Fermions distinguish between matter (leptons and quarks) and anti-matter.
Leptons are extremely small particles (less than 10-15 m radius) that have no known size or internal structure. They have tiny masses, travel very fast and are best described by wave functions. The best known examples of leptons are the electron and the neutrino. The leptons have been classified into:
- electron-electron neutrino
- muon-muon neutrino
- tau-tau neutrino
Quarks are extremely small particles (less than 10-15 m radius) that participate in the strong nuclear force. Isolated (single) quarks have never been found, probably because they combine with each other so quickly. Quarks also have fractional electric charges. They are classified as follows:
- down (d) - charge = -1/3
- up (u) - charge = +2/3
- strange (s) - charge = -1/3
- charm (c) - charge = +2/3
- bottom (b) - charge = -1/3
- top (t) - charge = +2/3 (most massive, discovered in 1995)
As of now, quarks are thought to be the most fundamental particles.
Not much is known about antimatter. The first anti-matter particle discovered was the positron, which has a mass similar to an electron but with a positive charge. This area of particle physics is currently under investigation.