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color charge
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{{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=November 2023}}{{Standard model of particle physics}}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 meanings of color and charge. The term color and the labels red, green, and blue became popular simply because of the loose analogy to the primary colors.- the content below is remote from Wikipedia
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History
Shortly after the existence of quarks was proposed by Murray Gell-Mann and George Zweig in 1964, Moo-Young Han and Yoichiro Nambu introduced a hidden internal degree of freedom in which quark wave functions were antisymmetric, thus solving the spin-statistics problem of the Gell Mann-Zweig quark model. 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 colorâanticolor combinations to be unique; see eight gluon colors for an explanation.All three colors mixed together, or any one of these colors and its complement (or negative), 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.The following illustrates the coupling constants for color-charged particles:Image:Quark_Colors_with_white.svg|The quark colors (red, green, blue) combine to be colorlessImage:Quark_Anticolors.svg|The quark anticolors (antired, antigreen, antiblue) also combine to be colorlessImage:QCD Intermediate 1.png|A hadron with 3 quarks (red, green, blue) before a color changeImage:QCD Intermediate 2.png|Blue quark emits a blueâantigreen gluonImage:QCD Intermediate 3.png|Green quark has absorbed the blueâantigreen gluon and is now blue; color remains conservedFile: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.Field lines from color charges
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).{{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=684|isbn=978-0-471-87373-0|url=https://archive.org/details/quantumphysicsof00eisb/page/684}} This effect confines quarks within hadrons.File:Qcd fields field (physics).svg|400px|center|thumb|Fields due to color charges of quarks (G is the gluon field strength tensor) in “colorless” combinations.Top: Color charge has “ternary neutral states” as well as binary neutrality (analogous to (electric charge]]).Bottom: Quark/antiquark combinations.{{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}}{{Citation |author= M. Mansfield, C. O’Sullivan|title= Understanding Physics|edition= 4th |year= 2011|publisher= John Wiley & Sons|isbn=978-0-47-0746370}})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 representation under the gauge group. For example, the electron has charge â1 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|Ï(x)}} is applied in electrodynamics, then one finds (using tensor index notation):begin{align}A_mu &to A_mu + partial_mu,phi(x) ~, psi &to expleft[+i,Qphi(x)right]; psi ~, barpsi &to expleft[-i,Qphi(x)right] ; barpsiend{align}where A_mu is the photon field, and {{mvar|Ï}} is the electron field with {{math|1=Q = â1}} (a bar over {{mvar|Ï}} denotes its antiparticle â the positron). Since QCD is a 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 running coupling is usually denoted by alpha_s. Each flavour of quark belongs to the fundamental representation (3) and contains a triplet of fields together denoted by psi. The antiquark field belongs to the complex conjugate representation (3*) and also contains a triplet of fields. We can write
psi = begin{pmatrix}psi_1 psi_2 psi_3end{pmatrix} and overlinepsi = begin{pmatrix}{overlinepsi}^*_1 {overlinepsi}^*_2 {overlinepsi}^*_3end{pmatrix}.
{mathbf A}_mu = A_mu^alambda_a.
See also
{{Wiktionary}}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=Colour 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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