Three generations of matter.
A lepton is an elementary, half-integer spin (spin 1⁄2) particle that does not undergo strong interactions. Particles that do participate in strong interactions are called hadrons.
There are six leptons in the present structure, the electron, muon, and tau particles and their associated neutrinos. Leptons are said to be elementary particles; that is, they do not appear to be made up of smaller units of matter. They behave as point-like particles. All leptons are fermions, i.e. leptons are spin- 1⁄2 particles and thus that they are subject to the Pauli exclusion principle. This fact has key implications for the building up of the periodic table of elements.
Any of the six elementary particles that (with their antiparticles) are not quarks are leptons. Two main classes of leptons exist:
- Charged leptons. Charged leptons can combine with other particles to form various composite particles such as atoms and positronium.
- Electron. The electron is a negatively charged particle with a mass that is approximately 1/1836 that of the proton. Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom. The electron is only one member of a class of elementary particles, which forms an atom.
- Muon. The muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of ½. Muons are heavier, having more than 200 times as much mass as electrons. The muon is an unstable subatomic particle with a mean lifetime of 2.2 µs.
- Tau. The tau (τ), also called the tau lepton, tau particle, or tauon, is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of ½. Taus are approximately 3,700 times more massive than electrons. Tau leptons have a lifetime of 2.9×10−13 s.
- Neutral leptons (better known as neutrinos) are electrically neutral particles that rarely interact with anything, and are consequently rarely observed. A neutrino is an elementary subatomic particle with infinitesimal mass (less than 0.3 eV..?) and with no electric charge. Neutrinos are weakly interacting subatomic particles with ½ unit of spin.
- Electron neutrino. The electron neutrino is a subatomic lepton elementary particle which has the symbol νe. It has no net electric charge and a spin of ½. Together with the electron it forms the first generation of leptons, hence the name electron neutrino.
- Muon neutrino. The muon neutrino is a subatomic lepton elementary particle which has the symbol νμ. It has no net electric charge and a spin of ½. Together with the muon it forms the second generation of leptons, hence the name muon neutrino.
- Tau neutrino. The tau neutrino is a subatomic lepton elementary particle which has the symbol ντ. It has no net electric charge and a spin of ½. Together with the tauon it forms the third generation of leptons, hence the name tau neutrino.