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

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 (such as the electron neutrino). All six of these particles have corresponding antiparticles (such as 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. true dat.

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 [1]
Muon		−1 / +1	105.7		Muon neutrino / Muon antineutrino		0	< 0.17 <ref name="neutrinomass" />
Tau lepton		−1 / +1	1777		Tau neutrino / Tau antineutrino		0	< 15.5 <ref name="neutrinomass" />

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 2007. However there have been measured (indirectly based on the oscillation periods) the differences of the mass squares between the neutrinos, which have been estimated and . 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

The names "mu" and "tau" seem to have been selected due to their places in the Greek alphabet; mu is seven letters after epsilon (electron), whereas tau is seven letters after mu.

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.^[2]

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

See also

• List of particles

In Popular Culture

"Lepton Radiation" is said to be emitted by devices that transport people to other dimensions in the TV series Stargate SG-1. (Crystal Skull, Arthur's Mantle)

"What are you looking at, lepton?" Little Man Tate (1991)

The Björk Song by Lore Sjöberg contains a lyric (and a discussion of said lyric) involving a lepton. "She's small and she's odd like a lepton or quark."

References

1. ^ Laboratory measurements and limits for neutrino properties.
2. ^ Rosenfeld, Léon (1948). Nuclear Forces. Interscience Publishers, New York, xvii. 

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.

•  • [ e] Particles in physics
Elementary particles	Elementary fermions:  Quarks: u d s c b t • Leptons: e μ τ νe νμ ντ Elementary bosons:  Gauge bosons: γ g W Z0 • Ghosts
Composite particles	Hadrons:  Baryons(list)/Hyperons/Nucleons: p n Δ Λ Σ Ξ Ω Ξb • Mesons(list)/Quarkonia: π K ρ J/ψ Υ Other:  Atomic nucleus • Atoms • Molecules • Positronium
Hypothetical elementary particles	Superpartners: Axino Dilatino Chargino Gluino Gravitino Higgsino Neutralino Sfermion Slepton Squark Other: Axion Dilaton Goldstone boson Graviton Higgs boson Tachyon X Y W' Z'
Hypothetical composite particles	Exotic hadrons:  Exotic baryons: Pentaquark • Exotic mesons: Glueball Tetraquark Other:  Mesonic molecule
Quasiparticles	Davydov soliton Exciton Magnon Phonon Plasmon Polariton Polaron

spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point.

In classical mechanics, the spin angular momentum
..... Click the link for more information.

In particle physics, fermions are particles with half-integer spin, such as protons and electrons. They are named after Enrico Fermi. In the Standard Model there are two types of elementary fermions: quarks and leptons.
..... Click the link for more information.

The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD).
..... Click the link for more information.

For the novel, see .

In particle physics, an elementary particle or fundamental particle is a not known to have substructure; that is, it is not known to be made up of smaller particles.
..... Click the link for more information.

quark (pronounced IPA: /kwɔrk/) is one of the two basic constituents of matter (the other is the lepton). Quarks make up protons and neutrons, with there being exactly three quarks within each kind of particle.
..... Click the link for more information.

Flavour in particle physics

 

..... Click the link for more information.

Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
..... Click the link for more information.

Muon

The Moon's cosmic ray shadow, as seen in secondary muons detected 700m below ground, at the Soudan II detector.
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: Second
Interaction: Gravity, Electromagnetic,
Weak
..... Click the link for more information.

Tau Lepton
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: Third
Interaction: Gravity, Electromagnetic,
Weak
Antiparticle: Antitauon
Mass: 1776.99±0.
..... Click the link for more information.

Flavour in particle physics

 

..... Click the link for more information.

Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
..... Click the link for more information.

Neutrino
Composition: Elementary particle
Family: Fermion
Group: Lepton
Interaction: weak force and gravity
Antiparticle: Antineutrino (possibly identical to the neutrino)
Theorized: 1930 by Wolfgang Pauli
..... Click the link for more information.

Neutrino
Composition: Elementary particle
Family: Fermion
Group: Lepton
Interaction: weak force and gravity
Antiparticle: Antineutrino (possibly identical to the neutrino)
Theorized: 1930 by Wolfgang Pauli
..... Click the link for more information.

Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charges. (The exceptions are massless gauge bosons such as the photon.) Even electrically neutral particles, such as the neutron, are not identical to their antiparticle.
..... Click the link for more information.

Positron
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
Interaction: Gravity, Electromagnetic, Weak
Antiparticle: Electron
Theorized: Paul Dirac, 1928
Discovered: Carl D.
..... Click the link for more information.

Antineutrino
Composition: Elementary particle
Family: Fermion
Group: Lepton, Anti-Lepton
Interaction: weak force and gravity
Antiparticle: Neutrino
Theorized: 1930
Discovered: 1956
Symbol: , and
No.
..... Click the link for more information.

The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron.
..... Click the link for more information.

spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point.

In classical mechanics, the spin angular momentum
..... Click the link for more information.

helicity is the projection of the spin onto the direction of momentum, :

Because the spin with respect to an axis has discrete values, helicity is discrete, too.
..... Click the link for more information.

helicity is the projection of the spin onto the direction of momentum, :

Because the spin with respect to an axis has discrete values, helicity is discrete, too.
..... Click the link for more information.

helicity is the projection of the spin onto the direction of momentum, :

Because the spin with respect to an axis has discrete values, helicity is discrete, too.
..... Click the link for more information.

This unexplained relation was discovered by Yoshio Koide in 1981, and relates the masses of the three leptons so well that it predicted the mass of the tau lepton.

Let

It is clear that from the definition.
..... Click the link for more information.

Flavour in particle physics

 

..... Click the link for more information.

Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvo whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor.
..... Click the link for more information.

Standard Model of particle physics is a theory which describes three of the four known fundamental interactions between the elementary particles that make up all matter. It is a quantum field theory developed between 1970 and 1973 which is consistent with both quantum mechanics and
..... Click the link for more information.

A chiral anomaly is the anomalous nonconservation of a chiral current. In some theories of fermions with a chiral symmetry the quantization may lead to the breaking of this (global) chiral symmetry. In that case, the charge associated with the chiral symmetry is not conserved.
..... Click the link for more information.

In particle physics, gauge bosons are bosonic particles which act as carriers of the fundamental forces of Nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual
..... Click the link for more information.

Given an assembly of elements, the number of which decreases ultimately to zero, the lifetime (also called the mean lifetime) is a certain number that characterizes the rate of reduction ("decay") of the assembly.
..... Click the link for more information.

SLC may refer to, among other things:

• St Laurence's College, Brisbane Australia
• Salt Lake City, Utah, USA
• Salt Lake City International Airport (IATA Airport Code)
• SLC Punk!

..... Click the link for more information.

LEP tunnel at CERN, now being filled with magnets for the LHC]] The Large Electron-Positron Collider (LEP) was one of the largest particle accelerators ever made. It was built at CERN, a multi-national center for research in nuclear and particle physics.
..... Click the link for more information.