Quantum Chromodynamics | Research & Encyclopedia Articles

Quantum Chromodynamics

The theory of quantum electrodynamics (QED) has proved to be enormously successful in explaining interactions involving both electromagnetic and weak forces. So it is hardly surprising that physicists have considered the possibility of a similar theory that will explain interactions involving the strong force. That theory is now called quantum chromodynamics (QCD). The theory was developed in the early 1970s, partly as the result of research by Murray Gell-Mann on the basic structure of matter.

The fundamental components of quantum chromodynamics are quarks and gluons. Quarks are thought to be one of the two fundamental kinds of particles ( leptons are the other) of which all matter is made. Gluons are mediating particles that transmit the strong force between quarks and between themselves.

One of the fundamental problems leading to the development of QCD was finding particles containing identical quarks. The D⁺⁺⁺ particle, for example, consists of three up quarks (uuu). This situation should be impossible according to the Wolfgang Pauli principle, which states that no two particles with identical characteristics can occupy the same quantum state.

To resolve this dilemma, Gell-Mann proposed that quarks posses an as-yet unrecognized property that he described as color. Any one "flavor" of quark (that is, any one type of quark, such as any one up quark) could, he proposed, occur in one of three possible "color" states, "red," "green," or "blue." The term color as used here has nothing to do with the colors we think of in everyday life. They are simply a way of describing three different states of a quark. If such is the case, the three quarks in the D⁺⁺⁺ particle are not really identical to each other, but are of three different colors, denoted as u_r, u_g, and u_b.

According to this formulation, gluons also have the property of color. A gluon might, for example, carry the color "red" and the anticolor "green," denoted as G_rg. When a quark absorbs or emits a gluon, it may change color. For example, if a " blue" quark absorbs a G_rg gluon, it changes to "red." This exchange of colors during the absorption and emission of gluons is thought to be responsible for the strong force.

The role of color in the interaction is responsible for the name chromodynamics (Greek, chromo-= " color") in the term quantum chromodynamics. At present, there are thought to be six color-changing gluons and two color-preserving gluons.

Current theory suggests that neither quarks nor gluons can exist by themselves. One goal of QCD is to discover a theoretical basis for that presumption or to show that it is not true.

Thus far, relatively little empirical evidence has been obtained to support QCD. The mathematics involved in the theory is very complex, making experimental tests difficult to suggest. Some authorities have even questioned whether the analysis proposed by QCD can be accomplished by other, simpler theories that already exist.