## Quick Reference

(QCD)

A gauge theory that describes the strong interaction in terms of quarks and antiquarks and the exchange of massless gluons between them (see also elementary particles). Quantum chromodynamics is similar to quantum electrodynamics (QED), with colour being analogous to electric charge and the gluon being the analogue of the photon. The gauge group of QCD is non-Abelian and the theory is much more complicated than quantum electrodynamics; the gauge symmetry in QCD is not a broken symmetry.

QCD has the important property of asymptotic freedom – that at very high energies (and, hence, short distances) the interactions between quarks tend to zero as the distance between them tends to zero. Because of asymptotic freedom, perturbation theory may be used to calculate the high energy aspects of strong interactions, such as those described by the parton model.

*Subjects:*
Physics.

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