Lepton

Generations of matter<table><tbody><tr><th colspan="2"><a href="/facts/Fermion/LEckdDsR">Fermion</a> categories</th><th colspan="3"><a href="/facts/Elementary_particle/CTFDM97U">Elementary particle</a> <a href="/facts/Generation_(particle_physics)/hAbgvUfV">generation</a></th></tr><tr><th> Type </th><th> Subtype </th><th> First </th><th> Second </th><th> Third </th></tr><tr><th rowspan="2"><a href="/facts/Quark/hwuuVkV5">Quarks</a>(<a href="/facts/Color_(quantum_mechanics)/aWS1xsCR">colored</a>)</th><th>down-type</th><td><a href="/facts/Down_quark/Nq10ayEE">down</a></td><td><a href="/facts/Strange_quark/VDWd3ky0">strange</a></td><td><a href="/facts/Bottom_quark/2s4rJRSo">bottom</a></td></tr><tr><th>up-type</th><td><a href="/facts/Up_quark/Aw3Vp3vk">up</a></td><td><a href="/facts/Charm_quark/RLduBtpL">charm</a></td><td><a href="/facts/Top_quark/82qd7NPI">top</a></td></tr><tr><th rowspan="2">Leptons (color-free) </th><th>charged </th><td><a href="/facts/Electron/L0KBo5cW">electron</a></td><td><a href="/facts/Muon/BP2fLVIR">muon</a></td><td><a href="/facts/Tau_(particle)/6DW8j6PC">tau</a></td></tr><tr><th> <a href="/facts/Neutrino/yGWVfSg2">neutral</a> </th><td> <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrino</a> </td><td> <a href="/facts/Muon_neutrino/A8kPHDKd">muon neutrino</a> </td><td> <a href="/facts/Tau_neutrino/GGIb4W5b">tau neutrino</a> </td></tr></tbody></table>

In <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, a lepton is an <a href="/facts/Elementary_particle/CTFDM97U">elementary particle</a> of <a href="/facts/Half-integer_spin/LEckdDsR">half-integer spin</a> (<a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a> ⁠1/2⁠) that does not undergo <a href="/facts/Strong_interaction/JjRsKcLg">strong interactions</a>. Two main classes of leptons exist: <a href="/facts/Electric_charge/xxsBfmlB">charged</a> leptons (also known as the <a href="/facts/Electron/L0KBo5cW">electron</a>-like leptons or muons), including the <a href="/facts/Electron/L0KBo5cW">electron</a>, <a href="/facts/Muon/BP2fLVIR">muon</a>, and <a href="/facts/Tauon/6DW8j6PC">tauon</a>, and neutral leptons, better known as <a href="/facts/Neutrino/yGWVfSg2">neutrinos</a>. Charged leptons can combine with other particles to form various <a href="/facts/Composite_particle/FwR0Ab6u">composite particles</a> such as <a href="/facts/Atom/FoQ4kGN5">atoms</a> and <a href="/facts/Positronium/F35JBo9C">positronium</a>, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the <a href="/facts/Electron/L0KBo5cW">electron</a>.
There are six types of leptons, known as <a href="/facts/Flavour_(particle_physics)/RR7rsuTD">flavours</a>, grouped in three <a href="/facts/Generation_(particle_physics)/hAbgvUfV">generations</a>. The <a href="/facts/Standard_Model/LhVGf9ow">first-generation</a> leptons, also called electronic leptons, comprise the <a href="/facts/Electron/L0KBo5cW">electron</a> (e−) and the <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrino</a> (νe); the second are the muonic leptons, comprising the <a href="/facts/Muon/BP2fLVIR">muon</a> (μ−) and the <a href="/facts/Muon_neutrino/A8kPHDKd">muon neutrino</a> (νμ); and the third are the tauonic leptons, comprising the <a href="/facts/Tau_(particle)/6DW8j6PC">tau</a> (τ−) and the <a href="/facts/Tau_neutrino/GGIb4W5b">tau neutrino</a> (ντ). 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 <a href="/facts/Particle_decay/3koAz34P">particle decay</a>: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the <a href="/facts/Universe/XbfpnDmM">universe</a>, whereas muons and taus can only be produced in <a href="/facts/High_energy_physics/YgIuyj2t">high-energy</a> collisions (such as those involving <a href="/facts/Cosmic_ray/uxCVdTKd">cosmic rays</a> and those carried out in <a href="/facts/Particle_accelerator/Pkuc08xC">particle accelerators</a>).
Leptons have various <a href="/facts/Intrinsic_properties/5WJ1zN7q">intrinsic properties</a>, including <a href="/facts/Electric_charge/xxsBfmlB">electric charge</a>, <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a>, and <a href="/facts/Mass/HHdc8XmF">mass</a>. Unlike <a href="/facts/Quark/hwuuVkV5">quarks</a>, however, leptons are not subject to the <a href="/facts/Strong_interaction/JjRsKcLg">strong interaction</a>, but they are subject to the other three <a href="/facts/Fundamental_interaction/WrFEJaFl">fundamental interactions</a>: <a href="/facts/Gravitation/bqAN7DdQ">gravitation</a>, the <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a>, and to <a href="/facts/Electromagnetism/oAkmALHT">electromagnetism</a>, 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 <a href="/facts/Antiparticle/7Ic43ThI">antiparticle</a>, known as an antilepton, that differs from the lepton only in that some of its properties have <a href="/facts/Charge_conjugation/YJ1vjdfD">equal magnitude but opposite sign</a>. According to certain theories, neutrinos may be <a href="/facts/Majorana_fermion/39gzL0Nv">their own antiparticle</a>. 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 and was discovered in 1897 by <a href="/facts/J._J._Thomson/Y4jyndJF">J. J. Thomson</a>. The next lepton to be observed was the <a href="/facts/Muon/BP2fLVIR">muon</a>, discovered by <a href="/facts/Carl_D._Anderson/rIMqD9ax">Carl D. Anderson</a> in 1936, which was classified as a <a href="/facts/Meson/9R0kIc1q">meson</a> at the time. 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. The first neutrino, the electron neutrino, was proposed by <a href="/facts/Wolfgang_Pauli/Ugk4ubXk">Wolfgang Pauli</a> in 1930 to explain certain characteristics of <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>. It was first observed in the <a href="/facts/Cowan%E2%80%93Reines_neutrino_experiment/2XaJzDRC">Cowan–Reines neutrino experiment</a> conducted by <a href="/facts/Clyde_Cowan/kdySD9zb">Clyde Cowan</a> and <a href="/facts/Frederick_Reines/sdWH4hNg">Frederick Reines</a> in 1956. The muon neutrino was discovered in 1962 by <a href="/facts/Leon_M._Lederman/3UT17nBB">Leon M. Lederman</a>, <a href="/facts/Melvin_Schwartz/UApASDpn">Melvin Schwartz</a>, and <a href="/facts/Jack_Steinberger/JUd5e2pJ">Jack Steinberger</a>, and the tau discovered between 1974 and 1977 by <a href="/facts/Martin_Lewis_Perl/g1BeBJ5z">Martin Lewis Perl</a> and his colleagues from the <a href="/facts/Stanford_Linear_Accelerator_Center/CzX2afas">Stanford Linear Accelerator Center</a> and <a href="/facts/Lawrence_Berkeley_National_Laboratory/tvjPSwC9">Lawrence Berkeley National Laboratory</a>. The <a href="/facts/Tau_neutrino/GGIb4W5b">tau neutrino</a> remained elusive until July 2000, when the <a href="/facts/DONUT/vpjkhsLV">DONUT</a> collaboration from <a href="/facts/Fermilab/8yyTx3oj">Fermilab</a> announced its discovery.
Leptons are an important part of the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>. Electrons are one of the components of <a href="/facts/Atom/FoQ4kGN5">atoms</a>, alongside <a href="/facts/Proton/clCjepXP">protons</a> and <a href="/facts/Neutron/oRCM1geb">neutrons</a>. <a href="/facts/Exotic_atom/FrPmnXvk">Exotic atoms</a> with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as <a href="/facts/Positronium/F35JBo9C">positronium</a>.

Lepton open-in-new

Lepton