Gluon

Gluon (g)
classification
Elementary particle boson gauge boson
properties
electric charge	neutral
Dimensions	(theoretically) 0  kg
Spin parity	1 -

In particle physics are gluons (Engl. To glue  = glue) elementary particles that indirectly for attracting protons and neutrons in a nucleus are responsible. The symbol for the gluon is . ${\displaystyle \mathrm {g} }$

The gluons thus form the exchange particles of the strong interaction . There are eight different gluons that are exchanged between quarks , the building blocks of hadrons ( baryons , e.g. protons and neutrons, and mesons ). Gluons can also interact directly with other gluons, so that particles could exist that only consist of gluons, the glueballs .

properties

Gluons are electrically neutral and are assumed to be massless within the Standard Model , while a mass of a few MeV cannot be excluded experimentally . They have a color charge , which is always composed of a "color" and an "anti-color". This allows the different gluons to be distinguished.

The possible combinations of colors and anti-colors in gluons result from considerations of group theory :

${\displaystyle 3\otimes {\bar {3}}=8\oplus 1\,.}$

(In words, the direct product of the color triplets with the anti-color triplet results in the direct sum consisting of octet and a singlet).

The singlet is unable to change the color of a quark because it represents a totally symmetrical state. One can imagine this fact in analogy to spin states . All gluons occurring in nature have a "gross color" (corresponds to: the total spin is different from zero). Below there are two gluons (the last two in the following list) that have no “net color” (corresponds to: the z component of the spin is zero); but they too have “gross color”. In contrast, the singlet is genuinely colorless ( total spin 0), like a real or complex number factor, and if it existed, it would not be bound by confinement due to its lack of color charge , i.e. H. there would be a component of the strong interaction with infinite range that is not observed in nature. For this reason, this combination is not realized, and quantum chromodynamics is described by the symmetry group . So while the total has generators and would therefore have 9  gauge fields (gluons), one only gets generators (the Gell-Mann matrices ) for the group , and the usual eight gluon wave functions result : ${\displaystyle (r{\bar {r}}+b{\bar {b}}+g{\bar {g}})/{\sqrt {3}}}$ ${\displaystyle {\hat {=}}}$ ${\displaystyle SU(3)_{C}}$ ${\displaystyle U(3)_{C}}$ ${\displaystyle N^{2}=3^{2}=9}$ ${\displaystyle SU(N=3)}$ ${\displaystyle N^{2}-1=8}$

${\displaystyle {\begin{aligned}\psi _{1}&=|r{\bar {g}}\rangle \,,\quad &\psi _{2}&=|r{\bar {b}}\rangle \,,\\\psi _{3}&=|g{\bar {r}}\rangle \,,\quad &\psi _{4}&=|g{\bar {b}}\rangle \,,\\\psi _{5}&=|b{\bar {r}}\rangle \,,\quad &\psi _{6}&=|b{\bar {g}}\rangle \,,\\\psi _{7}&={\tfrac {1}{\sqrt {2}}}\left(|r{\bar {r}}\rangle -|g{\bar {g}}\rangle \right)\,,\;&\psi _{8}&={\tfrac {1}{\sqrt {6}}}\left(|r{\bar {r}}\rangle +|g{\bar {g}}\rangle -2|b{\bar {b}}\rangle \right)\,.\end{aligned}}}$

Here, for example, the 1st combination means that the gluon can react with a green quark and change its color to red.

The ratios are analogous to the two-particle spin product with or or d. H. with two base states or one can form four independent linear combinations from them ; three of them, as well as , result in a related 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 = The additional complication of gluons, compared to this analogy, is that one considers N = 3 instead of  N = 2 and that one has an octet instead of the triplet at the base states. ${\displaystyle \langle \psi _{i}(1)\psi _{k}(2)\rangle \,,}$ ${\displaystyle \psi _{i}}$ ${\displaystyle \psi _{k}={\mathord {\uparrow }}}$ ${\displaystyle {\mathord {\downarrow }}\,,}$ ${\displaystyle \psi ={\mathord {\uparrow }}}$ ${\displaystyle \psi ={\mathord {\downarrow }}\,.}$ ${\displaystyle \psi _{1}={\mathord {\uparrow }}{\mathord {\uparrow }}\,,\psi _{2}=({1/{\sqrt {2}}})(\uparrow \downarrow +\downarrow \uparrow )}$ ${\displaystyle \psi _{3}={\mathord {\downarrow }}{\mathord {\downarrow }}}$ ${\displaystyle ({1/{\sqrt {2}}})(\uparrow \downarrow -\downarrow \uparrow )\,,}$

The attraction mediated by the gluons between the quarks - and consequently between protons and neutrons - is responsible for the stability of the atomic nucleus (cohesion of the protons and neutrons in the atomic nucleus; otherwise the protons would repel each other due to their identical electrical charge ).

The quantum chromodynamics  (QCD) is the currently accepted theory describing the strong interaction . In it, gluon forces mediate between particles that carry a color charge. When a gluon is exchanged between two quarks, the color charge of the quarks involved changes. The gluon carries an anti-color charge to compensate for the original color charge of the quark and the new color charge of the quark. Since the gluon itself also carries a color charge, it can interact with other gluons. This self-interaction , i.e. H. the interaction of the particles that mediate the interaction makes the mathematical analysis of the strong interaction very complicated.

Discovery / Evidence

The first experimental evidence of the existence of gluons was obtained in 1979, when events with a clear three- jet structure were found at DESY in Hamburg with the accelerator PETRA . The third jet was traced back to the emission of a gluon by one of the quarks produced.

Web links

Wiktionary: Gluon  - explanations of meanings, word origins, synonyms, translations

• Entry Gluon (PDF file; 31 kB) in the particle list of the Particle Data Group (PDF file; 31 kB)

Individual evidence

1. ^ David J. Griffiths: Introduction to Elementary Particles. Wiley, John & Sons, New York 1987, ISBN 0-471-60386-4 .
2. ↑ The index  C should mean color (the English word for color).
3. ↑ Another choice of base can be found in the English WP, also under ' Gluon '.
4. ^ John Ellis: Those were the days: discovering the gluon . In: CERN Courier . tape 49 , no. 6 , 2009, p. 15–18 (English, cern.ch ).