Color charge

Color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD).

Additionally, there are three "anti-colors", commonly called anti-red, anti-green, and anti-blue.

Unlike electric charge, color charge is never observed in nature: in all cases, red, green, and blue (or anti-red, anti-green, and anti-blue) or any color and its anti-color combine to form a "color-neutral" system.

The "color charge" of quarks and gluons is completely unrelated to the everyday meaning of color, which refers to the frequency of photons, the particles that mediate a different fundamental force, electromagnetism.

Shortly after the existence of quarks was proposed by Murray Gell-Mann and George Zweig in 1964, color charge was implicitly introduced the same year by Oscar W.

[1] In 1965, Moo-Young Han and Yoichiro Nambu explicitly introduced color as a gauge symmetry.

[1] Han and Nambu initially designated this degree of freedom by the group SU(3), but it was referred to in later papers as "the three-triplet model".

One feature of the model (which was originally preferred by Han and Nambu) was that it permitted integrally charged quarks, as well as the fractionally charged quarks initially proposed by Zweig and Gell-Mann.

Somewhat later, in the early 1970s, Gell-Mann, in several conference talks, coined the name color to describe the internal degree of freedom of the three-triplet model, and advocated a new field theory, designated as quantum chromodynamics (QCD) to describe the interaction of quarks and gluons within hadrons.

In quantum chromodynamics (QCD), a quark's color can take one of three values or charges: red, green, and blue.

An antiquark can take one of three anticolors: called antired, antigreen, and antiblue (represented as cyan, magenta, and yellow, respectively).

The following illustrates the coupling constants for color-charged particles: Analogous to an electric field and electric charges, the strong force acting between color charges can be depicted using field lines.

However, the color field lines do not arc outwards from one charge to another as much, because they are pulled together tightly by gluons (within 1 fm).

In a quantum field theory, a coupling constant and a charge are different but related notions.

Specifically, if a local gauge transformation ϕ(x) is applied in electrodynamics, then one finds (using tensor index notation):

Since QCD is a non-abelian theory, the representations, and hence the color charges, are more complicated.

Each flavour of quark belongs to the fundamental representation (3) and contains a triplet of fields together denoted by

Gluons have a combination of two color charges (one of red, green, or blue and one of antired, antigreen, or antiblue) in a superposition of states that are given by the Gell-Mann matrices.

Formally, these states are written as While "colorless" in the sense that they consist of matched color-anticolor pairs, which places them in the centre of a weight diagram alongside the truly colorless singlet state, they still participate in strong interactions - in particular, those in which quarks interact without changing color.

In the simple language introduced previously, the three indices "1", "2" and "3" in the quark triplet above are usually identified with the three colors.

One simple way of doing this is to look at the interaction vertex in QCD and replace it by a color-line representation.

The color-line diagrams can be restated in terms of conservation laws of color; however, as noted before, this is not a gauge invariant language.

In the electroweak theory, the W also carries electric charge, and hence interacts with a photon.