Gluon

Feynmann Diagram Gluon Radiation

Feynman Diagram Y-3g

Gluon is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the photon in the electromagnetic force but acts only on particles that carry color charge, such as quarks and other gluons. Gluons are an integral part of the theory of Quantum Chromodynamics (QCD), which is a component of the Standard Model of particle physics.

Overview

Gluons are massless particles and, like photons, they travel at the speed of light. They have a spin of 1, making them bosons. The strong force that gluons mediate is the strongest of the four fundamental forces at the distances typical in atomic nuclei, and it is responsible for holding quarks together to form protons, neutrons, and other hadron particles.

One of the unique properties of the strong force is color confinement, which means that color-charged particles (such as quarks and gluons) cannot be isolated and observed individually at low energies. Instead, they are always found in color-neutral combinations. This property is a direct consequence of the behavior of gluons, which, unlike photons, carry the color charge and can interact with each other. This self-interaction is a key feature of QCD and leads to the phenomenon of asymptotic freedom, whereby quarks behave as free particles at high energies but are tightly bound at low energies.

Discovery

The existence of gluons was first postulated in the early 1970s as part of the development of QCD. Their existence was indirectly confirmed through experiments at particle accelerators that showed jets of particles consistent with the behavior predicted by QCD. Direct evidence of gluons came from deep inelastic scattering experiments and electron-positron annihilation experiments, which provided clear signatures of gluon activity.

Properties

Gluons are described by their color charge, which comes in three types and their corresponding anticolors, making for a total of eight gluons due to the combinations of color charges. This is because the color singlet combination does not correspond to a physical gluon (it would be color neutral, which is not possible for gluons).

Gluons play a crucial role in the dynamics of the quark-gluon plasma, a state of matter thought to have existed just after the Big Bang, and which can be recreated in particle accelerators. In this state, quarks and gluons are not confined within hadrons but exist as a free fluid.

In the Standard Model

Within the Standard Model, gluons are responsible for mediating the strong force in the theory of Quantum Chromodynamics. They are one of the four gauge bosons in the model, alongside the photon, the W and Z bosons of the weak force, and the hypothetical graviton of gravity (which has not yet been observed).

See also

• Quark
• Standard Model
• Quantum Chromodynamics
• Strong force
• Hadron
• Particle accelerator