# Lepton

leptonsantileptondileptonlepton universality leptons and anti-leptonsanti-leptonsantileptonsleptaleptonicleptonic sector
In particle physics, a lepton is an elementary particle of half-integer spin (spin 1⁄2) that does not undergo strong interactions.wikipedia
294 Related Articles

### Elementary particle

elementary particlesparticleparticles
In particle physics, a lepton is an elementary particle of half-integer spin (spin 1⁄2) that does not undergo strong interactions.
Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are "matter particles" and "antimatter particles", as well as the fundamental bosons (gauge bosons and the Higgs boson), which generally are "force particles" that mediate interactions among fermions.

### Electron

electronse − electron mass
Two main classes of leptons exist, charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). The first-generation leptons, also called electronic leptons, comprise the electron and the electron neutrino ; the second are the muonic leptons, comprising the muon and the muon neutrino ; and the third are the tauonic leptons, comprising the tau and the tau neutrino.
Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure.

### Neutrino

neutrinosantineutrinoneutrino mass
Two main classes of leptons exist, charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos).
The weak force has a very short range, the gravitational interaction is extremely weak, and neutrinos, as leptons, do not participate in the strong interaction.

### Flavour (particle physics)

flavorflavourflavors
There are six types of leptons, known as flavours, grouped in three generations.
The Standard Model counts six flavours of quarks and six flavours of leptons.

### Spin (physics)

spinnuclear spinspins
In particle physics, a lepton is an elementary particle of half-integer spin (spin 1⁄2) that does not undergo strong interactions.

### Generation (particle physics)

generationgenerationssecond generation
There are six types of leptons, known as flavours, grouped in three generations.
Each generation is divided into two types of leptons and two types of quarks.

### Particle physics

high energy physicsparticle physicisthigh-energy physics
In particle physics, a lepton is an elementary particle of half-integer spin (spin 1⁄2) that does not undergo strong interactions. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

### Particle accelerator

particle acceleratorsacceleratoraccelerators
Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
These typically entail particle energies of many GeV, and interactions of the simplest kinds of particles: leptons (e.g. electrons and positrons) and quarks for the matter, or photons and gluons for the field quanta.

### Leon M. Lederman

Leon LedermanLeon Max LedermanLederman, Leon M.
The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.
Leon Max Lederman (July 15, 1922 – October 3, 2018) was an American experimental physicist who received the Wolf Prize in Physics in 1982, along with Martin Lewis Perl, for their research on quarks and leptons, and the Nobel Prize in Physics in 1988, along with Melvin Schwartz and Jack Steinberger, for their research on neutrinos.

### Quark

quarksantiquarkantiquarks
Unlike quarks, however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, the weak interaction, and to electromagnetism, of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos.
Unlike leptons, quarks possess color charge, which causes them to engage in the strong interaction.

### Cosmic ray

Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, leptons, and hadrons, such as electrons, positrons, muons, and pions.

### Universe

physical worldThe Universeuniverses
Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
Ordinary matter is composed of two types of elementary particles: quarks and leptons.

### Exotic atom

muonic atomexoticExotic atoms
Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium.
In a muonic atom (previously called a mu-mesic atom, now known to be a misnomer as muons are not mesons), an electron is replaced by a muon, which, like the electron, is a lepton.

### Melvin Schwartz

Schwartz
The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.
He shared the 1988 Nobel Prize in Physics with Leon M. Lederman and Jack Steinberger for their development of the neutrino beam method and their demonstration of the doublet structure of the leptons through the discovery of the muon neutrino.

### Meson

mesonsmesotronmeson physics
The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, which was classified as a meson at the time.
It was eventually found that the "mu meson" did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the electron, and was eventually classed as a lepton like the electron, rather than a meson.

### Standard Model

standard model of particle physicsThe Standard ModelStandard Model of Physics
The first-generation leptons, also called electronic leptons, comprise the electron and the electron neutrino ; the second are the muonic leptons, comprising the muon and the muon neutrino ; and the third are the tauonic leptons, comprising the tau and the tau neutrino. The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed, apart from the Higgs boson, which has been discovered in 2012.
This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons.

### Sterile neutrino

sterile neutrinosHeavy neutrinoNeutral heavy lepton
Right-handed neutrinos and left-handed anti-neutrinos have no possible interaction with other particles (see sterile neutrinos) and so are not a functional part of the Standard Model, although their exclusion is not a strict requirement; they are sometimes listed in particle tables to emphasize that they would have no active role if included in the model.
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that interact only via gravity and do not interact via any of the fundamental interactions of the Standard Model.

### Weak interaction

weak forceweakweak nuclear force
Unlike quarks, however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, the weak interaction, and to electromagnetism, of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos.
In one type of charged current interaction, a charged lepton (such as an electron or a muon, having a charge of −1) can absorb a boson (a particle with a charge of +1) and be thereby converted into a corresponding neutrino (with a charge of 0), where the type ("flavor") of neutrino (electron, muon or tau) is the same as the type of lepton in the interaction, for example:

### Martin Lewis Perl

Martin PerlMartin L. Perl
The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.
Perl chose to look for answers to these questions in experiments on high-energy charged leptons.

### Neutrino oscillation

neutrino oscillationsoscillationsoscillate
However, it is known from experiments—most prominently from observed neutrino oscillations —that neutrinos do in fact have some very small mass, probably less than 2 eV/c2.
Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton family number ("lepton flavor": electron, muon, or tau) can later be measured to have a different lepton family number.

### Quantum field theory

quantum field theoriesquantum fieldquantum theory
In many quantum field theories, such as quantum electrodynamics and quantum chromodynamics, left- and right-handed fermions are identical.
By combining the earlier theory of Glashow, Salam, and Ward with the idea of spontaneous symmetry breaking, Steven Weinberg wrote down in 1967 a theory describing electroweak interactions between all leptons and the effects of the Higgs boson.

### Jack Steinberger

J. SteinbergerJack H. SteinbergerSteinberger
The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz, and Jack Steinberger, and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.
Among the ALEPH experiment's initial accomplishments was the precise measurement of the number of families of leptons and quarks in the Standard Model through the measurement of the decays of the Z boson.

### Seesaw mechanism

neutrino massessee-saw mechanismseesaw
The currently most favoured extension is the so-called seesaw mechanism, which would explain both why the left-handed neutrinos are so light compared to the corresponding charged leptons, and why we have not yet seen any right-handed neutrinos.
In the theory of grand unification of particle physics, and, in particular, in theories of neutrino masses and neutrino oscillation, the seesaw mechanism is a generic model used to understand the relative sizes of observed neutrino masses, of the order of eV, compared to those of quarks and charged leptons, which are millions of times heavier.

### Lepton number

lepton number conservationelectron number, mu number, tau numberfamily number
The members of each generation's weak isospin doublet are assigned leptonic numbers that are conserved under the Standard Model.
In particle physics, lepton number (historically also called lepton charge) is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction.

### Higgs boson

Higgs fieldHiggs particleGod particle
The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed, apart from the Higgs boson, which has been discovered in 2012.
The Higgs field is pivotal in generating the masses of quarks and charged leptons (through Yukawa coupling) and the W and Z gauge bosons (through the Higgs mechanism).