Quantum Chromo-Dynamics and the Strong interaction

Quantum chromo-dynamics (or QCD) is a theory in physics that attempts to account for the behaviour of quarks and gluons when they combine to form hadrons. The prefix chromo refers to the word "colour", the term "colour" in this sense has nothing to do with the colours of the everyday world, but rather refers to a mathematical property which is an attribute of quarks and gluons. 'Colour-charge' refers to the charge associated with the force carried by gluons, called the strong interaction. The mathematics of colour charge is quite different from the ordinary arithmetic that is obeyed by electric charges.

There are three possible colour-charges for quarks. A group of three quarks, with one quark of each colour, is a colour neutral (also called a colour-singlet) state. It is this combination which makes up the baryon. One way to think about colour-charge, is to think about the three primary colours of paint, by mixing the three primary colours together you get white (or colourless paint). This is a good analogy for the colour neutral combination of three quarks. The colour mathematics always works out so that at any instant the entire hadron system is colour neutral.

Just as photons carry electromagnetic force (e.g. light), gluons transmit the force that binds quarks together. So that put another way colour is to strong interaction what electric charge is to electromagnetic interaction. Hence quantum chromo-dynamics can be thought of as the theory of strong interaction, and indeed is often described as such. It is this property of colour that makes QCD different to other quantum theories.

The strong interaction is an important part of our project since unlike most other forces which you may be familiar with, the strong interaction carried by gluon particles actually gets stronger the further apart the quarks are pulled. This is in direct contrast to the electromagnetic force, which grows weaker in proportion to the square of the distance between the interacting bodies.

However the strong interaction model is not quite as simple as this. The reason is that as you pull the quarks apart, the energy that you're putting in, in order to separate them is used to create new hadrons between the separated particles. This process is known as hadronisation. After a high energy collision (such as the ones we are analysing in our project) a quark or gluon starts to move away from the rest of the formerly colour-neutral object that contained it. A region of 'colour force -field' is produced between the two parts. The energy density in this colour force field is sufficient to produce additional quarks and anti-quarks. The forces between the colour-charged particles quickly cause the collection of quarks and anti quarks to be rearranged into colour neutral combinations. What emerges, far enough from the collision point to be detected, is always a collection or 'jet' of colour-neutral hadrons (hadronisation), never the initial high-energy quark or gluon alone.

This hadronisation is the reason why in our project we are only concerned with short-range strong interaction. Since the focus of our project is looking at the structure of the photon, the formation of hadrons resulting from the action of the strong interaction force at long distances would interfere with our examination of this structure. Thus we confine our project to looking at the strong interaction over short distances where the formation of new hadrons does not interfere with our analysis.