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.

Experimental tests

The notion of quark flavours was prompted by the necessity of explaining the properties of hadrons during the development of the quark model. The notion of colour was necessiated by the puzzle of the Δ++. This has been dealt with in the section on Quantum chromodynamics#History.

Related Topics:
Flavours - Hadron - Quark model - Quantum chromodynamics#History

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The first evidence for quarks as real constiutent elements of hadrons was obtained in deep inelastic scattering experiments at SLAC. The first evidence for gluons came in three-jet events at PETRA.

Related Topics:
Quark - Hadron - Deep inelastic scattering - SLAC - Three-jet events - PETRA

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Good quantitative tests of perturbative QCD are

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• the running of the QCD coupling as deduced from many observations
• scaling violation in polarized and unpolarized deep inelastic scattering
• vector boson production at colliders (this includes the Drell-Yan process)
• jet cross sections in colliders
• event shape observables at the LEP
• heavy-quark production in colliders

Quantitative tests of non-perturbative QCD are fewer, because the predictions are harder to make. The best is probably the running of the QCD coupling as probed through lattice computations of heavy-quarkonium spectra. There is a recent claim about the mass of the heavy meson Bc http://www.aip.org/pnu/2005/split/731-1.html. Other non-perturbative tests are currently at the level of 5% at best. Continuing work on masses and form factors of hadrons and their weak matrix elements are promising candidates for future quantitative tests. The whole subject of quark matter and the quark-gluon plasma is a non-perturbative test bed for QCD which still remains to be properly exploited.

Related Topics:
Running of the QCD coupling - Lattice - Heavy-quarkonium spectra - Form factor - Weak matrix elements - Quark matter - Quark-gluon plasma

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