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Colour (as in quantum chromodynamics)

  It is very important to realize that the quantum mechanical term colour has nothing whatsoever to do with the word colour in everyday use. It is simply a label used to provide a distinction between particles.

To understand why the concept of colour was introduced, it is necessary to understand the nature of fermions. Particles are divided into fermions and bosons. Fermions must obey the Pauli Exclusion Principle, which states that no two fermions in a system have the same set of quantum numbers. Thus, a particle made up of two or more identical fermions may not exist.

However, some particles composed of quarks appeared to violate this principle. An example is the particle which is made up of three seemingly identical strange quarks. In 1964, Greenburg proposed a solution to this problem, by suggesting that each quark possesses a property which he called colour. Thus the three quarks could each have a different colour quantum number (red, green or blue), but be identical in every other respect, and still remain fermions. This proposal means that there are three times as many quarks as were initially thought. However, this tripling has been tested in systems such as the neutral meson decay rate and found to be the case.

Colour was not detected before this because all quarks obey confinement. This means that quarks only combine in ways that are colour neutral overall. This is slightly analogous to charged particles. A negative particle and a positive one attract each other, and will combine to form a neutral one. Similarly, red, green and blue quarks combine to form colour neutral (white) particles. The essential difference is that we may observe positive and negative charges, but we never see ‘coloured’ systems. Some particles are colour neutral but have only two quarks. This is achieved by saying that each colour has a corresponding anti-colour, which cancels it out. Thus a particle could be composed of a red and an anti-red quark, and be colour neutral. JJ



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