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by Scot Trembley-Freytag, Lucent Technologies and Ernie Malamud, Fermilab
The force between quarks, the strong nuclear force, can be represented metaphorically by color. Each of the 6 kinds (flavors) of quarks can exist in one of 3 primary colors: red, green, blue. Each of the 6 kinds (flavors) of anti-quarks can exist in one of 3 anti-colors: cyan, magenta, yellow.
Gluons are exchanged between the quarks. The quark flavors remain the same. However, the quark colors change each time a gluon is exchanged. This is a metaphor for how quarks are held together.
E.g. a proton has 3 quarks: up-up-down. These have colors: red, green, blue. Gluon exchange between the quarks swaps the colors. An anti-proton has 3 quarks: antiup-antiup-antidown. These have colors: cyan, magenta, yellow. Gluon exchange between the quarks swaps the colors.
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For the experts: the complete family of gluons has 8 members
Click on a Colored Quark above to show a gluon exchange!
GLUONS have zero mass.
So in the real world the GLUON
Our pictures make the quarks look very large! In reality, the quarks are tiny, smaller than 1/1000th the size of the particles they make. It is unknown just how small the quarks (and gluons) really are. That is one of the challenges of current experimental particle physics.
Can a quark and an anti-quark combine to form a particle? YES.
These particles are called mesons and there are hundreds of different ones. The quark and anti-quark in a meson are also held together by the exchange of colored gluons.
Of course, nature is more complicated. Many gluons are being exchanged all the time. Nice mathematical models showing gluon exchange in nucleons (protons, neutrons etc.) and the fractal structure of a nucleon have been put on the Web by the Ecole Polytechnique in Paris. They also have a lovely animation.
The metaphor of color mixing is used to explain which particles are found in nature and which ones are not. If you combine the 3 primary colors using spotlights (ADDITIVE color mixing) you get white.
| Red + Green + Blue makes white. |  |
If you combine the 3 anti-primary colors,
| Cyan + Magenta + Yellow = White |  |
If you combine a color with its own anti-color you get white.
| Blue Quark + Yellow (anti-blue) quark = White Meson |  |
| Red Quark + Turquoise (anti-red) quark = White Meson |  |
| Green Quark + Magenta (anti-green) quark = White Meson |  |
Only those combinations that make White are observed in nature. This is why we do not find particles that are, for example, made of 4 quarks, or 2 quarks, or 3 quarks and one anti-quark.
| Force | Messenger Particle or Force Carrier | Force is between | Relative strength |
| Strong (or Color) force | "Colored" Gluons - 8 different kinds | quarks - holds nuclei together | Very strong |
| Electromagnetic | photons (light, radio waves, X-rays etc.) | electrically charged particles | about 100 x weaker than strong force |
| Weak Nuclear | W, Z bosons -- very massive | leptons (electrons, mu-leptons etc.) | very weak - causes radioactive decay and the sun to burn |
| Gravity | Graviton (this quantum of gravity has never been detected experimentally) | particles with mass | weakest of them all |
Comments or questions: malamud@fnal.gov |
rev. 5/2/02 E.M. |
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