- Gluon
Infobox Particle

bgcolour =

name = Gluon

caption =

num_types = 8

composition =Elementary particle

family =Boson

group =Gauge boson

generation =

interaction =Strong interaction

theorized =

discovered =

symbol = g

mass = 0 (theoretical value; experimentally at most a fewMeV /c^{2}[*cite journal|doi=10.1016/0370-2693(94)01677-5|title=Limits on the mass of the gluon*1|year=1995|author=Yndurain, F|journal=Physics Letters B|volume=345|pages=524*] )

decay_time =

decay_particle =

electric_charge = 0 [*http://pdg.lbl.gov/2007/tables/gxxx.pdf W.-M. Yao et al., J. Phys. G 33, 1 (2006)*] Retrieved December, 2007]

color_charge = octet (8 types)

spin = 1

num_spin_states =**Gluons**("glue " and the suffix "-on") areelementary particle s that causequarks to interact, and are indirectly responsible for the binding ofproton s andneutron s together inatomic nuclei .In technical terms, they are vector gauge bosons that mediate strong

color charge interactions ofquark s inquantum chromodynamics (QCD). Unlike the neutralphoton ofquantum electrodynamics (QED), gluons themselves participate in strong interactions. The gluon has the ability to do this as it carries the color charge and so interacts with itself, making QCD significantly harder to analyze than QED.**Properties**The gluon is a vector boson; like the

photon , it has a spin of 1. While massive spin-1 particles have three polarization states, massless gauge bosons like the gluon have only two polarization states becausegauge invariance requires the polarization to be transverse.Inquantum field theory , unbroken gauge invariance requires that gauge bosons have zero mass (experiment limits the gluon's mass to less than a few MeV).The gluon has negative intrinsic parity and zeroisospin . It is its ownantiparticle .Fact|date=October 2008**Numerology of gluons**Unlike the single

photon of QED or the threeW and Z bosons of theweak interaction , there are eight independent types of gluon in QCD.This may be difficult to understand intuitively.

Quark s carry three types ofcolor charge ; antiquarks carry three types of anticolor. Gluons may be thought of as carrying both color and anticolor, but to correctly understand how they are combined, it is necessary to consider the mathematics of color charge in more detail.**Color charge and superposition**In

quantum mechanics , the states of particles may be added according to the principle of superposition; that is, they may be in a "combined state" with a "probability", if some particular quantity is measured, of giving several different outcomes. A relevant illustration in the case at hand would be a gluon with a color state described by::$(rar\{b\}+bar\{r\})/sqrt\{2\}$

This is read as "red-antiblue plus blue-antired." (The factor of the square root of two is required for normalization, a detail which is not crucial to understand in this discussion.) If one were somehow able to make a direct measurement of the color of a gluon in this state, there would be a 50% chance of it having red-antiblue color charge and a 50% chance of blue-antired color charge.

**Color singlet states**It is often said that the stable strongly-interacting particles observed in nature are "colorless," but more precisely they are in a "color singlet" state, which is mathematically analogous to a "spin" singlet state. [

*"Griffiths", 280-281 (footnote)*] Such states allow interaction with other color singlets, but not with other color states; because long-range gluon interactions do not exist, this illustrates that gluons in the singlet state do not exist either. [*"Griffiths", 281 (first complete footnote)*]The color singlet state is [

*"Griffiths", 280*] ::$(rar\{r\}+bar\{b\}+gar\{g\})/sqrt\{3\}$

In words, if one could measure the color of the state, there would be equal probabilities of it being red-antired, blue-antiblue, or green-antigreen.

**Eight gluon colors**There are eight remaining independent color states, which correspond to the "eight types" or "eight colors" of gluons. Because states can be mixed together as discussed above, there are many ways of presenting these states, which are known as the "color octet." One commonly used list is [

*"Griffiths", 280*] :These are equivalent to the

Gell-Mann matrices ; the translation between the two is that red-antired is the upper-left matrix entry, red-antiblue is the left middle entry, blue-antigreen is the bottom middle entry, and so on. The critical feature of these particular eight states is that they arelinearly independent , and also independent of the singlet state; there is no way to add any combination of states to produce any other. (It is also impossible to add them to make $rar\{r\}$, $gar\{g\}$, or $bar\{b\}$ [*[*] ; if it were, then the forbidden singlet state could also be made.) There are many other possible choices, but all are mathematically equivalent, at least equally complex, and give the same physical results.*http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/gluons.html Why are there eight gluons and not nine?*]**Group theory details**Technically, QCD is a

gauge theory withSU(3) gauge symmetry. Quarks are introduced as spinor fields in "N_{f}" flavours, each in thefundamental representation (triplet, denoted**3**) of the colour gauge group, SU(3). The gluons are vector fields in theadjoint representation (octets, denoted**8**) of colour SU(3). For a general gauge group, the number of force-carriers (like photons or gluons) is always equal to the dimension of the adjoint representation. For the simple case of SU("N"), the dimension of this representation is "N"^{2}−1.In terms of group theory, the assertion that there are no color singlet gluons is simply the statement that

quantum chromodynamics has anSU(3) rather than a U(3) symmetry. There is no known "a priori" reason for one group to be preferred over the other, but as discussed above, the experimental evidence supports SU(3). [*"Griffiths", 281 (second complete footnote)*]**Confinement**Since gluons themselves carry color charge (again, unlike the

photon which is electrically neutral), they participate in strong interactions. These gluon-gluon interactions constrain color fields to string-like objects called "flux tubes", which exert constant force when stretched. Due to this force,quark s are confined withincomposite particle s calledhadron s. This effectively limits the range of the strong interaction to 10^{-15}meters, roughly the size of anatomic nucleus . (Beyond a certain distance, the energy of the flux tube binding two quarks increases linearly. At a large enough distance, it becomes energetically more favorable to pull a quark-antiquark pair out of the vacuum rather than increase the length of the flux tube.)Gluons also share this property of being confined within hadrons. One consequence is that gluons are not directly involved in the

nuclear force s between hadrons. The force mediators for these are other hadrons calledmeson s.Although in the

normal phase of QCD single gluons may not travel freely, it is predicted that there existhadron s which are formed entirely of gluons — called. There are also conjectures about otherglueball sin which real gluons (as opposed to virtual ones found in ordinary hadrons) would be primary constituents. Beyond the normal phase of QCD (at extreme temperatures and pressures),exotic hadron squark gluon plasma forms. In such a plasma there are no hadrons; quarks and gluons become free particles.**Experimental observations**The first direct experimental evidence of gluons was found in 1979 when three-jet events were observed at the electron-positron collider called

PETRA atDESY inHamburg .Experimentally, confinement is verified by the failure of

free quark search es. Neither free quarks nor free gluons have ever been observed. Although there have been hints of exotic hadrons, no glueball has been observed either. Quark-gluon plasma has been found recently at theRelativistic Heavy Ion Collider (RHIC) atBrookhaven National Laboratories (BNL).Fact|date=October 2008**See also***

Quark

*Hadron

*Gauge boson

*Glueball

*Exotic hadron s

*Quark model

*Quantum chromodynamics

*Standard model

* Three-jet events

*Deep inelastic scattering **References and external links***

*Kaufmann(ed), Scientific American: Particles & Fields (special edition), 1980

* [*http://pdg.lbl.gov/2004/tables/contents_tables.html Summary tables in the "Review of particle physics"*]

* [*http://www.desy.de/pr-info/desyhome/html/presse/glossary.html#G DESY glossary*]

* [*http://www.symmetrymag.org/cms/?pid=1000160 Logbook of gluon discovery*]

* [*http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/gluons.html Why are there eight gluons and not nine?*]

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