Lepton

From Wikipedia, the free encyclopedia

For other uses, see Lepton (disambiguation).

In physics, a lepton is a particle with spin, 1/2 (a fermion) that does not experience the strong nuclear force. The leptons form a family of elementary particles that are distinct from the other known family of fermions, the quarks.

Contents

[edit] Properties of leptons

There are three known flavors of lepton: the electron, the muon and the tau. Each flavor is represented by a pair of particles called a weak doublet. One is a massive charged particle that bears the same name as its flavor (like the electron). The other is a nearly massless neutral particle called a neutrino (like the electron neutrino). All six of these particles have corresponding antiparticles (like the positron or the electron antineutrino). All known charged leptons have a single unit of negative or positive electric charge (depending on whether they are particles or antiparticles) and all of the neutrinos and antineutrinos have zero electric charge. The charged leptons have two possible spin states, while only one helicity is observed for the neutrinos (all the neutrinos are left-handed, and all the antineutrinos are right-handed).

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 SLC and LEP experiments.

[edit] Table of the leptons

Particle / antiparticle Neutrino / antineutrino
Name Symbol Charge (e) Mass (MeV) Name Symbol Charge (e) Mass (MeV)
Electron / Positron e^- \, / \, e^+ −1 / +1 0.511 Electron neutrino / Electron antineutrino \nu_e \, / \, \overline{\nu}_e 0 <0.000003
Muon / Antimuon \mu^- \, / \, \mu^+ −1 / +1 105.6 Muon neutrino / Muon antineutrino \nu_\mu \, / \, \overline{\nu}_\mu 0 <0.19
Tau lepton / Antitau \tau^- \, / \, \tau^+ −1 / +1 1777 Tau neutrino / Tau antineutrino \nu_\tau \, / \, \overline{\nu}_\tau 0 <18.2

[edit] Etymology

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

Following a suggestion of Prof. C. Møller, I adopt — as a pendant to "nucleon" — the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass.[1]

The name originates from before the discovery in the 1970s of the heavy tau lepton, which is nearly twice the mass of a proton.

[edit] See also

[edit] References

  1. ^ Rosenfeld, Léon (1948). Nuclear Forces. Interscience Publishers, New York, xvii.

[edit] External links

  • The Particle Data Group who compile authoritative information on particle properties.
  • Leptons from the Georgia State University is a small summary of the lepton.
 v • d • e 
Particles in physics - elementary particles
Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos)
Gauge bosons: Photon | W and Z bosons | Gluons
Not yet observed: Higgs boson | Graviton | Other hypothetical particles
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