Lepton

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 frac|1|2 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 frac|1|2 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.

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

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$Delta\; m^2\_\{12\}\; =\; 80mbox\{\; meV\}^2$ and $Delta\; m^2\_\{23\}\; approx\; Delta\; m^2\_\{13\}\; =\; 2400mbox\{\; meV\}^2$. This leads to the following conclusions:
* "SubatomicParticle|Muon neutrino" and "SubatomicParticle|Tau neutrino" are lighter than 2.2 eV (as "SubatomicParticle|Electron neutrino" 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.

ee also

* List of particles

References

External links

* [http://pdg.lbl.gov The "Particle Data Group"] who compile authoritative information on particle properties.
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html Leptons] from the Georgia State University is a small summary of the lepton.