View source for Lepton

{{short description|Class of elementary particles}}
{{Distinguish|leptin|Lipton}}
{{other uses}}
{{GenerationsOfMatter}}
{{Infobox Particle
| bgcolour        =
| name            = Lepton
| image           = [[File:Beta Negative Decay.svg|200px]]
| caption         = Leptons are involved in several processes such as [[beta decay]].
| num_types       = 6 ([[electron]], [[electron neutrino]], [[muon]], [[muon neutrino]], [[tau (particle)|tau]], [[tau neutrino]])
| composition     = [[Elementary particle]]
| statistics      = [[Fermionic]]
| group           =
| generation      = 1st, 2nd, 3rd
| interaction     = [[Electromagnetism]], [[gravitation]], [[Weak interaction|weak]]
| particle        =
| antiparticle    = Antilepton ({{SubatomicParticle|Antilepton}})
| theorized       =
| discovered      =
| symbol          = {{SubatomicParticle|Lepton}}
| baryon_number   = 0
| mass            =
| decay_time      =
| decay_particle  =
| electric_charge = +1&nbsp;[[elementary charge|''e'']], 0&nbsp;''e'', −1&nbsp;''e''
| color_charge    = No
| spin            = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
| num_spin_states =
}}
{{Special characters}}

In [[particle physics]], a '''lepton''' is an [[elementary particle]] of [[half-integer spin]] ([[Spin (physics)|spin]] {{sfrac|1|2}}) that does not undergo [[strong interaction]]s.<ref>{{cite encyclopedia |title=Lepton (physics) |encyclopedia=[[Encyclopædia Britannica]] |url=https://www.britannica.com/EBchecked/topic/336940/lepton |access-date=29 September 2010}}</ref> Two main classes of leptons exist: [[electric charge|charged]] leptons (also known as the [[electron]]-like leptons or muons), including the [[electron]], [[muon]], and [[tauon]], and neutral leptons, better known as [[neutrino]]s. Charged leptons can combine with other particles to form various [[composite particle]]s such as [[atom]]s and [[positronium]], while neutrinos rarely interact with anything, and are consequently rarely observed.  The best known of all leptons is the [[electron]].

There are six types of leptons, known as ''[[flavour (particle physics)|flavours]]'', grouped in three ''[[Generation (particle physics)|generations]]''.<ref name="HyperphysicsLepton">{{cite web |author=Nave |first=R. |title=Leptons |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/lepton |access-date=29 September 2010 |work=[[HyperPhysics]] |publisher=[[Georgia State University]], Department of Physics and Astronomy}}</ref>  The [[Standard Model|first-generation]] leptons, also called ''electronic leptons'', comprise the [[electron]] ({{SubatomicParticle|Electron-}}) and the [[electron neutrino]] ({{SubatomicParticle|Electron neutrino}}); the second are the ''muonic leptons'', comprising the [[muon]] ({{SubatomicParticle|Muon-}}) and the [[muon neutrino]] ({{SubatomicParticle|Muon neutrino}}); and the third are the ''tauonic leptons'', comprising the [[Tau (particle)|tau]] ({{SubatomicParticle|Tau-}}) and the [[tau neutrino]] ({{SubatomicParticle|Tau neutrino}}). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of [[particle decay]]: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the [[universe]], whereas muons and taus can only be produced in [[high energy physics|high-energy]] collisions (such as those involving [[cosmic ray]]s and those carried out in [[particle accelerator]]s).

Leptons have various [[intrinsic properties]], including [[electric charge]], [[Spin (physics)|spin]], and [[mass]]. Unlike [[quark]]s, however, leptons are not subject to the [[strong interaction]], but they are subject to the other three [[fundamental interaction]]s: [[gravitation]], the [[weak interaction]], and to [[electromagnetism]], of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos.

For every lepton flavor, there is a corresponding type of [[antiparticle]], known as an antilepton, that differs from the lepton only in that some of its properties have [[charge conjugation|equal magnitude but opposite sign]]. According to certain theories, neutrinos may be [[Majorana fermion|their own antiparticle]]. It is not currently known whether this is the case.

The first charged lepton, the electron, was theorized in the mid-19th century by several scientists<ref name="farrar">{{cite journal |author=Farrar |first=W. V. |year=1969 |title=Richard Laming and the Coal-Gas Industry, with His Views on the Structure of Matter |journal=[[Annals of Science]] |volume=25 |issue=3 |pages=243–254 |doi=10.1080/00033796900200141}}</ref><ref name="arabatzis">{{cite book |author=Arabatzis |first=T. |url=https://books.google.com/books?id=rZHT-chpLmAC&pg=PA70 |title=Representing Electrons: A Biographical Approach to Theoretical Entities |publisher=[[University of Chicago Press]] |year=2006 |isbn=978-0-226-02421-9 |pages=70–74}}</ref><ref name="buchwald1">{{cite book |last1=Buchwald |first1=J. Z. |url=https://books.google.com/books?id=1yqqhlIdCOoC&pg=PA195 |title=Histories of the Electron: The Birth of Microphysics |last2=Warwick |first2=A. |publisher=[[MIT Press]] |year=2001 |isbn=978-0-262-52424-7 |pages=195–203}}</ref> and was discovered in 1897 by [[J. J. Thomson]].<ref name="thomson">{{cite journal |author=Thomson |first=J. J. |author-link=J. J. Thomson |year=1897 |title=Cathode Rays |url=http://web.lemoyne.edu/~GIUNTA/thomson1897.html |journal=[[Philosophical Magazine]] |volume=44 |issue=269 |page=293 |doi=10.1080/14786449708621070}}</ref> 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.<ref>{{harvnb|Neddermeyer|Anderson|1937}}</ref> After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particles to be proposed.<ref name=LAS /> The first neutrino, the electron neutrino, was proposed by [[Wolfgang Pauli]] in 1930 to explain certain characteristics of [[beta decay]].<ref name=LAS /> It was first observed in the [[Cowan–Reines neutrino experiment]] conducted by [[Clyde Cowan]] and [[Frederick Reines]] in 1956.<ref name=LAS>
{{cite journal
 |year=1997
 |title=The Reines-Cowan Experiments: Detecting the Poltergeist
 |journal=[[Los Alamos Science]]
 |volume=25 |page=3
 |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |access-date=2010-02-10
}}</ref><ref>
{{cite journal
 |first1=F. |last1=Reines
 | first2=C.L. Jr. |last2=Cowan
 |year=1956
 |title=The Neutrino
 |journal=[[Nature (journal)|Nature]]
 |volume=178 |issue=4531 |page=446
 |doi=10.1038/178446a0
 |bibcode = 1956Natur.178..446R
 |s2cid=4293703
}}</ref> The muon neutrino was discovered in 1962 by [[Leon M. Lederman]], [[Melvin Schwartz]], and [[Jack Steinberger]],<ref name="slac.stanford.edu">{{cite journal |author=Danby |first=G. |display-authors=etal |year=1962 |title=Observation of high-energy neutrino reactions and the existence of two kinds of neutrinos |url=http://inspirehep.net/search?p=PRLTA,9,36 |journal=[[Physical Review Letters]] |volume=9 |issue=1 |page=36 |bibcode=1962PhRvL...9...36D |doi=10.1103/PhysRevLett.9.36}}</ref> 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]].<ref>{{harvnb|Perl|1975}}</ref> The [[tau neutrino]] remained elusive until July 2000, when the [[DONUT]] collaboration from [[Fermilab]] announced its discovery.<ref name=tauonpress>
{{cite press release
 |date=20 July 2000
 |title=Physicists find first direct evidence for tau neutrino at Fermilab
 |url=http://www.fnal.gov/pub/presspass/press_releases/donut.html
 |publisher=[[Fermilab]]
}}</ref><ref name=tauonpaper>{{harvnb|Kodama|2001}}</ref>

Leptons are an important part of the [[Standard Model]]. Electrons are one of the components of [[atom]]s, alongside [[proton]]s and [[neutron]]s. [[Exotic atom]]s with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as [[positronium]].

== Etymology ==
The name ''lepton'' comes from the [[Ancient Greek|Greek]] {{lang|el|λεπτός}} ''leptós'', "fine, small, thin" ([[Grammatical gender|neuter]] nominative/accusative singular form: λεπτόν ''leptón'');<ref>[http://www.etymonline.com/index.php?term=lepton "lepton"]. ''[[Online Etymology Dictionary]]''.</ref><ref>{{LSJ|lepto/s|λεπτός|ref}}.</ref> the earliest attested form of the word is the [[Mycenaean Greek]] {{lang|gmy|𐀩𐀡𐀵}}, ''re-po-to'', written in [[Linear B]] syllabic script.<ref>Found on the [[Knossos|KN]] L 693 and [[Pylos|PY]] Un 1322 tablets. {{cite web|title=The Linear B word re-po-to|publisher=Palaeolexicon. Word study tool of ancient languages|url=http://www.palaeolexicon.com/ShowWord.aspx?Id=16917}} {{cite web|url=http://minoan.deaditerranean.com/resources/linear-b-sign-groups/re/re-po-to/|title=re-po-to|work=Minoan Linear A & Mycenaean Linear B|last=Raymoure|first=K.A.|publisher=Deaditerranean|access-date=2014-03-22|archive-date=2016-01-16|archive-url=https://web.archive.org/web/20160116072258/http://minoan.deaditerranean.com/resources/linear-b-sign-groups/re/re-po-to/|url-status=dead}} {{cite web|title=KN 693 L (103)|url=https://www2.hf.uio.no/damos/Index/item/chosen_item_id/630}} {{cite web|title=PY 1322 Un + fr. (Cii)|website=DĀMOS: Database of Mycenaean at Oslo|url=https://www2.hf.uio.no/damos/Index/item/chosen_item_id/5021|publisher=[[University of Oslo]]}}</ref> ''Lepton'' was first used by physicist [[Léon Rosenfeld]] in 1948:<ref>{{harvnb|Rosenfeld|1948}}</ref>
<blockquote>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.</blockquote>

Rosenfeld chose the name as the common name for electrons and (then hypothesized) neutrinos. Additionally, the muon, initially classified as a meson, was reclassified as a lepton in the 1950s. The masses of those particles are small compared to nucleons—the mass of an electron ({{val|0.511|ul=MeV/c2}})<ref name="Electron">{{harvnb|Amsler|2008}}: [http://pdg.lbl.gov/2008/listings/s003.pdf Particle listings—{{SubatomicParticle|Electron-}}]</ref> and the mass of a muon (with a value of {{val|105.7|u=MeV/c2}})<ref name="Muon">{{harvnb|Amsler|2008}}: [http://pdg.lbl.gov/2008/listings/s004.pdf Particle listings—{{SubatomicParticle|Muon-}}]</ref> are fractions of the mass of the "heavy" proton ({{val|938.3|u=MeV/c2}}), and the mass of a neutrino is nearly zero.<ref name="Proton">{{harvnb|Amsler|2008}}: [http://pdg.lbl.gov/2008/listings/s016.pdf Particle listings—{{SubatomicParticle|Proton+}}]</ref> However, the mass of the tau (discovered in the mid-1970s) ({{val|1777|u=MeV/c2}})<ref name="Tauon">{{harvnb|Amsler|2008}}: [http://pdg.lbl.gov/2008/listings/s035.pdf Particle listings—{{SubatomicParticle|Tau-}}]</ref> is nearly twice that of the proton and {{physconst|mtau/me|round=0}} times that of the electron.

== History ==
{{See also|Electron#Discovery|Muon#History|Tau (particle)#History}}
[[File:Feynman diagram of muon to electron decay.svg|left|200px|thumb|A muon transmutes into a [[muon neutrino]] by emitting a [[W boson|{{SubatomicParticle|W boson-}} boson]]. The {{SubatomicParticle|W boson-}} boson subsequently decays into an [[electron]] and an [[electron antineutrino]].]]
{| class="wikitable"  style="float:right; font-size:90%; margin:.5em 0 .5em 1em;"
|+Lepton nomenclature
|-
!Particle name !! Antiparticle name
|-
|electron || antielectron<br />positron
|-
|electron neutrino || electron antineutrino
|-
|muon<br />mu lepton<br />mu || antimuon<br />antimu lepton<br />antimu
|-
|muon neutrino<br />muonic neutrino<br />mu neutrino || muon antineutrino<br />muonic antineutrino<br />mu antineutrino
|-
|tauon<br />tau lepton<br />tau || antitauon<br />antitau lepton<br />antitau
|-
|tauon neutrino<br />tauonic neutrino <br />tau neutrino|| tauon antineutrino<br />tauonic antineutrino<br />tau antineutrino
|}

The first lepton identified was the electron, discovered by [[J.J. Thomson]] and his team of British physicists in 1897.<ref>{{harvnb|Weinberg|2003}}</ref><ref>{{harvnb|Wilson|1997}}</ref> Then in 1930, [[Wolfgang Pauli]] postulated the [[electron neutrino]] to preserve [[conservation of energy]], [[conservation of momentum]], and [[conservation of angular momentum]] in [[beta decay]].<ref>{{harvnb|Riesselmann|2007}}</ref> Pauli theorized that an undetected particle was carrying away the difference between the [[energy]], [[momentum]], and [[angular momentum]] of the initial and observed final particles. The electron neutrino was simply called the neutrino, as it was not yet known that neutrinos came in different flavours (or different "generations").

Nearly 40 years after the discovery of the electron, the [[muon]] was discovered by [[Carl D. Anderson]] in 1936.  Due to its mass, it was initially categorized as a [[meson]] rather than a lepton.<ref>{{harvnb|Neddermeyer|Anderson|1937}}</ref>  It later became clear that the muon was much more similar to the electron than to mesons, as muons do not undergo the [[strong interaction]], and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles—the leptons. In 1962, [[Leon M. Lederman]], [[Melvin Schwartz]], and [[Jack Steinberger]] showed that more than one type of neutrino exists by first detecting interactions of the [[muon]] neutrino, which earned them the [[Nobel Prize in Physics|1988 Nobel Prize]], although by then the different flavours of neutrino had already been theorized.<ref>{{harvnb|Anicin|2005}}</ref>

The [[tau (particle)|tau]] was first detected in a series of experiments between 1974 and 1977 by [[Martin Lewis Perl]] with his colleagues at the [[SLAC]] [[Lawrence Berkeley National Laboratory|LBL group]].<ref>{{harvnb|Perl|1975}}</ref> Like the electron and the muon, it too was expected to have an associated neutrino. The first evidence for tau neutrinos came from the observation of "missing" energy and momentum in tau decay, analogous to the "missing" energy and momentum in beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the [[DONUT]] collaboration at [[Fermilab]], making it the second-to-latest particle of the [[Standard Model]] to have been directly observed,<ref name="obs">{{harvnb|Kodama|2001}}</ref> with [[Higgs boson]] being discovered in 2012.

Although all present data is consistent with three generations of leptons, some particle physicists are searching for a fourth generation. The current lower limit on the mass of such a fourth charged lepton is {{val|100.8|ul=GeV/c2}},<ref>{{harvnb|Amsler|2008}} [http://pdg.lbl.gov/2008/listings/s025.pdf Heavy Charged Leptons Searches]</ref> while its associated neutrino would have a mass of at least {{val|45.0|ul=GeV/c2}}.<ref>{{harvnb|Amsler|2008}} [http://pdg.lbl.gov/2008/listings/s077.pdf Searches for Heavy Neutral Leptons]</ref>

== Properties ==

=== Spin and chirality ===
[[File:Right left helicity.svg|thumb|200px|right|Left-handed and right-handed helicities]]

Leptons are [[Spin (physics)|spin]]&nbsp;{{sfrac|1|2}} particles. The [[spin-statistics theorem]] thus implies that they are [[fermion]]s and thus that they are subject to the [[Pauli exclusion principle]]: no two leptons of the same species can be in the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down.

A closely related property is [[chirality (physics)|chirality]], which in turn is closely related to a more easily visualized property called [[helicity (particle physics)|helicity]]. The helicity of a particle is the direction of its spin relative to its [[momentum]]; particles with spin in the same direction as their momentum are called ''right-handed'' and they are otherwise called ''left-handed''. When a particle is massless, the direction of its momentum relative to its spin is the same in every reference frame, whereas for massive particles it is possible to 'overtake' the particle by choosing a faster-moving [[Lorentz transformation|reference frame]]; in the faster frame, the helicity is reversed. Chirality is a technical property, defined through transformation behaviour under the [[Poincaré group]], that does not change with reference frame. It is contrived to agree with helicity for massless particles, and is still well defined for particles with mass.

In many [[quantum field theories]], such as [[quantum electrodynamics]] and [[quantum chromodynamics]], left- and right-handed fermions are identical. However, the Standard Model's [[weak interaction]] treats left-handed and right-handed fermions differently: only left-handed fermions (and right-handed anti-fermions) participate in the weak interaction. This is an example of [[parity violation]] explicitly written into the model. In the literature, left-handed fields are often denoted by a capital <small>L</small> subscript (e.g. the normal electron e{{su|b=L|p=−}}) and right-handed fields are denoted by a capital <small>R</small> subscript (e.g. a positron e{{su|b=R|p=+}}).

Right-handed neutrinos and left-handed anti-neutrinos have no possible interaction with other particles (see ''[[Sterile neutrino]]'') 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. Even though electrically charged right-handed particles (electron, muon, or tau) do not engage in the weak interaction specifically, they can still interact electrically, and hence still participate in the [[Electroweak unification|combined electroweak force]], although with different strengths ([[Weak hypercharge|{{math|''Y''}}<sub>W</sub>]]).
{{clear}}

=== Electromagnetic interaction ===
[[File:Lepton-interaction-vertex-eeg.svg|thumb|right|Lepton–photon interaction]]
One of the most prominent properties of leptons is their [[electric charge]], {{mvar|Q}}. The electric charge determines the strength of their [[electromagnetic interaction]]s. It determines the strength of the [[electric field]] generated by the particle (see [[Coulomb's law]]) and how strongly the particle reacts to an external electric or magnetic field (see [[Lorentz force]]). Each generation contains one lepton with {{nowrap|1={{math|''Q''}} = −1 [[elementary charge|''e'']]}} and one lepton with zero electric charge. The lepton with electric charge is commonly simply referred to as a ''charged lepton'' while a neutral lepton is called a ''neutrino''. For example, the first generation consists of the electron {{SubatomicParticle|electron}} with a negative electric charge and the electrically neutral electron neutrino {{SubatomicParticle|electron neutrino}}.

In the language of quantum field theory, the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with the quantum of the electromagnetic field, the [[photon]]. The [[Feynman diagram]] of the electron–photon interaction is shown on the right.

Because leptons possess an intrinsic rotation in the form of their spin, charged leptons generate a magnetic field. The size of their [[magnetic dipole moment]] {{mvar|μ}} is given by
: <math>\mu = g\, \frac{\; Q \hbar \;}{4 m} \ ,</math>
where {{mvar|m}} is the mass of the lepton and {{mvar|g}} is the so-called [[g-factor (physics)|"{{mvar|g}}&nbsp;factor"]] for the lepton. First-order quantum mechanical approximation predicts that the {{mvar|g}}&nbsp;factor is 2 for all leptons. However, higher-order quantum effects caused by loops in Feynman diagrams introduce corrections to this value. These corrections, referred to as the ''[[anomalous magnetic dipole moment]]'', are very sensitive to the details of a quantum field theory model, and thus provide the opportunity for precision tests of the Standard Model. The theoretical and measured values for the ''electron'' anomalous magnetic dipole moment are within agreement within eight significant figures.<ref>{{harvnb|Peskin|Schroeder|1995|p=197}}</ref> The results for the ''muon'', however, [[Muon g-2|are problematic]], hinting at a small, persistent discrepancy between the Standard Model and experiment.

=== Weak interaction ===
{|  style="float:right; margin: 0em 0em 1em 1em "
|-
|
{|class=wikitable style="font-size:90%;"
|[[File:Lepton-interaction-vertex-evW.svg|frameless|upright=.7]]
|[[File:Lepton-interaction-vertex-pvW.svg|frameless|upright=.7]]
|[[File:Lepton-interaction-vertex-eeZ.svg|frameless|upright=.7]]
|-
|colspan=3|{{center|The [[weak interaction]]s of the first generation leptons.}}
|}
|}
In the Standard Model, the left-handed charged lepton and the left-handed neutrino are arranged in [[Doublet (physics)|doublet]] {{nowrap||(ν{{sub|e {{sc|L}}}}, e{{small|{{su|p=−|b={{sc|L}} }} }})}} that transforms in the [[spinor]] representation ({{nowrap|{{math|''T''}} {{=}} {{sfrac| 1 |2}}}}) of the [[weak isospin]] [[SU(2)]] gauge symmetry. This means that these particles are eigenstates of the isospin projection {{math|''T''}}{{sub|3}} with eigenvalues {{sfrac|+| 1 |2}} and {{sfrac|−| 1 |2}} respectively. In the meantime, the right-handed charged lepton transforms as a weak isospin scalar ({{nowrap|{{math|''T''}} {{=}} 0}}) and thus does not participate in the [[weak interaction]], while there is no evidence that a right-handed neutrino exists at all.

The [[Higgs mechanism]] recombines the gauge fields of the weak isospin SU(2) and the [[weak hypercharge]] U(1) symmetries to three massive vector bosons ({{SubatomicParticle|W boson+}}, {{SubatomicParticle|W boson-}}, {{SubatomicParticle|Z boson0}}) mediating the [[weak interaction]], and one massless vector boson, the photon (&gamma;), responsible for the electromagnetic interaction. The electric charge {{mvar|Q}} can be calculated from the isospin projection {{mvar|T}}{{sub|3}} and weak hypercharge {{math|''Y''}}{{sub|W}} through the [[Gell-Mann–Nishijima formula]],
: {{nowrap|{{math|''Q'' {{=}} ''T''{{sub|3}} + {{sfrac| 1 |2}} ''Y''{{sub|W}}}}.}}

To recover the observed electric charges for all particles, the left-handed weak isospin doublet {{nowrap|(ν{{sub|eL}}, e{{su|p=−|b=L}})}} must thus have {{nowrap|''Y''{{sub|W}} {{=}} −1}}, while the right-handed isospin scalar {{nowrap|e{{su|p=−|b=R}}}} must have {{nowrap|{{math|''Y''}}{{sub|W}} {{=}} −2}}. The interaction of the leptons with the massive weak interaction vector bosons is shown in the figure on the right.

=== Mass ===
In the [[Standard Model]], each lepton starts out with no intrinsic mass. The charged leptons (i.e. the electron, muon, and tau) obtain an effective mass through interaction with the [[Higgs field]], but the neutrinos remain massless. For technical reasons, the masslessness of the neutrinos implies that there is no mixing of the different generations of charged leptons as [[CKM matrix|there is for quarks]]. The zero mass of neutrino is in close agreement with current direct experimental observations of the mass.<ref>{{harvnb|Peskin|Schroeder|1995|p=27}}</ref>

However, it is known from indirect experiments—most prominently from observed [[neutrino oscillation]]s<ref>{{harvnb|Fukuda|1998}}</ref>—that neutrinos have to have a nonzero mass, probably less than {{val|2|ul=eV/c2}}.<ref name="Neutrino" /> This implies the existence of physics [[beyond the Standard Model]]. 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.

=== Lepton flavor quantum numbers ===
{{Main|Lepton number}}
The members of each generation's [[weak isospin]] [[doublet (physics)|doublet]] are assigned [[lepton number|leptonic numbers]] that are conserved under the Standard Model.<ref name="MartinShaw">{{harvnb|Martin|Shaw|1992}}</ref> Electrons and electron neutrinos have an ''electronic number'' of {{nowrap|{{math|''L''}}<sub>e</sub> {{=}} 1}}, while muons and muon neutrinos have a ''muonic number'' of {{nowrap|{{math|''L''}}<sub>μ</sub> {{=}} 1}}, while tau particles and tau neutrinos have a ''tauonic number'' of {{nowrap|{{math|''L''<sub>τ</sub>}} {{=}} 1}}. The antileptons have their respective generation's leptonic numbers of&nbsp;−1.

Conservation of the leptonic numbers means that the number of leptons of the same type remains the same, when particles interact. This implies that leptons and antileptons must be created in pairs of a single generation. For example, the following processes are allowed under conservation of leptonic numbers:
[[File:Lepton isodoublets fixed.png|thumb|right|Each generation forms a [[weak isospin]] [[Doublet (physics)|doublet]].]]
: {{SubatomicParticle|link=yes|Photon}} &nbsp; &nbsp;→&nbsp;&nbsp; {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Antielectron}},
: {{SubatomicParticle|link=yes|Z boson0}} &nbsp;→&nbsp;&nbsp; {{SubatomicParticle|link=yes|Tau}} + {{SubatomicParticle|link=yes|Antitau}},
but none of these:
: {{SubatomicParticle|link=yes|Photon}} &nbsp; &nbsp; &nbsp;→&nbsp;&nbsp; {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Antimuon}},
: {{SubatomicParticle|link=yes|W boson-}} &nbsp;→&nbsp;&nbsp; {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Tau neutrino}},
: {{SubatomicParticle|link=yes|Z boson0}} &nbsp;&nbsp; →&nbsp;&nbsp; {{SubatomicParticle|link=yes|Muon}} + {{SubatomicParticle|link=yes|Antitau}}.

However, [[neutrino oscillation]]s are known to violate the conservation of the individual leptonic numbers. Such a violation is considered to be smoking gun evidence for [[physics beyond the Standard Model]]. A much stronger conservation law is the conservation of the total number of leptons ({{mvar|L}} {{small|with ''no'' subscript}}), conserved even in the case of neutrino oscillations, but even it is still violated by a tiny amount by the [[chiral anomaly]].

== Universality ==
{{See also|LHCb experiment#Lepton flavour universality}}

The coupling of leptons to all types of [[gauge boson]] are flavour-independent: The interaction between leptons and a gauge boson measures the same for each lepton.<ref name="MartinShaw" /> This property is called '''lepton universality'''<!--boldface per WP:R#PLA--> and has been tested in measurements of the [[muon]] and [[tau lepton|tau]] [[mean lifetime|lifetimes]] and of {{SubatomicParticle|link=yes|Z boson}} boson partial [[decay width]]s, particularly at the [[Stanford Linear Collider]] (SLC) and [[Large Electron–Positron Collider]] (LEP) experiments.<ref name="Cumalat1993">
{{cite book
 |last=Cumalat |first=J.P.
 |year=1993
 |title=Physics in Collision
 |volume=12
 |publisher=Atlantica Séguier Frontières
 |isbn=978-2-86332-129-4
}}</ref>{{rp|pages=241–243}}<ref name="Fraser1998">
{{cite book
 |last=Fraser |first=G.
 |date=1 January 1998
 |title=The Particle Century
 |publisher=CRC Press
 |isbn=978-1-4200-5033-2
 |url=https://books.google.com/books?id=pgasJfizmocC
 |via=Google Books
}}</ref>{{rp|page=138}}

The decay rate (<small><math>\Gamma</math></small>) of muons through the process {{nowrap|{{SubatomicParticle|link=yes|muon-}} → {{SubatomicParticle|link=yes|electron-}} + {{SubatomicParticle|link=yes|electron antineutrino}} + {{SubatomicParticle|link=yes|muon neutrino}}}} is approximately given by an expression of the form (see [[muon decay]] for more details)<ref name="MartinShaw" />
: <math>\Gamma \left ( \mu^- \rarr e^- + \bar{\nu_e} +\nu_\mu \right ) \approx K_2\,  G_\text{F}^2\,  m_\mu^5 ~,</math>
where {{mvar|K}}{{sub|2}} is some constant, and {{mvar|G}}{{sub|F}} is the [[Fermi coupling constant]]. The decay rate of tau particles through the process {{nowrap|{{SubatomicParticle|link=yes|tau-}} → {{SubatomicParticle|link=yes|electron-}} + {{SubatomicParticle|link=yes|electron antineutrino}} + {{SubatomicParticle|link=yes|tau neutrino}}}} is given by an expression of the same form<ref name="MartinShaw" />
: <math>\Gamma \left ( \tau^- \rarr e^- + \bar{\nu_e} +\nu_\tau \right ) \approx K_3\, G_\text{F}^2\, m_\tau^5 ~,</math>
where {{mvar|K}}{{sub|3}} is some other constant. Muon–tauon universality implies that {{nowrap|{{mvar|K}}{{sub|2}} ≈ {{mvar|K}}{{sub|3}}}}. On the other hand, electron–muon universality implies<ref name="MartinShaw" />
: <math>0.9726 \times \Gamma \left( \tau^- \rarr e^- + \bar{\nu_e} +\nu_\tau \right) = \Gamma \left( \tau^- \rarr \mu^- + \bar{\nu_\mu} +\nu_\tau \right) ~.</math>

The [[branching ratio]]s for the electronic mode (17.82%) and muonic (17.39%) mode of tau decay are not equal due to the mass difference of the final state leptons.<ref name="Tauon" />

Universality also accounts for the ratio of muon and tau lifetimes. The lifetime <math>\Tau_\ell</math> of a lepton <math>\ell</math> (with <math>\ell</math> = "{{math|μ}}" or "{{math|τ}}") is related to the decay rate by<ref name="MartinShaw" />
: <math>\Tau_\ell = \frac{\; \mathcal{B} \left( \ell^- \rarr e^- + \bar{\nu_e} +\nu_\ell \right) \; }{ \Gamma \left( \ell^- \rarr e^- + \bar{\nu_e} +\nu_\ell \right)}\,</math>,
where <math>\; \mathcal{B} (x \rarr y) \;</math> denotes the branching ratios and <math>\;\Gamma(x \rarr y) \;</math> denotes the [[decay width|resonance width]] of the process <math>\; x \rarr y ~,</math> with {{mvar|x}} and {{mvar|y}} replaced by two different particles from "{{math|e}}" or "{{math|μ}}" or "{{math|τ}}".

The ratio of tau and muon lifetime is thus given by<ref name="MartinShaw" />
: <math>\frac{\, \Tau_\tau \,}{\Tau_\mu} = \frac{\; \mathcal{B} \left( \tau^- \rarr e^- + \bar{\nu_e} +\nu_\tau \right) \;}{ \mathcal{B} \left( \mu^- \rarr e^- + \bar{\nu_e} +\nu_\mu \right) }\, \left(\frac{m_\mu}{m_\tau}\right)^5 ~.</math>

Using values from the 2008 ''[[Review of Particle Physics]]'' for the branching ratios of the muon<ref name="Muon" /> and tau<ref name="Tauon" /> yields a lifetime ratio of ~&nbsp;{{val|1.29|e=-7}}, comparable to the measured lifetime ratio of ~&nbsp;{{val|1.32|e=-7}}. The difference is due to {{mvar|K}}{{sub|2}} and {{mvar|K}}{{sub|3}} not ''actually'' being constants: They depend slightly on the mass of leptons involved.

Recent tests of lepton universality in [[B meson|{{Subatomic particle|B}} meson]] decays, performed by the [[LHCb]], [[BaBar experiment|BaBar]], and [[Belle experiment|Belle]] experiments, have shown consistent deviations from the Standard Model predictions. However the combined statistical and systematic significance is not yet high enough to claim an observation of [[new physics]].<ref name=Ciezarek2017>
{{cite journal
 |vauthors=Ciezarek G, Franco Sevilla M, Hamilton B, Kowalewski R, Kuhr T, Lüth V, Sato Y
 |year=2017
 |title=A challenge to lepton universality in B&nbsp;meson decays
 |journal=Nature
 |volume=546 |issue=7657 |pages=227–233
 |doi=10.1038/nature22346|pmid=28593973 |arxiv=1703.01766
 |bibcode=2017Natur.546..227C
 |s2cid=4385808
}}</ref>

In July 2021 results on lepton flavour universality have been published testing W decays, previous measurements by the LEP had given a slight imbalance but the new measurement by the [[ATLAS experiment|ATLAS]] collaboration have twice the precision and give a ratio of <math>R_W^{\tau/\mu}=\mathcal{B} (W\rarr \tau\nu_\tau)/\mathcal{B}( W\rarr \mu\nu_\mu)=0.992\pm0.013</math>, which agrees with the standard-model prediction of unity.<ref>{{Cite journal|last1=Aad|first1=G.|last2=Abbott|first2=B.|last3=Abbott|first3=D. C.|last4=Abud|first4=A. Abed|last5=Abeling|first5=K.|last6=Abhayasinghe|first6=D. K.|last7=Abidi|first7=S. H.|last8=AbouZeid|first8=O. S.|last9=Abraham|first9=N. L.|last10=Abramowicz|first10=H.|last11=Abreu|first11=H.|date=5 July 2021|title=Test of the universality of τ and μ lepton couplings in W-boson decays with the ATLAS detector|url=https://www.nature.com/articles/s41567-021-01236-w|journal=Nature Physics|language=en|volume=17|issue=7|pages=813–818|doi=10.1038/s41567-021-01236-w|issn=1745-2481|arxiv=2007.14040|bibcode=2021NatPh..17..813A|s2cid=220831347}}</ref><ref>{{Cite journal|last=Middleton|first=Christine|date=2021-07-09|title=ATLAS measurement supports lepton universality|journal=Physics Today|volume=2021|issue=1|pages=0709a|url=https://physicstoday.scitation.org/do/10.1063/PT.6.1.20210709a/abs/|language=EN|doi=10.1063/PT.6.1.20210709a|bibcode=2021PhT..2021a.709.|s2cid=242888088}}</ref><ref>{{Cite web|title=New ATLAS result addresses long-standing tension in the Standard Model|url=https://atlas.cern/updates/briefing/addressing-long-standing-tension-standard-model|access-date=2021-07-12|website=ATLAS|language=en}}</ref> In 2024 a preprint by the ATLAS collaboration has published a new value of <math>R_W^{\mu/e}=\mathcal{B} ( W\rarr \mu\nu_\mu)/\mathcal{B}( W\rarr e\nu_e)=0.9995\pm0.0045</math> the most precise ratio so far testing the lepton flavour universality.<ref>{{cite arXiv |last=ATLAS Collaboration |title=Precise test of lepton flavour universality in ''W''-boson decays into muons and electrons in ''pp'' collisions at &radic;''s'' = 13 TeV with the ATLAS detector |date=2024-03-04 | class=hep-ex |eprint=2403.02133}}</ref><ref>{{Cite web |date=2024-09-18 |title=The delicate balance of lepton flavours |url=https://home.cern/news/news/physics/delicate-balance-lepton-flavours |access-date=2024-09-20 |website=CERN |language=en}}</ref>

== Table of leptons ==
: {| class="wikitable" style="text-align:center"
|+Properties of leptons
|-
!rowspan=2| [[Spin (physics)|Spin<br />{{mvar|J}}]] <small>{{bracket|[[reduced Planck constant|''ħ'']]}}</small>
!rowspan=2| Particle or <br /> antiparticle name
!rowspan=2| Symbol
!rowspan=2| [[Electric charge|Charge<br />{{mvar|Q}}]] <small>{{bracket|[[elementary charge|''e'']]}}</small>
!colspan=3| Lepton flavor number
!rowspan=2| Mass<br /><small>{{bracket|[[MeV]]/''c''<sup>2</sup>}}</small>
!rowspan=2| Lifetime<br /><small>{{bracket|[[second|s]]}}</small>
|-
! {{mvar|L}}{{sub|e}}
! {{mvar|L}}{{sub|μ}}
! {{mvar|L}}{{sub|τ}}
<!-- ! Common decay -->
|-
|rowspan=13| {{sfrac| 1 |2}}
|style="text-align:left"| [[electron]]<ref name="Electron" />
| {{SubatomicParticle|link=yes|Electron}}
| −1
| +1
|rowspan=2| 0
|rowspan=2| 0
|rowspan=2 style="text-align:right"| {{val|0.510998910|(13)}}
|rowspan=2| stable
<!-- |rowspan=2| stable -->
|-
|style="text-align:left"| [[positron]]<ref name="Electron" />
| {{SubatomicParticle|link=yes|Positron}}
| +1
| −1
|-
<!-- |rowspan=2| {{frac| 1 |2}} -->
|style="text-align:left"| [[muon]]<ref name="Muon" />
| {{SubatomicParticle|link=yes|Muon}}
| −1
|rowspan=2| 0
| +1
|rowspan=2| 0
|rowspan=2 style="text-align:right"| {{val|105.6583668|(38)}}
|rowspan=2 style="text-align:right"| {{val|2.197019|(21)|e=−6}}<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<!-- |rowspan=2| {{SubatomicParticle|link=yes|electron-}} + {{SubatomicParticle|link=yes|electron antineutrino}} + {{SubatomicParticle|link=yes|muon neutrino}} -->
|-
<!-- |rowspan=2| {{frac|1|2}} -->
|style="text-align:left"|[[antimuon]]<ref name="Muon" />
| {{SubatomicParticle|link=yes|Antimuon}}
| +1
| −1
|-
|style="text-align:left"| [[Tau (particle)|tau]]<ref name="Tauon" />
| {{SubatomicParticle|link=yes|Tau}}
| −1
|rowspan=2| 0
|rowspan=2| 0
| +1
|rowspan=2 style="text-align:right"| {{val|1776.84|(17)}}
|rowspan=2 style="text-align:right"| {{val|2.906|(10)|e=−13}}
<!-- |rowspan=2| See <small>[http://pdg.lbl.gov/2008/listings/s035.pdf ''τ decay modes'']</small> -->
|-
|style="text-align:left"| [[antitau]]<ref name="Tauon" />
| {{SubatomicParticle|link=yes|Antitau}}
| +1
| −1
|-
!colspan=8|
|-
<!-- |rowspan=2| {{frac|1|2}} -->
|style="text-align:left"| [[electron neutrino]]<ref name="Neutrino">{{harvnb|Amsler|2008}}: [http://pdg.lbl.gov/2008/listings/s066.pdf Particle listings — Neutrino properties]</ref>
| {{SubatomicParticle|link=yes|Electron neutrino}}
| rowspan="6" | 0
| +1
|rowspan=2| 0
|rowspan=2| 0
|rowspan=2 style="text-align:left"| <&nbsp;{{val|0.0000022}}<ref name=PeltoniemiSarkamo2005>
 {{cite web
  |first1=J. |last1=Peltoniemi |first2=J. |last2=Sarkamo |year=2005
  |url=http://cupp.oulu.fi/neutrino/nd-mass.html
  |title=Laboratory measurements and limits for neutrino properties
  |work=The Ultimate Neutrino Page
  |access-date=2008-11-07
  |df=dmy-all
 }}</ref>
|rowspan=2| unknown
<!-- |rowspan=2| -->
|-
<!-- |rowspan=2| {{frac|1|2}} -->
|style="text-align:left"| [[electron antineutrino]]
| {{SubatomicParticle|link=yes|Electron antineutrino}}
| −1
|-
|style="text-align:left"| [[muon neutrino]]<ref name="Neutrino" />
| {{SubatomicParticle|link=yes|Muon neutrino}}
|rowspan=2| 0
| +1
|rowspan=2| 0
|rowspan=2 style="text-align:left"| <&nbsp;0.17<ref name=PeltoniemiSarkamo2005 />
|rowspan=2| unknown
<!-- |rowspan=2| -->
|-
|style="text-align:left"| [[muon antineutrino]]<ref name="Neutrino" />
| {{math| {{SubatomicParticle|link=yes|Muon antineutrino}} }}
| −1
|-
<!-- |rowspan=2| {{frac|1|2}} -->
|style="text-align:left"| [[tau neutrino]]<ref name="Neutrino" />
| {{math| {{SubatomicParticle|link=yes|Tau neutrino}} }}
|rowspan=2| 0
|rowspan=2| 0
| +1
|rowspan=2 style="text-align:left"| <&nbsp;15.5<ref name=PeltoniemiSarkamo2005 />
|rowspan=2| unknown
<!-- |rowspan=2| -->
|-
|style="text-align:left"| [[tau antineutrino]]<ref name="Neutrino" />
| {{math| {{SubatomicParticle|link=yes|Tau antineutrino}} }}
| −1
|-
|}

== See also ==
* [[Koide formula]]
* [[List of particles]]
* [[Preon]]s – hypothetical particles that were once postulated to be subcomponents of quarks and leptons

== Notes ==
{{notelist}}

== References ==
{{reflist|25em|refs=}}

=== Bibliography ===
* {{cite journal
 |last1=Amsler |first1=C.
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2008
 |title=Review of Particle Physics
 |journal=[[Physics Letters B]]
 |volume=667 |issue=1 |page=1
 |doi=10.1016/j.physletb.2008.07.018|bibcode = 2008PhLB..667....1A
 |url=http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf
 |hdl=1854/LU-685594
 |s2cid=227119789
 |hdl-access=free
}}
* {{cite arXiv
 |last=Anicin |first=I. V.
 |year=2005
 |title=The Neutrino: Its Past, Present and Future
 |eprint=physics/0503172
}}
* {{cite journal
 |last1=Fukuda |first1=Y.
 |display-authors=etal
 |year=1998
 |title=Evidence for Oscillation of Atmospheric Neutrinos
 |journal=Physical Review Letters
 |volume=81 |issue=8 |pages=1562–1567
 |arxiv=hep-ex/9807003
 |doi=10.1103/PhysRevLett.81.1562
 |bibcode = 1998PhRvL..81.1562F
 |s2cid=7102535
}}
* {{cite journal
 |last1=Kodama |first1=K.
 |display-authors=etal
 |year=2001
 |title=Observation of tau neutrino interactions
 |journal=Physics Letters B
 |volume=504 |issue=3 |pages=218–224
 |doi=10.1016/S0370-2693(01)00307-0|arxiv = hep-ex/0012035 |bibcode = 2001PhLB..504..218D
 |s2cid=119335798
}}
* {{cite book
 |last1=Martin |first1=B. R.
 |last2=Shaw |first2=G.
 |year=1992
 |chapter=Chapter&nbsp;2: Leptons, quarks and hadrons
 |title=Particle Physics
 |chapter-url=https://archive.org/details/particlephysics0000mart/page/23
 |chapter-url-access=registration
 |pages=23–47
 |publisher=[[John Wiley & Sons]]
 |isbn=978-0-471-92358-9
}}
* {{cite journal
 |last1=Neddermeyer |first1=S. H.
 |last2=Anderson |first2=C. D.
 |year=1937
 |title=Note on the Nature of Cosmic-Ray Particles
 |journal=[[Physical Review]]
 |volume=51 |issue=10 |pages=884–886
 |doi=10.1103/PhysRev.51.884
 |bibcode=1937PhRv...51..884N
 |url=https://authors.library.caltech.edu/8618/1/NEDpr37.pdf
}}
* {{cite journal
 |last1=Perl |first1=M. L.
 |display-authors=etal
 |year=1975
 |title=Evidence for Anomalous Lepton Production in e<sup>+</sup>–e<sup>−</sup> Annihilation
 |journal=[[Physical Review Letters]]
 |volume=35 |issue=22 |pages=1489–1492
 |doi=10.1103/PhysRevLett.35.1489
 |bibcode = 1975PhRvL..35.1489P
}}
* {{cite book
 |last1=Peskin |first1=M. E.
 |last2=Schroeder |first2=D. V.
 |title=Introduction to Quantum Field Theory
 |year=1995
 |publisher=[[Westview Press]]
 |isbn=978-0-201-50397-5
 |url-access=registration
 |url=https://archive.org/details/introductiontoqu0000pesk
}}
* {{cite journal
 |last1=Riesselmann |first1=K.
 |year=2007
 |title=Logbook: Neutrino Invention
 |url=https://www.symmetrymagazine.org/article/march-2007/neutrino-invention
 |journal=Symmetry Magazine
 |volume=4 |issue=2
 |url-status=dead
 |archive-url=https://web.archive.org/web/20090531073123/http://www.symmetrymagazine.org/cms/?pid=1000450
 |archive-date=2009-05-31 |df=dmy-all
}}
* {{cite book
 |last1=Rosenfeld |first1=L.
 |year=1948
 |title=Nuclear Forces
 |page=xvii
 |publisher=[[Interscience Publishers]]
}}
* {{cite book
 |last1=Shankar |first1=R.
 |year=1994 |edition=2nd
 |chapter=Chapter 2: Rotational Invariance and Angular Momentum
 |title=Principles of Quantum Mechanics
 |pages=305–352
 |publisher=[[Springer (publisher)|Springer]]
 |isbn=978-0-306-44790-7
}}
* {{cite book
 |last1=Weinberg |first1=S.
 |year=2003
 |title=The Discovery of Subatomic Particles
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-82351-7
 |url-access=registration
 |url=https://archive.org/details/discoveryofsubat00wein_0
}}
* {{cite book
 |last1=Wilson |first1=R.
 |year=1997
 |title=Astronomy Through the Ages: The Story of the Human Attempt to Understand the Universe
 |page=138
 |publisher=[[CRC Press]]
 |isbn=978-0-7484-0748-4
}}

== External links ==
{{Commons category|Leptons}}
{{Wiktionary|lepton}}
* {{cite web |url=http://pdg.lbl.gov |title=Particle Data Group homepage}} – The PDG compiles authoritative information on particle properties.
* {{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html |title=Leptons |publisher=Georgia State University |department=Physics & Astronomy |agency=[[Hyperphysics]]}} – a summary of leptons.

{{particles}}

{{Authority control}}

[[Category:Leptons| ]]
[[Category:Elementary particles]]
[[Category:1897 in science]]