A lepton is an elementary fermion that does not participate in the strong interaction. In the Standard Model of particle physics, leptons form one of the two classes of elementary matter particles, alongside quarks. The six known lepton flavors are the electron, muon, tau, and their three associated neutrinos. Leptons are treated as particles without internal constituents, rather than composite objects such as protons or neutrons. All have spin ½, expressed in units of the reduced Planck constant. (nobelprize.org)
Classification and generations
The leptons are organized into three generations, each containing a negatively charged particle and an electrically neutral neutrino. This arrangement parallels the three generations of quarks. The charged particles share the same electric charge, −e, where e is the magnitude of the elementary charge, but differ substantially in mass. Their neutral partners are called the electron neutrino, muon neutrino, and tau neutrino. (home.web.cern.ch)
| Generation | Charged lepton | Associated neutrino |
|---|---|---|
| First | Electron, e⁻ | Electron neutrino, νₑ |
| Second | [[muon | Muon]], μ⁻ |
| Third | [[tau-lepton | Tau]], τ⁻ |
“Flavor” identifies a particle type, not a sensory property. Neutrino flavor is operationally associated with the charged lepton produced or absorbed in a weak interaction. An electron neutrino can produce an electron in an appropriate interaction, whereas a muon neutrino can produce a muon. Neutrinos are detected through these interaction products, not by directly observing an isolated neutrino. (neutrinos.fnal.gov)
Charged leptons have distinct antiparticles with positive charge. The electron’s antiparticle is the positron; the corresponding particles for the muon and tau are the positive muon and positive tau. Neutrinos and antineutrinos exhibit different interaction behavior, but whether they are fundamentally distinct particles remains unresolved. (hst-archive.web.cern.ch)
Interactions and elementary character
Leptons lack color charge, the charge associated with the strong interaction. They therefore do not form strongly bound composite particles in the manner of quarks. The absence of strong interactions, rather than low mass, is their defining distinction: the tau is substantially heavier than several strongly interacting particles. (hst-archive.web.cern.ch)
All known leptons participate in the weak interaction. Charged leptons also interact through electromagnetism, whose force carrier is the photon. Neutrinos carry no electric charge and do not couple to photons through the ordinary electric-charge interaction. The Standard Model describes these electromagnetic and weak processes within the electroweak interaction. Gravity is not incorporated into that model, although leptons are not exempt from gravitational effects. (home.web.cern.ch)
The charged leptons’ common electroweak coupling structure is called lepton universality. It does not imply identical observed decay rates: different masses alter available final states and kinematics. Comparisons between processes involving electrons, muons, and taus must account for those differences. Such comparisons provide tests of the Standard Model and searches for additional interactions. (home.cern)
Masses and decay
The electron mass is approximately 0.511 MeV/c². The muon is about 207 times heavier than the electron and has a mean lifetime of approximately 2.197 microseconds at rest. Unlike the electron, which is stable in the Standard Model, the muon and tau undergo weak decay. Their greater masses permit final states unavailable to a free electron. (pdg.lbl.gov)
The principal negative-muon decay is
A tau can decay into an electron or muon accompanied by neutrinos, or into hadrons and a tau neutrino. Hadronic tau decay does not mean that the tau itself experiences the strong interaction: the decay proceeds through the weak interaction, while the resulting quarks and hadrons participate in strong-interaction processes. (pdg.lbl.gov)
Neutrino masses require a more careful description. The three flavor states are not identical to the states with definite mass. Neutrino oscillation—a change in the probability of detecting different flavors during propagation—establishes that neutrino masses cannot all be zero. This observation goes beyond the original minimal Standard Model, which assigned neutrinos zero mass. (nobelprize.org)
Lepton number and flavor
Lepton number is a quantum-number bookkeeping convention assigning +1 to leptons and −1 to antileptons. It is conserved in familiar weak decays such as muon decay. Separate electron, muon, and tau family numbers are also useful for describing many reactions, but neutrino oscillation demonstrates that individual neutrino flavors are not conserved during propagation. Flavor change should not be confused with violation of total lepton number. (arxiv.org)
Whether total lepton number is an exact conservation law remains an experimental question. If neutrinos are their own antiparticles, they would be Majorana particles, allowing lepton-number-violating processes. Searches for neutrinoless double beta decay, in which a nucleus emits two electrons without accompanying neutrinos, investigate this possibility. (arxiv.org)
Discovery
The electron was discovered in 1897, and the muon was identified in cosmic-ray studies in 1936. Wolfgang Pauli proposed a neutral particle in 1930 to account for the missing energy and angular momentum in beta decay. In 1956, Clyde Cowan and Frederick Reines detected electron antineutrinos from a nuclear reactor. The muon neutrino was discovered in 1962, establishing that neutrinos associated with different charged leptons were distinguishable. (neutrinos.fnal.gov)
Martin Perl and collaborators discovered the tau in 1975. The DONUT collaboration at Fermilab announced the first direct evidence for tau-neutrino interactions on July 21, 2000. Perl and Reines shared the 1995 Nobel Prize in Physics for experimental contributions to lepton physics, specifically the discovery of the tau and detection of the neutrino. (nobelprize.org)