Particle Physics

Quantum Chromo Dynamics is all about the strong interactions between Quarks (and Gluons, which are the charge carrier) and how it affects the Hadrons made up of Quarks (if you have not read the section on the Standard Model, hope here endith!)

Have you ever wondered why the nucleus of an atom doesn't just blow apart? After all, the only forces you really hear about are the electrostatic repulsions of the Protons in the nucleus. Neutrons are electrically neutral, so they don't hold the nucleus together. It's all about the Quarks...

Quarks have something called colour charge. It creates a very stong attraction between any particles with it. The force particle carrier in this interaction is the Gluon (which oddly enough also has colour charge...) and Quarks exchange Gluons like there's no tomorrow. This is happening constantly, therefore the colour-force field (no, not something out of Star Trek) is incredibly strong. Inventively, this force is called... Strong. And its strength increases as the distance between the Quarks increases (i.e. strength is proportional to distance, which is unusual). If a Quark in a Hadron decides it wants to get away, the colour-force field becomes so strong it "snaps" to form another Quark/Anti-Quark pair. This means that Quarks cannot exist on their own.

So what's this got to do with nuclei? Well, quite a lot! The quarks in the protons are attracted to each other by the Strong force, which usefully overpowers the repulsive force, keeping our nuclei together! Which in turn keeps the Universe together. Wayhey! You now know the meaning of life! Well ok, not quite. Incidentally, up to a critical number the number of protons in the nucleus and the number of neutrons are the same. However, after this number there are then more neutrons that protons as the distances get bigger and the strong force only has a short finite range (5fm) so there needs to be more electrostatic attraction between proton-neutron pairs to overcome the electrostatic repulsion between proton-proton pairs.

But what exactly IS colour charge?

First things first, it is important to note that Quarks do not exchange little blobs of colour. It's a nice idea, but sadly it doesn't happen. The colours are an easy way for Particle Physicists to explain their complicated calculations. Ok, this may sound a little odd.

Each quark has a colour charge. It could be blue, green or red (it is constantly changing, for reasons I shall go into in a moment). If a quark has combinations of blue, red and green, it is colour neutral. Bit like white light. Anti-Quarks have anticolour charges, which are known as antired, antigreen and antiblue. Combinations of these 3 also make neutral colour charge (think anti-white light). Gluons have colour charge as well (that's what makes this so strange- normally the force carrier particles do not have the charge that is being transferred. Gluons are just odd, if you haven't already guessed from the name). Gluons can be thought of as having a colour/anticolour charge, but they do not have neutral colour. So it all makes sense. The colour charge of Quarks is swapped between them by Gluons (well, exchanging them), so it is always changing.

However, although there should be 9 possible colour combinations for gluons, calculations have proved that there are only 8. The other possible combination is neutral, and this Gluon has never been observed, as it would have no effect on Quarks or other Gluons.