According to the standard model of particle physics, leptons are one of the two fundamental building blocks of matter, the other being quarks. A lepton is a spin 1/2 elementary particle that is not subject to the strong force (also called the chromodynamic force or color force). In other words, leptons are colorless.
Flavors and families
There are six leptons (apart from their antiparticles), sometimes referred to as flavors of lepton: the electron (e−), muon (μ−), tau (τ−), and their associated neutrinos: the electron neutrino (νe), muon neutrino (νμ), and tau neutrino (ντ). The leptons are divided into families, the e, μ, τ families.
Leptons can possess electric charge as in the case of the electron, muon, and tau (all negatively charged), and the corresponding antileptons (all positively charged), or can be electrically charge-neutral like the three flavors of neutrino and the corresponding antineutrinos.
Each family of leptons is assigned its own lepton number which is one for that family and zero for the others, and for all particles that are not leptons. For example, the electron and its neutrino are given an electron lepton number Le = 1, while all other particles have Le = 0. The antiparticles have the opposite sign of lepton number, i.e. either -1 or 0.
An over-all lepton number L is assigned to assemblies of leptons that is the sum of all the constituent lepton numbers,
Lepton number L is conserved in particle reactions, as in:
which conserves both L and individual family lepton numbers. Violations of this conservation law, called LFV for lepton flavor violation, have yet to be observed.
The standard model page
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