# quantum chromodynamics

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## quantum chromodynamics

**quantum chromodynamics** (QCD), quantum field theory that describes the properties of the strong interactions between quarks and between protons and neutrons in the framework of quantum theory. Quarks possess a distinctive property called color that governs their binding together to form other elementary particles. Analogous to electric charge in charged particles, color is of three varieties, arbitrarily designated as red, blue, and yellow, and—analogous to positive and negative charges—three anticolor varieties. Just as positively and negatively charged particles form electrically neutral atoms, colored quarks form particles with no net color. Quarks interact by emitting and absorbing massless particles called gluons, each of which carries a color-anticolor pair. Eight kinds of gluons are required to transmit the strong force between quarks, e.g., a blue quark might interact with a yellow quark by exchanging a blue-antiyellow gluon.

The concept of color was proposed by American physicist Oscar Greenberg and independently by Japanese physicist Yoichiro Nambu in 1964. The theory was confirmed in 1979 when quarks were shown to emit gluons during studies of high-energy particle collisions at the German national laboratory in Hamburg. QCD is nearly identical in mathematical structure to quantum electrodynamics (QED) and to the unified theory of weak and electromagnetic interactions advanced by American physicist Steven Weinberg and Pakistani physicist Abdus Salam.

### Bibliography

See F. J. Yndurain, *Quantum Chromodynamics: An Introduction to the Theory of Quarks and Gluons* (1983); G. Altarelli, *The Development of Perturbative QCD* (1994); W. Greiner and A. Schafer, *Quantum Chromodynamics* (1994).

## Quantum chromodynamics

A theory of the strong (“nuclear”) interactions among quarks, which are regarded as fundamental constituents of matter. Quantum chromodynamics (QCD) seeks to explain why quarks combine in certain configurations to form the observed patterns of subnuclear particles, such as the proton and pi meson. According to this picture, the strong interactions among quarks are mediated by a set of force particles known as gluons. Strong interactions among gluons may lead to new structures that correspond to as-yet-undiscovered particles. The long-studied nuclear force that binds protons and neutrons together in atomic nuclei is regarded as a collective effect of the elementary interactions among constituents of the composite protons and neutrons. *See* Nuclear structure

Quantum chromodynamics has not yet been subjected to precise experimental tests. Several qualitative predictions of quantum chromodynamics do seem to have been borne out. Part of the esthetic appeal of the theory is due to the fact that quantum chromodynamics is nearly identical in mathematical structure to quantum electrodynamics (QED) and to the unified theory of weak and electromagnetic interactions. This resemblance encourages the hope that a unified description of the strong, weak, and electromagnetic interactions may be at hand. *See* Electroweak interaction, Quantum electrodynamics, Weak nuclear interactions

#### Gauge theories

At the heart of current theories of the fundamental interactions is the idea of gauge invariance. It is widely believed that gauge theories constructed to embody various symmetry principles represent the correct quantum-mechanical descriptions of the strong, weak, and electromagnetic interactions. *See* Symmetry laws (physics)

#### Color

Although the idea that the strongly interacting particles are built up of quarks brought new order to hadron spectroscopy and suggested new relations among mesons and baryons, the constituent description brought with it a number of puzzles. According to the Pauli exclusion principle, identical spin-½ particles cannot occupy the same quantum state. As a consequence, the observed baryons such as Δ^{++} (*uuu*) and &OHgr;^{-} (*sss*), which would be composed of three identical quarks in the same state, would seem to be forbidden configurations. To comply with the Pauli principle, it is necessary to make the three otherwise identical quarks distinguishable by supposing that every flavor of quark exists in three varieties, fancifully labeled by the colors red, green, and blue. Color may be regarded as the strong-interaction analog of electric charge. Color cannot be created or destroyed by any of the known interactions. Like electric charge, it is said to be conserved. *See* Color (quantum mechanics), Exclusion principle

In the face of evidence that color could be regarded as the conserved charge of the strong interactions, it was natural to seek a gauge symmetry that would have color conservation as its consequence. An obvious candidate for the gauge symmetry group is the unitary group SU(3), now to be applied to color rather than flavor. The theory of strong interactions among quarks that is prescribed by local color gauge symmetry is known as quantum chromodynamics. The mediators of the strong interactions are eight massless spin-1 bosons, one for each generator of the symmetry group. These strong-force particles are named gluons because they make up the “glue” that binds quarks together into hadrons. Gluons also carry color and hence have strong interactions among themselves.

#### Asymptotic freedom

The theoretical description of the strong interactions has historically been inhibited by the very strength of the interaction, which renders low-order perturbative calculations untrustworthy. However, in 1973 it was found that in many circumstances the effective strength of the interaction in Yang-Mills theories becomes increasingly feeble at short distances, a property known as asymptotic freedom. For quantum chromodynamics, this remarkable observation implies that the interaction between quarks becomes weak at small separations. This discovery raises the hope that some aspects of the strong interactions might be treated by using familiar computational techniques that are predicated upon the smallness of the interaction strength.

#### Quarkonium

It was suggested in 1974 that the bound system of an extremely massive quark with its antiquark would be so small that the strong force would be extremely feeble. In this case, the binding between quark and antiquark is mediated by the exchange of a single massless gluon, and the spectrum of bound states resembles that of an exotic atom composed of an electron and an antielectron (positron) bound electromagnetically in a Coulomb potential generated by the exchange of a massless photon. Since the electron-positron atom is known as positronium, the heavy quark-antiquark atom has been called quarkonium. Two families of heavy quark-antiquark bound states, the &psgr;/J system composed of charmed quarks and the &Ugr; system made up of *b* quarks, have been discovered. Both have level schemes characteristic of atomic spectra, which have been analyzed by using tools of nonrelativistic quantum mechanics developed for ordinary atoms. The atomic analogy has proved extremely fruitful for studying the strong interaction. *See* Charm, J/psi particle, Positronium

#### Lattice models

To deal with the existence and properties of the hadrons themselves, it is necessary to devise a new computational approach that does not break down when the interaction becomes strong. The most promising method has been the crystal lattice formulation of the theory. By considering the values of the color field only on individual lattice sites, it is possible to use many of the techniques developed in statistical physics for the study of spin systems such as magnetic substances. *See* Elementary particle, Fundamental interactions, Gluons, Ising model, Quantum field theory, Quarks, Standard model, Statistical mechanics, Strong nuclear interactions