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From the Stanford Linear Accelerator Website:
The Standard Model is the name given to the current theory of fundamental particles and how they interact. This theory includes:
The theory does not include the effects of gravitational interactions. These effects are tiny under high-energy Physics situations, and can be neglected in describing the experiments. Eventually, we seek a theory that also includes a correct quantum version of gravitational interactions, but this is not yet achieved.
The Standard Model was the model of particle physics in the 1970's . It incorporated all that was known at that time and has since then successfully predicted the outcome of a large variety of experiments. Today, the Standard Model is a well established theory applicable over a wide range of conditions, but we know that there are limitations.
Matter Constituents as Understood in the 1930s
Matter is made of atoms, atoms are made of a nucleus plus electrons, and electrical forces between the nucleus and the electrons explain the structure and stability of the atom.
The nucleus is made of protons and neutrons, but what force holds them together in the nucleus?
Investigation of this questions led to the discovery of many more types of matter, and antimatter, and to the modern theory of matter -- known as the Standard Model.
The attempt to understand and classify the many particles discovered led to the recognition that protons and neutrons are not fundamental particles but are made of smaller objects called quarks.
Matter Constituents as Understood Today
There are two major classes of matter -- hadrons and leptons -- distinguished by their behaviors, that is by the types of interaction in which they participate.
Particles that participate in all known types of interaction -- strong, electromagnetic, weak and gravitational are called hadrons. Observable hadrons are composite objects made from quarks and antiquarks and gluons.
Particles that do not participate in strong interactions, but do participate in the other types are charged leptons --the most common of these being the electron.
Neutral leptons, i.e. those without electric charge are even more elusive -- they participate only in weak and gravitational interactions, and thus are rather difficult to detect. These particles are called neutrinos.
In the Standard Model, quarks and leptons are fundamental particles. As far as we know today there is no evidence that refutes this assumption, nor any evidence for a size or structure for any of these particles. All we can say from experiment is that any structure is at a size smaller than 10-18 meters.
Decays
The notion of a constituent or building block gets a new twist here, too. Normally if something can fall apart into certain other objects, then we say that these objects were constituents of the first.
With fundamental particles a new possibility occurs: an object which is fundamental -- that is, has no constituent parts -- can nevertheless be unstable --that is it can decay radioactively, disappearing itself and producing two or more other fundamental particles which fly apart.
Furthermore objects can be composite -- hadrons made from quarks -- but never can be broken apart into their constituent parts! Modern theory says quarks cannot exist in isolation but are only to be found inside hadrons.
