Gluon
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In particle physics, gluons mediate strong interactions of quarks in quantum chromodynamics. The first direct experimental evidence of gluons was found in the early 1980s when three-jet events were observed at the electron-positron collider called PETRA at DESY in Hamburg. Quantitative studies of deep inelastic scattering at the Stanford Linear Accelerator Center had established their existence a decade before that.
The failure of free quark searches has led to the notion of confinement of quarks. Gluons also share this property of being confined within hadrons. More properly, confinement is the property which says that free colour charge does not exist. One consequence is that gluons are not involved in the nuclear forces. The force mediators for these are other hadrons called mesons.
The gluon is a vector boson; it has spin one. Usually vector particles have three spin states, but gauge invariance reduces the number of spin states of a gluon to two. It has negative intrinsic parity, and has zero isospin. In quantum field theory, unbroken gauge invariance requires that gauge bosons have zero mass (although the experimental limit for gluon mass is a few MeV). Unlike the single photon of QED or the three W and Z bosons of the weak interaction, there are 8 independent types of gluon in QCD.
Technically QCD is a gauge theory with SU(3) gauge symmetry. Quarks are introduced as Dirac fields in Nf flavours, each in the fundamental representation (triplets) of the colour gauge group, SU(3). Gluons are vector fields in the adjoint representation (octets) of colour SU(3).
In the normal phase of QCD, it is predicted that there exist hadrons which are formed entirely of gluons — called glueballs. There are also conjectures about other exotics in which real gluons (as opposed to virtual ones found in ordinary hadrons) would be primary constituents. At extreme temperatures, quark gluon plasma forms, in which there are no hadrons, and gluons become free particles.
See also
- Quarks, hadrons, gauge bosons
- Glueballs, exotics, quark model
- Quantum chromodynamics, standard model
- Three jet events, deep inelastic scattering
References and external links
- Summary tables in the "Review of particle physics" (http://pdg.lbl.gov/2004/tables/contents_tables.html)
- DESY glossary (http://www.desy.de/pr-info/desyhome/html/presse/glossary.html#G)
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| Fermions : Quarks | Leptons | |
| Gauge bosons : Photon | W+, W- and Z0 bosons | Gluons | |
| Not yet observed: | |
| Higgs boson | Graviton | |
| Supersymmetric Partners : Neutralinos | Charginos | Gravitino | Gluinos | Squarks | Sleptons | |
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