Particle Physics

Under Construction

Most matter is composed of molecules (groups of atoms) e.g. water, salt, proteins or atoms themselves e.g. hydrogen, iron.

Atoms are made up of protons & neutrons in a nucleus and electrons which surround the nucleus. This is the Bohr model of the atom based on the discovery of the neutron in 1921.

The current, 2005, structure, called the Standard Model of Fundamental Particles and Interactions, describes neutrons and protons made up of quarks. Based on decades of experiments showing the existance of quarks first proposed in 1964.
Electrons are a type of lepton.

There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.)

Quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called Hadrons.
There are two classes of hadrons:
baryons are any hadron which is made of three quarks (qqq).
mesons contain one quark (q) and one antiquark ().

Protrons and neutrons are baryons:
protons are made of two up quarks and one down quark (uud),
neutrons are made of one up quark and two down quarks (udd).

One example of a meson is a pion (π⁺), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (π^-) is made up a down quark and an up antiquark.

Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K^-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.

There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e-). The other two charged leptons are the muon() and the tau(), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos (). They have no electrical charge, very little mass, and they are very hard to find.

Where do these particles, which are not part of atoms, exist?
They are created when other particles decay or are broken up by high speed collisions, created in the laboratory or from cosmic rays hitting the atmosphere. There may be other sources; I couldn't find a good website that explains it.