign problem: A computational obstacle in which quantum Monte Carlo weights become complex or nonpositive. It limits first-principles lattice calculations of QCD at the high densities relevant to the phase.
Hadron-quark transition: A possible transition between matter composed of hadrons and matter with deconfined quarks. Color superconductivity concerns the quark-matter side, whose onset relative to hadrons is unknown.

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