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.

History

With the invention of bubble chambers and spark chambers in the 1950s, experimental particle physics discovered a large and ever-growing number of particles called hadrons. It seemed that such a large number of particles could not all be fundamental. First, the particles were classified by charge and isospin; then (in 1953) according to strangeness by Murray Gell-Mann and Kazuhiko Nishijima. To gain greater insight, the hadrons were sorted into groups having similar properties and masses using the eightfold way, invented in 1961 by Gell-Mann and Yuval Ne'eman. Gell-Mann and George Zweig went on to propose in 1963 that the structure of the groups could be explained by the existence of three flavors of smaller particles inside the hadrons: the quarks.

Related Topics:
Bubble chamber - Spark chamber - Particle physics - Hadrons - Fundamental - Isospin - Strangeness - Murray Gell-Mann - Kazuhiko Nishijima - Eightfold way - Yuval Ne'eman - George Zweig - Flavor - Quark

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At this stage, one particle, the Δ++ remained mysterious; in the quark model, it is composed of three up quarks with parallel spins. However, since quarks are fermions, this combination is forbidden by the Pauli exclusion principle. In 1965, Moo-Young Han and Yoichiro Nambu resolved the problem by proposing that quarks possess an additional SU(3) gauge degree of freedom, later called color charge. They noted that quarks would interact via an octet of vector gauge bosons: the gluons.

Related Topics:
Fermion - Pauli exclusion principle - Moo-Young Han - Yoichiro Nambu - SU(3) - Gauge - Degree of freedom - Gauge boson - Gluon

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Since free quark searches consistently failed to turn up any evidence for the new particles, it was then believed that quarks were merely convenient mathematical constructs, not real particles. Richard Feynman argued that high energy experiments showed quarks to be real: he called them partons (since they were parts of hadrons). James Bjorken proposed that certain relations should then hold in deep inelastic scattering of electrons and protons, which were spectacularly verified in experiments at SLAC in 1969.

Related Topics:
Richard Feynman - James Bjorken - Deep inelastic scattering - Electron - SLAC - 1969

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Although the study of the strong interaction remained daunting, the discovery of asymptotic freedom by David Gross, David Politzer and Frank Wilczek allowed people to make precise predictions of the results of many high energy experiments using the techniques of perturbation theory (quantum mechanics). Evidence of gluons was discovered in three-jet events at PETRA in 1979. These experiments became more and more precise, culminating in the verification of perturbative QCD at the level of a few percent at the LEP in CERN.

Related Topics:
Asymptotic freedom - David Gross - David Politzer - Frank Wilczek - Perturbation theory (quantum mechanics) - Gluon - Three-jet events - PETRA - Perturbative QCD - LEP - CERN

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The other side of asymptotic freedom is confinement. Since the force between color charges does not decrease with distance, it is believed that quarks and gluons can never be liberated from hadrons. This aspect of the theory is verified within lattice QCD computations, but is not mathematically proven. One of the Millennium Prizes announced by the Clay Mathematics Institute requires a claimant to produce such a proof. Other aspects of non-perturbative QCD are the exploration of phases of quark matter, including the quark-gluon plasma.

Related Topics:
Confinement - Hadrons - Lattice QCD - Clay Mathematics Institute - Non-perturbative QCD - Quark matter - Quark-gluon plasma

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