Gluon

In particle physics, gluons (from glue + -on) are elementary particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei.

In technical terms, they are vector gauge bosons that mediate strong color charge interactions of quarks in quantum chromodynamics (QCD). Unlike the neutral photon of quantum 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.

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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 because gauge invariance requires the polarization to be transverse. In quantum 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 zero isospin. It is its own antiparticle.

Numerology of gluons

Unlike the single photon of QED or the three W and Z bosons of the weak interaction, there are eight independent types of gluon in QCD.

This may be difficult to understand intuitively. Quarks may carry three types of color charge; antiquarks carry three types of anticolor. Gluons may be thought of as carrying both color and anticolor or as describing how quark color changes during interactions, so because gluons carry nonzero color charge it may be thought that there are only six gluons.

Technically, QCD is a gauge theory with SU(3) gauge symmetry. Quarks are introduced as spinor fields in N_f flavours, each in the fundamental representation (triplet, denoted 3) of the colour gauge group, SU(3). The gluons are vector fields in the adjoint 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 N²−1.

Confinement

Main article: Colour 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, quarks are confined within composite particles called hadrons. This effectively limits the range of the strong interaction to 10^-15 meters, roughly the size of an atomic nucleus.

Gluons also share this property of being confined within hadrons. One consequence is that gluons are not directly involved in the nuclear forces. The force mediators for these are other hadrons called mesons.

Although in the normal phase of QCD single gluons may not travel freely, it is predicted that there exist hadrons which are formed entirely of gluons — called glueballs. There are also conjectures about other exotic hadrons in 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), quark 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 at DESY in Hamburg. Quantitative studies of deep inelastic scattering at the Stanford Linear Accelerator Center had established their existence a decade before that.

Experimentally, confinement is verified by the failure of free quark searches. 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 the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratories (BNL).

See also

• Quark
• Hadron
• Gauge boson
• Glueball
• Exotic hadrons
• Quark model
• Quantum chromodynamics
• Standard model
• Three-jet events
• Deep inelastic scattering

References and external links

• Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4. 
• Kaufmann(ed), Scientific American: Particles & Fields (special edition), 1980
• Summary tables in the "Review of particle physics"
• DESY glossary
• Logbook of gluon discovery
• Why are there eight gluons and not nine?