Definitions

# lepton

[lep-ton]
lepton [Gr.,=light (i.e., lightweight)], class of elementary particles that includes the electron and its antiparticle, the muon and its antiparticle, the tau and its antiparticle, and the neutrino and antineutrino associated with each of these particles. Leptons are the lightest class of particles having nonzero rest mass. From a technical point of view, they are defined by their behavior, being weakly interacting fermions, i.e., leptons can result from the slow decay of nuclear particles such as the neutron but do not experience a strong attraction toward the nuclear particles; they are described by the Fermi-Dirac statistics, which apply to all particles restricted by the Pauli exclusion principle. This means that two identical leptons cannot occupy the same quantum state. However, one muon and one electron are allowed to occupy the same state. The muon was originally classed as a meson because of its mass, about 200 times that of the electron, but the subsequent reclassification of particles on the basis of their behavior placed it with the electron in the lepton category. The electron and the muon are almost twins, except for their large mass difference; each is negatively charged, has a positively charged antiparticle, and has an associated neutrino and antineutrino. Separate laws govern the conservation of electron family number and of muon family number, the number being +1 for ordinary particles of either family and -1 for antiparticles (see conservation laws, in physics).

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.

Leptons are a family of fundamental subatomic particles, comprising the electron, the muon, and the tauon (or tau particle), as well as their associated neutrinos (electron neutrino, muon neutrino, and tau neutrino). Leptons are spin particles, and as such are fermions. Leptons do not strongly interact, in contrast to the quarks.

## Properties of leptons

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.

All leptons have corresponding antiparticles. It is possible that neutrinos are their own antiparticles.

Particle Antiparticle
electron antielectron
(or positron)
muon antimuon
tauon
(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.

## Table of the leptons

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 $Delta m^2_\left\{12\right\} = 80mbox\left\{ meV\right\}^2$ and $Delta m^2_\left\{23\right\} approx Delta m^2_\left\{13\right\} = 2400mbox\left\{ meV\right\}^2$. This leads to the following conclusions:

• and are lighter than 2.2 eV (as is and the mass differences between the neutrinos are of order of millielectronvolts)
• one (or more) of the neutrinos is heavier than 0.040 eV
• two (or three) of the neutrinos are heavier than 0.008 eV

## Etymology

According to the Oxford English Dictionary, the name "lepton" (from Greek leptos meaning 'thin') was first used by physicist Léon Rosenfeld in 1948:

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.