QCD vacuum

The lowest-energy state of quantum chromodynamics, the theory of quarks and gluons. It is a structured quantum state with nonzero condensates and nontrivial topology.

Quantum chromodynamics: The quantum field theory describing quarks, gluons, and the strong interaction. The vacuum is QCD’s lowest-energy state, defined by this theory’s fields and dynamics.
Quantum chromodynamics vacuum angle: The parameter multiplying the topological term in the QCD Lagrangian. It weights topological sectors and governs possible violation of charge-parity symmetry.
QCD sum rules: A method relating hadron properties to short-distance QCD expansions and vacuum condensates. They made vacuum condensates quantitative inputs to hadron phenomenology.
Strong CP problem: The puzzle of why QCD appears to conserve charge-parity symmetry despite allowing its violation. Its unresolved parameter is the vacuum angle, whose observed value is exceptionally small.
Quark: A fundamental fermion that carries color charge and participates in the strong interaction. Quark fields contribute to the vacuum and form quark–antiquark condensates.
Instanton: A finite-action solution of Euclidean Yang–Mills equations that connects topologically distinct gauge configurations. Instantons illustrate how tunneling between gauge-field configurations shapes the vacuum.
Shifman–Vainshtein–Zakharov sum rules: QCD sum rules developed by Mikhail Shifman, Arkady Vainshtein, and Valentin Zakharov. Their 1979 work established a systematic role for condensates in QCD vacuum descriptions.
Axion: A hypothetical light pseudoscalar particle proposed to solve the strong CP problem. An axion field could dynamically relax the effective QCD vacuum angle.
Gluon: A gauge boson that mediates the strong interaction and carries color charge. Gluon fields fluctuate in the vacuum and generate its nonperturbative structure.
Topological susceptibility: A measure of fluctuations in the topological charge of a gauge theory. It quantifies topological fluctuations in the QCD vacuum.

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