Any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Leptons have a half-integral spin and obey the Pauli exclusion principle. They can either carry one unit of electric charge or be neutral. The charged leptons are the electrons, muons, and taus. Each has a negative charge and a distinct mass. Each charged lepton has an associated neutral partner, or neutrino, which has no electric charge and very little if any mass.
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There are three flavours of lepton, electronic leptons (electron, electron neutrino), muonic (muon, muon neutrino), and tauonic (tauon, tau neutrino). Each flavour pair forms a weak isospin doublet. Each doublet comprises one massive particle (which is often called "electron-like lepton") and one massless particle (neutrino).
Electron-like leptons have a charge of −1 e, while neutrinos are neutral particles (with a charge of 0 e). Since leptons are spin particles, they have two possible helicities, although all observed neutrinos have been left-handed.
(or tau particle)
| antitauon |
|electron neutrino (electrino)||electron antineutrino|
|muon neutrino (or mutrino)||muon antineutrino|
|tau neutrino (or tautrino)||tau antineutrino (or tautrino)|
The masses of the leptons also obey a simple relation, known as the Koide formula, but at present this relationship cannot be explained.
When particles interact, generally the number of leptons of the same type (electrons and electron neutrinos, muons and muon neutrinos, tau leptons and tau neutrinos) remains the same. This principle is known as conservation of lepton number. Conservation of the number of leptons of different flavors (for example, electron number or muon number) may sometimes be violated (as in neutrino oscillation). A much stronger conservation law is the total number of leptons of all flavors, which is violated by a tiny amount in the Standard Model by the so-called chiral anomaly.
The couplings of the leptons to gauge bosons are flavor-independent. This property is called lepton universality and has been tested in measurements of the tau and muon lifetimes and of Z-boson partial decay widths, particularly at the Stanford Linear Collider and Large Electron-Positron Collider (LEP) experiments.
|Charged lepton / antiparticle||Neutrino / antineutrino|
|Name||Symbol||Electric charge (e)||Mass (MeV/c2)||Name||Symbol||Electric charge (e)||Mass (MeV/c2)|
|Electron / Positron||/||−1 / +1||0.511||Electron neutrino / Electron antineutrino||/||0||< 0.0000022|
|Muon||/||−1 / +1||105.7||Muon neutrino / Muon antineutrino||/||0||< 0.17|
|Tau lepton||/||−1 / +1||1777||Tau neutrino / Tau antineutrino||/||0||< 15.5|
Note that the neutrino masses are known to be non-zero because of neutrino oscillation, but their masses are sufficiently light that they have not been measured directly as of 2008. However, the differences of the mass squares between the neutrinos have been measured (indirectly based on the oscillation periods), which are estimated to be and . This leads to the following conclusions:
The etymology incorrectly implies that all the leptons are light, since they were named before the discovery in the 1970s of the heavy tau lepton, which is nearly twice the mass of a proton.