by Andrew J. Bennieston

Created on: March 01, 2009

The Standard Model (SM) of particle physics describes the elementary particles which make up the universe, as well as the interactions between them. Aspects of the standard model have been tested to incredible precision - for example, quantum electrodynamics (QED) is the most precisely tested theory in physics, to date - and while the theory is not complete, it provides a framework for the understanding of a great many particle interactions and physical phenomena. The standard model describes three kinds of elementary particle; quarks, leptons and gauge bosons. We'll start with quarks.

Quarks are elementary particles (i.e. particles which are not composed of other, smaller particles, but are believed to be a single, indivisible entity) which possess electric charge and one of six quark flavours. The flavour is just a quantum mechanical marker that determines the "kind" of quark. Quark flavours are given the names up (u), down (d), charm (c), strange (s), top (t) and bottom (b) (or beauty). They form three generations:

( u, d ) ( c, s ) ( t, b )

The first generation, consisting of the up and down quark, are the quarks found in the nuclei of the atoms making up the universe around us, while the other two generations are simply heavier (more massive) versions of the up and down quark. The up quark has electric charge of +2/3 and the down has charge -1/3. The charm and top quarks have the same charge as the up quark, and similarly with the down, strange and bottom quarks.

Each quark also carries a quantity known as colour. The colour of a quark is not related to any kind of visible colouration, but to the so-called strong interaction, described by quantum chromodynamics (QCD). QCD dictates that free particles (those not part of a bound state) should be colourless. This mean that quarks cannot exist in isolation, but must be combined together to make colourless combinations. Quarks can possess one of three colours (commonly referred to as red, green and blue).

Quarks can be combined in one of two ways; as bound states of three quarks, where they make particles known as baryons, or as bound states of quark and anti-quark, where they make particles known as mesons.

Baryons include the proton, composed of two up quarks and one down quark, and the neutron, composed of two down quarks and one up. Adding together the charges on each quark provide the well-known properties that the proton has electric charge +1, and the neutron has zero charge. Baryons make colourless combinations

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