The Standard Model is the combination of two theories of particle physics into a single framework to describe all interactions of subatomic particles, except those due to gravity. The two components of the standard model are electroweak theory, which describes interactions via the electromagnetic and weak forces, and quantum chromodynamics, the theory of the strong nuclear force. Both these theories are gauge field theories, which describe the interactions between particles in terms of the exchange of intermediary "messenger" particles that have one unit of intrinsic angular momentum, or spin.
In addition to these force-carrying particles, the standard model encompasses two families of subatomic particles that build up matter and that have spins of one-half unit. These particles are the quarks and the leptons, and there are six varieties, or "flavours," of each, related in pairs in three "generations" of increasing mass. Everyday matter is built from the members of the lightest generation: the "up" and "down" quarks that make up the protons and neutrons of atomic nuclei; the electron that orbits within atoms and participates in binding atoms together to make molecules and more complex structures; and the electron-neutrino that plays a role in radioactivity and so influences the stability of matter. Heavier types of quark and lepton have been discovered in studies of high-energy particle interactions, both at scientific laboratories with particle accelerators and in the natural reactions of high-energy cosmic-ray particles in the atmosphere.
The standard model has proved a highly successful framework for predicting the interactions of quarks and leptons with great accuracy. Yet it has a number of weaknesses that lead physicists to search for a more complete theory of subatomic particles and their interactions. The present standard model, for example, cannot explain why there are three generations of quarks and leptons. It makes no predictions of the masses of the quarks and the leptons nor of the strengths of the various interactions. Physicists hope that, by probing the standard model in detail and making highly accurate measurements, they will discover some way in which the model begins to break down and thereby find a more complete theory. This may prove to be what is known as a grand unified theory, which uses a single theoretical structure to describe the strong, weak, and electromagnetic forces.
Excerpt from the Encyclopedia Britannica without permission.