Gluons (from the English noun "glue") are the mediators of the strong interaction. They hold together quarks to form bound states called hadrons, such as protons and neutrons.
Gluons, together with quarks, carry a quantum number called colour charge. Quarks can be red, blue or green (their antiparticle is correspondingly anti-red, anti-blue or anti-green), while gluons carry two colours. Hadrons are always colourless, meaning that their quarks must have balancing colours.
While all the other natural forces weaken with distance, the strong force between two particles increases with distance. For this reason, gluons and quarks have never been observed isolated, but always in colourless bound states. If one tries to separate two quarks, the strong force between them becomes so strong that a new quark-antiquark pair emerges from the vacuum. This happens, for example, in a detector, in which this process is repeated several times and the large amount of produced hadrons forms a "hadron shower", also called “jet”.
The gluon was discovered in 1979 at the TASSO experiment at DESY. The scattering between two electrons generated three jets, indicating that a gluon was radiated by one of the two quarks. Such an event in the TASSO experiment with three "jets": two from quarks and an additional one from a gluon, is shown on the sign.
