Gluon

Gluons are vector gauge bosons that mediate strong color charge interactions of quarks in quantum chromodynamics. As such, they are the source of the fundamental source that makes basic particles like quarks form larger structures, known as hadrons.

Unlike the neutral photon of quantum electrodynamics, gluons themselves participate in strong interactions. The gluon has the ability to do this as it itself carries the colour charge and so interacts with itself, making QCD a little harder to analyse than QED.

Properties

The gluon is a vector boson like the photon; it has spin 1. 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. The gluon is its own antiparticle.

Colors and flavors

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

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.

Hadron 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 roughly the size of an atomic nucleus, and is the force that keeps the most basic of the fundamental particles together.

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, exotic hadrons exist which are formed entirely of gluons — called glueballs. There are also other types of 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.