Color charge
Color charge
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Color charge

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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). Like electric charge, it determines how quarks and gluons interact through the strong force; however, rather than there being only positive and negative charges, there are three "charges", commonly called red, green, and blue. 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. For example, the three quarks making up any baryon universally have three different color charges, and the two quarks making up any meson universally have opposite color charge.

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. The term color and the labels red, green, and blue became popular simply because of the loose but convenient analogy to the primary colors.

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

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 Gell-Mann's QCD, each quark and gluon has fractional electric charge, and carries what came to be called color charge in the space of the color degree of freedom.

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). Gluons are mixtures of two colors, such as red and antigreen, which constitutes their color charge. QCD considers eight gluons of the possible nine color–anticolor combinations to be unique; see eight gluon colors for an explanation.

All three colors mixed together, all three anticolors mixed together, or a combination of a color and its anticolor is "colorless" or "white" and has a net color charge of zero. Due to a property of the strong interaction called color confinement, free particles must have a color charge of zero.

A baryon is composed of three quarks, which must be one each of red, green, and blue colors; likewise an antibaryon is composed of three antiquarks, one each of antired, antigreen and antiblue. A meson is made from one quark and one antiquark; the quark can be any color, and the antiquark has the matching anticolor.

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