Strong interaction

The strong interaction (sometimes called the strong force) is nowadays understood to mean the interactions between quarks and gluons which is decribed by quantum chromodynamics. Before the 1970s, when the proton and neutron were thought to be fundamental, this phrase meant nuclear force. The strong interactions has always been regarded as a fundamental interaction of nature: the change only relates to a change in perception about the fundamental constituents of matter.

For some time after the realization that the true fundamental particles making up the atomic nucleus were quarks (and gluons) this older notion was referred to by the name residual strong force (as the new strong interaction was passingly called colour force), but now this term has also fallen out of fashion. This older problem, once the very definition of strong interactions, is now seen as merely one of a host of problems dealing with effective field theories of hadronic physics which should be derived from quantum chromodynamics.

Strong interactions deal with fermions called quarks, their antiparticles, called antiquarks, and bosons called gluons. These are unobservable, as has been shown by failed free quark searches. This property of the unobservability of the fundamental particles is called confinement. The theory of the strong interactions is called quantum chromodynamics. This theory should predict that the observable particles are hadrons (ie, mesons and baryons).

Quantum chromodynamics, a part of the standard model of particle physics, is a typical non-Abelian gauge theory based on a local (gauge) symmetry group called SU(3). All the particles in this theory interact with each other. The strength of the interaction is parametrized by the strong coupling constant. This strength is, as usual, modified by the gauge color charge of the particle. This really refers to a group theoretical property whose meaning is explained in the article on color charge. Quarks and gluons are the only fundamental particles which carry non-vanishing color charge, and hence participate in the strong interactions.

Quantum chromodynamics presents peculiar problems of its own which are distinguish it from other parts of the standard model. Foremost among these is the property that the coupling constant is large at long distances and becomes weak at short distances: called asymptotic freedom. The notion of confinement has already been touched upon; it is a consequence of the strong coupling in QCD. This also gives rise to an extremely non-trivial structure of the QCD vacuum, whose excitations are the known hadrons and conjectured exoticss. In addition, it turns out that under extreme conditions of density and temperature quarks may aggregate in different ways. Such novel forms of matter, the search for which is underway, are called quark matter in general, the quark-gluon plasma being the most widely investigated form.

See also

• Weak interaction, electromagnetic interaction and gravity
• Standard model of particle physics and its field theoretical formulation.
• Quantum field theory and gauge theory
• Quantum chromodynamics and Quark matter
• Internucleon force and nuclear physics

References and external links

• Introduction to Elementary Particles, by David J. Griffiths (New York: John Wiley & Sons, 1987) [ISBN 0471603864]
• The Last Sorcerers: The Path from Alchemy to the Periodic Table, by Richard Morris (Washington, DC: Joseph Henry Press, 2003) [ISBN 0309505933]

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