Editing Color charge

{{Short description|Quantum number related to the strong force}}
{{Multiple issues|{{More footnotes needed|date=March 2009}}{{Page numbers needed|date=March 2009}}}}
{{Use American English|date=January 2025}}
{{Standard model of particle physics}}

'''Color charge''' is a property of [[quark]]s and [[gluon]]s that is related to the particles' [[strong interaction]]s 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 [[photon]]s, 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 color]]s.

== History ==
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]].<ref name=":0">{{Citation |last=Greenberg |first=Oscar Wallace |title=Color Charge Degree of Freedom in Particle Physics |date=2009 |work=Compendium of Quantum Physics |pages=109–111 |editor-last=Greenberger |editor-first=Daniel |url=https://link.springer.com/chapter/10.1007/978-3-540-70626-7_32 |access-date=2024-09-17 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-540-70626-7_32 |isbn=978-3-540-70626-7 |editor2-last=Hentschel |editor2-first=Klaus |editor3-last=Weinert |editor3-first=Friedel|url-access=subscription }}</ref> In 1965, [[Moo-Young Han]] and [[Yoichiro Nambu]] explicitly introduced color as a gauge symmetry.<ref name=":0" />  

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.

== Red, green, and blue ==
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 ''[[Gluon#Eight color states |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 particle]]s 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.

The following illustrates the [[coupling constant]]s for color-charged particles:

<gallery>
Image:Quark_Colors_with_white.svg|The quark colors (red, green, blue) combine to be colorless
Image:Quark_Anticolors.svg|The quark anticolors (antired, antigreen, antiblue) also combine to be colorless
</gallery>

<gallery>
Image:QCD Intermediate 1.png|A hadron with 3 quarks (red, green, blue) before a color change
Image:QCD Intermediate 2.png|Blue quark emits a blue–antigreen gluon, becoming green
Image:QCD Intermediate 3.png|The first green quark has absorbed the blue–antigreen gluon and is now blue; color remains conserved
File:Neutron QCD Animation.gif|An animation of the interaction inside a neutron. The gluons are represented as circles with the color charge in the center and the anti-color charge on the outside.
</gallery>

=== Field lines from color charges ===
{{main|Field (physics)}}
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 [[femtometre|fm]]).<ref>{{Citation|title=Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles|edition=2nd|author=R. Resnick, R. Eisberg|publisher=John Wiley & Sons|year=1985|page=[https://archive.org/details/quantumphysicsof00eisb/page/684 684]|isbn=978-0-471-87373-0|url=https://archive.org/details/quantumphysicsof00eisb/page/684}}</ref> This effect [[Color confinement|confines]] [[quark]]s within [[hadron]]s.

[[File:Qcd fields field (physics).svg|400px|center|thumb|Fields due to color charges of [[quark]]s ('''G''' is the [[gluon field strength tensor]]) in "colorless" combinations.<br/>''Top'': Color charge has "ternary neutral states" as well as binary neutrality (analogous to [[electric charge]]).<br/>''Bottom'': Quark/antiquark combinations.<ref>{{Citation|title=McGraw Hill Encyclopaedia of Physics|first1=C.B.|last1=Parker|edition=2nd|publisher=Mc Graw Hill|year=1994|isbn=978-0-07-051400-3|url=https://archive.org/details/mcgrawhillencycl1993park}}</ref><ref>{{Citation |author= M. Mansfield, C. O’Sullivan|title= Understanding Physics|edition= 4th |year= 2011|publisher= John Wiley & Sons|isbn=978-0-47-0746370}}</ref>]]

== Coupling constant and charge ==
In a [[quantum field theory]], a [[coupling constant]] and a charge are different but related notions. The coupling constant sets the magnitude of the force of interaction; for example, in [[quantum electrodynamics]], the [[fine-structure constant]] is a coupling constant. The charge in a [[gauge theory]] has to do with the way a particle transforms under the gauge symmetry; i.e., its [[group representation|representation]] under the gauge group. For example, the [[electron]] has charge −1 and the [[positron]] has charge +1, implying that the gauge transformation has opposite effects on them in some sense. Specifically, if a local [[gauge transformation]] {{math|''ϕ''(''x'')}} is applied in electrodynamics, then one finds (using [[tensor index notation]]):<math display="block">\begin{align}
A_\mu &\to A_\mu + \partial_\mu\,\phi(x) \\
\psi &\to \exp\left[+i\,Q\phi(x)\right]\; \psi \\
\bar\psi &\to \exp\left[-i\,Q\phi(x)\right] \; \bar\psi ~,
\end{align}</math>
where <math>A_\mu</math> is the [[photon]] field, and {{math|''ψ''}} is the electron field with {{math|1=''Q'' = −1}} (a bar over {{mvar|ψ}} denotes its antiparticle&nbsp;– the positron). Since QCD is a [[non-abelian group|non-abelian]] theory, the representations, and hence the color charges, are more complicated. They are dealt with in the next section.

== Quark and gluon fields ==
[[File:Strong force charges.svg|300px|right|thumb|The pattern of strong charges for the three colors of quark, three antiquarks, and eight gluons (with two of zero charge overlapping).]]
In QCD the gauge group is the non-abelian group [[SU(3)]]. The ''[[coupling constant#Running coupling|running coupling]]'' is usually denoted by <math>\alpha_s</math>. Each [[flavour (particle physics)| flavour]] of quark belongs to the [[fundamental representation]] ('''3''') and contains a triplet of fields together denoted by <math>\psi</math>. The [[antiparticle|antiquark]] field belongs to the [[Hermitian conjugate|complex conjugate representation]] ('''3<sup>*</sup>''') and also contains a triplet of fields. We can write
: <math>\psi = \begin{pmatrix}\psi_1\\ \psi_2\\ \psi_3\end{pmatrix}</math> &nbsp;and&nbsp; <math>\overline\psi = \begin{pmatrix}{\overline\psi}^*_1\\ {\overline\psi}^*_2\\ {\overline\psi}^*_3\end{pmatrix}.</math>
The gluon contains an octet of fields (see [[gluon field]]), and belongs to the [[adjoint representation of a Lie group|adjoint representation]] ('''8'''), and can be written using the [[Gell-Mann matrices]] as
: <math>{\mathbf A}_\mu = A_\mu^a\lambda_a.</math>
(there is an [[Einstein notation|implied summation]] over ''a'' = 1, 2, ... 8). All other [[subatomic particle|particle]]s belong to the [[trivial representation]] ('''1''') of color [[SU(3)]]. The color charge of each of these fields is fully specified by the representations. Quarks have a color charge of red, green or blue and antiquarks have a color charge of antired, antigreen or antiblue. 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. All other particles have zero color charge.

The gluons corresponding to <math>\lambda_3</math> and <math>\lambda_8</math> are sometimes described as having "zero charge" (as in the figure). Formally, these states are written as
:<math>g_3 = \frac{1}{\sqrt{2}} (r\overline{r}-b\overline{b})</math> and <math>g_8 = \frac{1}{\sqrt{6}} (r\overline{r}+b\overline{b}-2g\overline{g})</math>
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.

Mathematically speaking, the color charge of a particle is the value of a certain quadratic [[Casimir operator]] in the representation of the particle.

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. The colorful language misses the following point. A gauge transformation in color SU(3) can be written as <math>\psi \to U \psi</math>, where <math>U</math> is a {{math|3 × 3}} matrix that belongs to the group SU(3). Thus, after gauge transformation, the new colors are linear combinations of the old colors. In short, the simplified language introduced before is not gauge invariant.

[[File:vertex.png|150px|left|Color-line representation of QCD vertex]]

Color charge is conserved, but the book-keeping involved in this is more complicated than just adding up the charges, as is done in quantum electrodynamics. One simple way of doing this is to look at the interaction vertex in QCD and replace it by a color-line representation. The meaning is the following. Let <math>\psi_i</math> represent the {{mvar|i}}th component of a quark field (loosely called the {{mvar|i}}th color). The ''color'' of a gluon is similarly given by <math>\mathbf{A}</math>, which corresponds to the particular Gell-Mann matrix it is associated with. This matrix has indices {{mvar|i}} and {{mvar|j}}. These are the ''color labels'' on the gluon. At the interaction vertex one has {{math|q<sub>''i''</sub> → g<sub>''ij''</sub> + q<sub>''j''</sub>}}. The ''color-line'' representation tracks these indices. Color [[charge conservation]] means that the ends of these color lines must be either in the initial or final state, equivalently, that no lines break in the middle of a diagram.

[[File:3gluon.png|150px|right|Color-line representation of 3-gluon vertex]]

Since gluons carry color charge, two gluons can also interact. A typical interaction vertex (called the three gluon vertex) for gluons involves g&nbsp;+&nbsp;g&nbsp;→&nbsp;g. This is shown here, along with its 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. Note that in a typical [[non-abelian gauge theory]] the [[gauge boson]] carries the charge of the theory, and hence has interactions of this kind; for example, the [[W&nbsp;boson]] in the electroweak theory. In the electroweak theory, the W also carries electric charge, and hence interacts with a photon.

== See also ==
{{Wiktionary}}
* [[Color confinement]]
* [[Gluon field strength tensor]]
* [[Electric charge]]

== References ==
{{reflist}}

== Further reading ==
* {{citation |first=Howard |last=Georgi |title=Lie algebras in particle physics |year=1999 |publisher=Perseus Books Group |isbn=978-0-7382-0233-4 }}.
* {{citation |first=David J. |last=Griffiths |title=Introduction to Elementary Particles |year=1987 |publisher=John Wiley & Sons |location=New York |isbn=978-0-471-60386-3 }}.
* {{citation |first=J. Richard |last=Christman |url=http://www.physnet.org/modules/pdf_modules/m283.pdf |title=Color and Charm |year=2001 |work=PHYSNET document MISN-0-283 }}.
* {{citation |first=Stephen |last=Hawking |title=A Brief History of Time |year=1998 |publisher=Bantam Dell Publishing Group |isbn=978-0-553-10953-5 }}.
* {{citation |first=Frank |last=Close |title=The New Cosmic Onion |year=2007 |publisher=Taylor & Francis |isbn=978-1-58488-798-0 }}.

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[[Category:Gluons]]
[[Category:Quantum chromodynamics]]