| QCD | |
|---|---|
| Name | Quantum chromodynamics |
| Caption | Diagrammatic depiction of gluon self-interaction |
| Field | Particle physics |
| Introduced | 1973 |
| Creators | Murray Gell-Mann; Georgi; development by David Gross and Frank Wilczek; H. David Politzer |
| Institutions | CERN, Fermilab, SLAC National Accelerator Laboratory, Brookhaven National Laboratory |
QCD
Quantum chromodynamics (QCD) is the quantum field theory describing the strong interaction between quarks and gluons, the fundamental constituents of hadronic matter. As a non-Abelian gauge theory based on the SU(3) symmetry group, QCD explains phenomena from the binding of protons and neutrons in nuclei to high-energy scattering observed at colliders, and is central to the Standard Model of particle physics.
QCD is one of the four fundamental interactions in the Standard Model alongside the electroweak interaction and electromagnetism. It provides the theoretical framework for understanding the dynamics of quark flavors such as up, down, strange, charm, bottom and top, and the exchange of gluon gauge bosons. QCD phenomena link to nuclear physics via Quantum hadrodynamics and to early-universe cosmology through the quark–gluon plasma phase studied in heavy-ion collisions at facilities like RHIC and the Large Hadron Collider. The theory interfaces with theoretical frameworks including renormalization group methods and effective field theory.
The central charge in QCD is color charge, carried by quarks in three types conventionally labeled red, green and blue. Gluons carry color–anticolor combinations and transform in the adjoint representation of SU(3), giving rise to gluon self-interactions absent in quantum electrodynamics (QED). This non-Abelian gauge symmetry underlies key properties such as asymptotic freedom and confinement. Foundational contributions include the classification of hadrons via the Eightfold Way and the quark model of Murray Gell-Mann and George Zweig. The gauge principle is implemented using local gauge symmetry and covariant derivatives in the QCD Lagrangian.
The QCD Lagrangian combines Dirac fields for quarks with the non-Abelian field-strength tensor for gluons, parametrized by the strong coupling constant g_s. In perturbative regimes at high energy, calculations use Feynman diagram techniques, dimensional regularization, and renormalization; pioneering perturbative results were derived by David Gross, Frank Wilczek, and H. David Politzer, earning a Nobel Prize in Physics for the discovery of asymptotic freedom. Perturbative QCD (pQCD) underlies predictions for processes measured at CERN's ATLAS and CMS experiments, and requires matching to parton distribution functions as encoded in global fits from collaborations like CTEQ and NNPDF.
Asymptotic freedom—decrease of the effective coupling at high momentum transfer—follows from the negative beta function computed in pQCD and explains why quarks behave as quasi-free partons in deep inelastic scattering experiments at SLAC and later at HERA. Confinement—the absence of free color-charged particles—remains a nonperturbative QCD property associated with flux-tube formation and the area-law behavior of Wilson loops introduced by Kenneth Wilson. The scale parameter Λ_QCD sets the transition between perturbative and nonperturbative regimes. Concepts such as the running coupling constant and beta function are central to renormalization-group analyses that connect low-energy hadronic physics to high-energy collider phenomenology.
QCD predicts that hadrons are color-singlet bound states: mesons (quark–antiquark) and baryons (three quarks). Spectroscopy of mesons and baryons is organized by quantum numbers and classified in potential models and lattice computations; notable states include the proton, neutron, pion, kaon, and heavier resonances such as the J/ψ and Υ (upsilon). Parton model descriptions and structure functions measured in deep inelastic scattering characterize the momentum and spin distributions of quarks and gluons inside hadrons. The proton spin crisis highlighted the role of gluon and orbital angular momentum contributions, studied by experiments at COMPASS, Jefferson Lab, and RHIC spin programs.
Nonperturbative QCD is primarily addressed by lattice gauge theory methods, discretizing Euclidean space-time to compute observables numerically using Monte Carlo algorithms. Lattice QCD provides first-principles determinations of hadron masses, decay constants, and matrix elements relevant for flavor physics and tests of the Cabibbo–Kobayashi–Maskawa matrix via collaborations like the MILC Collaboration and groups at RIKEN-BNL Research Center. Other approaches include QCD sum rules developed by Shifman–Vainshtein–Zakharov, chiral perturbation theory for low-energy dynamics, and models such as the Nambu–Jona-Lasinio model and potential models inspired by string theory flux tubes.
QCD has been tested across a range of experiments: scaling violations in deep inelastic scattering observed at SLAC and CERN; jet production and three-jet events confirming gluon emission at PETRA and later at LEP; quarkonium spectroscopy and heavy-flavor production at B-factories (BaBar, Belle) and at LHCb; and the creation of quark–gluon plasma in heavy-ion collisions at CERN SPS, RHIC, and ALICE at the LHC. Precision lattice and perturbative computations contribute to tests of CP violation and searches for physics beyond the Standard Model in rare decays studied by collaborations such as LHCb and Belle II.
Category:Quantum chromodynamics Category:Quantum field theory Category:Particle physics