Quantum chromodynamics

Quantum chromodynamics (QCD) is the theory describing one of the fundamental forces, the strong interaction. It describes the interactions of quarks and gluons and takes the form of a quantum field theory of a special kind called a non-abelian gauge theory. QCD forms an important part of the standard model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years.

Methods

Further analysis of the content of the theory is complicated. Various techniques have been developed to work with QCD. Some of them are discussed briefly below.

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Perturbative QCD

This approach is based on asymptotic freedom, which allows perturbation theory to be used accurately in experiments performed at very high energies. Although limited in scope, this approach has resulted in the most precise tests of QCD to date.

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Lattice QCD

Among non-perturbative approaches to QCD, the most well established one is lattice QCD. This approach uses a discrete set of space-time points (called the lattice) to reduce the analytically intractable path integrals of the continuum theory to a very difficult numerical computation which is then carried out on supercomputers. While it is a slow and resource-intensive approach, it has wide applicability, giving insight into parts of the theory inaccessible by other means.

Related Topics:
Lattice QCD - Supercomputers

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1/N expansion

A well-known approximation scheme, the 1/N expansion, starts from the premise that the number of colors is infinite, and makes a series of corrections to account for the fact that it is not. Until now it has been the source of qualitative insight rather than a method for quantitative predictions. Modern variants include the AdS/CFT approach.

Related Topics:
1/N expansion - AdS/CFT

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Effective theories

For specific problems some theories may be written down which seem to give qualitatively correct results. In the best of cases, these may then be obtained as systematic expansions in some parameter of the QCD Lagrangian. Among the best such effective models one should now count chiral perturbation theory (which expands around light quark masses near zero) and heavy quark effective theory (which expands around heavy quark mass near infinity). Other less accurate models are the Nambu-Jona-Lasinio model and the chiral model.

Related Topics:
Chiral perturbation theory - Heavy quark effective theory - Nambu-Jona-Lasinio model - Chiral model

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