Gluon

Gluon

In particle physics, gluons are ( engl. to glue stick = ) elementary particles that are indirectly responsible for the attraction of protons and neutrons in an atomic nucleus. The symbol for the gluon is.

Thus the gluons are the exchange particles of the strong interaction. There are 8 different gluons, which are between quarks, the building blocks of hadrons ( baryons, such as protons and neutrons, and mesons ), replaced. Gluons but can also interact directly with other gluons, so that particles called glueballs, could exist, which consist only of gluons.

Gluons are electrically neutral and are accepted within the standard model as a massless, while experimentally a mass of a few MeV can not be excluded. They have a color charge, which always made ​​up of a "color" and an " anti- color". Due to this the various gluons can be distinguished.

From group theoretical considerations, the possible combinations of colors and anti colors in gluons arise. The following relation holds: ( In words: The direct product of the color triplets with the anti - triplet color results in the direct sum consisting of octet and a singlet). Singlet is now not able to change the color of a curd, as it is a totally balanced condition. One can imagine this situation in analogy to spin states. All gluons occurring in nature carry " gross color" (corresponds to the total spin is different from zero). Among them are two gluons ( the last two in the list below ) that do not have " net color" (corresponds to the z- component of the spin is zero); but " gross color" also possess them. In contrast, the singlet is genuine colorless ( total spin 0) as a real or complex number factor, and if it existed, it would not be bound due to its lack of color charge by confinement, ie there would be a component of the strong interaction with infinite range exist, in the nature is not observed. For this reason, this combination is not realized and the quantum chromodynamics is described by the symmetry group. Thus, while the total has generators and would thus have 9 gauge fields ( gluons ), one obtains for the group only generators ( the so-called Gell- Mann matrices ), and there are the usual eight gluon wave functions:

For example, here is the first combination that the gluon can react with a green quark (or more precisely with its antiparticle ) and changes its color to red.

The conditions are analogous to the two-particle spin or product or that is, with two basic states or you can make it four independent linear combinations; three of them, and found a matched triplet ( total spin = gross spin S = 1; "magnetic quantum number " ( net spin) M = 1 or 0 or 1, a fourth function, belongs to the singlet state ( gross spin = net spin S = 0). the additional complication, as compared with this analogy is that considering here instead of N = 2 N = 3 and that, instead of the triplet has a structure with eight base states.

Mediated by gluons attraction between the quarks, and consequently between protons and neutrons, is responsible for the stability of atomic nuclei. ( Cohesion of the protons and neutrons in the nucleus; just the protons would otherwise repel due to their same electric charge ).

Quantum chromodynamics ( QCD) is the now accepted theory to describe the strong interaction. In her gluons mediate forces between particles carrying a color charge. When a gluon is exchanged between two quarks, the color charge of the quarks involved changes. The gluon contributes to one anti-color charge to compensate for the original color charge of the quarks and the new color charge of quarks. Since the gluon carries a color charge itself, it can interact with other gluons. This so-called self-interaction, that is, the interaction of the interaction -promoting particles together, making the mathematical analysis of the strong interaction is very complicated.

First experimental evidence for the existence of gluons was gained in 1979, when they found events with a clear three- jet structure in Hamburg at DESY the PETRA accelerator. The third jet was led to the radiation of a gluon back by one of the quarks produced.