Editing Lepton (section)

== 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>