Standard Model: Particles and Forces

The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces, as well as the fundamental particles that make up all matter. It is a quantum field theory, and consistent with both quantum mechanics and special relativity. To date, almost all experimental tests of the three forces described by the Standard Model have agreed with its predictions. However, the Standard Model is not a complete theory of fundamental interactions, primarily because it does not describe gravitation.

The Standard Model contains both fermionic and bosonic fundamental particles. Informally speaking, fermions are particles of matter and bosons are particles that transmit forces.

In the Standard Model, the theory of the electroweak interaction (which describes the weak and electromagnetic interactions) is combined with the theory of quantum chromodynamics. Each of these theories are gauge field theories, meaning that they model the forces between fermions by coupling them to bosons which mediate the forces. The Lagrangean of each set of mediating bosons is invariant under a transformation called a gauge transformation, so these mediating bosons are referred to as gauge bosons. The bosons in the Standard Model are:

Photons, which mediate the electromagnetic interaction.
W+ and W- and Z0 bosons, which mediate the weak nuclear force
Eight species of gluons, which mediate the strong nuclear force. Six of these gluons are labelled as pairs of "colors" and "anti-colors" (for example, a gluon can carry "red" and "anti-green".) The other two species are a more complicated mix of colors and anti-colors.
The Higgs bosons, which induce spontaneous symmetry breaking of the gauge groups and are responsible for the existence of inertial mass. The Higgs boson is the only boson in the theory which is not a gauge boson; it has a special status in the theory, and has been the subject of some controversy. Gravitons, the bosons believed to mediate the gravitational interaction, are not accounted for in the Standard Model.

There are twelve different types, or "flavours", of fermions in the Standard Model. Amongst the proton, neutron, and electron, those fermions which constituent the vast majority of matter, the Standard Model considers only the electron a fundamental particle. The proton and neutron are aggregates of smaller particles known as quarks, which are held together by the strong interaction.

The fermions can be arranged in three "generations", the first one consisting of the electron, the up and down quarks, and the electron neutrino. All ordinary matter is made from first generation particles; the higher generation particles decay quickly into the first generation ones and can only be generated for a short time in high-energy experiments.

The electron and the electron-neutrino, and their counterparts in the other generations, are called "leptons". Unlike the other fermions, they do not possess a quality called "color", and therefore their interactions (weak and electromagnetic) fall off rapidly with distance. On the other hand, the strong force between quarks gets stronger with distance, so that quarks are always found in colorless combinations called hadrons. These are either fermionic baryons composed of three quarks (the proton and neutron being the most familiar example) or bosonic mesons composed of a quark-antiquark pair (such as pions). The mass of such aggregates exceeds that of the components due to their binding energy.