Hadrons, the strongly interacting particles, can be further subdivided into two classes based on their internal composition: mesons and baryons. Originally, mesons and baryons were classified according to their masses. Baryons were heavier than mesons, and both were heavier than leptons. Today mesons and baryons are distinguished by their internal structure. Baryons have masses greater than the proton mass. All hadrons are composed of two or three fundamental particles, which came to be known as quarks. A quark is always combined with one or two other quarks.

According to the original model proposed by Murray Gell-Mann and George Zweig in 1963, there were three types of quarks indicated by the symbols u, d, and s. These were given the arbitrary names up, down, and sideways (now referred to as strange). Associated with each quark is an anti-quark, which are the antimatter equivalents

We are seeing only the conceptual results of this very mathematical theory, but we should realize that is it based on experimental evidence.

In addition to these force-carrying particles, the standard model includes 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 "flavors," of each, related in pairs in three "generations" of increasing mass. Everyday matter is built from the members of the lightest generation such as the "up" and "down" quarks that make up the protons and neutrons of atomic nuclei. 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 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 with 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.

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 pr

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