Leptons are elementary fermions that do not carry colour charge and therefore do not participate in the strong interaction as described by Quantum Chromodynamics. The known charged leptons are the electron, muon and tau, while the corresponding neutrinos are the electron neutrino, muon neutrino and tau neutrino. All leptons have spin one half. Charged leptons carry electric charge of negative one elementary charge, whereas neutrinos are electrically neutral. Leptons participate in the weak interaction, and charged leptons also participate in electromagnetism. Their lack of colour charge distinguishes them from quarks.
The electron was the first known lepton and was discovered in the late nineteenth century through experiments investigating cathode rays. The muon was discovered in cosmic ray studies in 1936, initially causing confusion because it appeared to resemble an electron while being much heavier. The tau was discovered in 1975 through electron positron collision experiments. Neutrinos were proposed to account for missing energy and momentum in beta decay and the electron neutrino was subsequently detected experimentally. Muon and tau neutrinos were later identified through their interactions. These discoveries established three charged lepton generations accompanied by three corresponding neutrino flavours.
Charged leptons have masses that increase dramatically from the electron to the muon and then the tau. The electron is stable under known interactions because there is no energetically allowed decay into lighter particles that preserves the relevant conserved quantities. The muon and tau are unstable and decay through the weak interaction. A muon commonly decays into an electron and two neutrinos, while tau decays have several possible channels. Neutrinos are much lighter than charged leptons and were historically treated as massless in the minimal Standard Model. Neutrino oscillation experiments demonstrated that neutrinos have nonzero masses and that flavour states are mixtures of mass states.
Leptons are organised into three generations. The first contains the electron and electron neutrino, the second contains the muon and muon neutrino, and the third contains the tau and tau neutrino. Each generation has the same general pattern of charges and interactions but different masses. The reason for the existence of three generations and the hierarchy of their masses is not explained by the Standard Model. Neutrino mixing is described through the Pontecorvo Maki Nakagawa Sakata framework, which relates neutrino flavour states to mass eigenstates. Oscillations occur because different mass eigenstates propagate with different phases, allowing a neutrino produced in one flavour state to later be detected as another flavour.
Neutrinos have exceptionally weak interactions with ordinary matter because they carry no electric charge and no colour charge. They interact through the weak interaction and possibly through gravity, making them extremely difficult to detect. Enormous numbers of neutrinos pass through matter with little interaction. Neutrino detectors therefore require large target masses and sensitive instrumentation. Neutrinos are produced in nuclear reactions, radioactive decays, stellar fusion, supernovae, cosmic ray interactions and particle accelerator processes. Their weakly interacting nature also makes them valuable astronomical messengers because they can escape dense environments that absorb or scatter electromagnetic radiation.
Lepton number is a useful concept in analysing particle reactions. Individual lepton family numbers are approximately conserved in many ordinary processes but neutrino oscillations demonstrate that individual flavour lepton numbers are not exact conserved quantities. The total lepton number is conserved in the minimal Standard Model at the perturbative level, although nonperturbative electroweak processes and possible physics beyond the Standard Model introduce important theoretical qualifications. Neutrino properties therefore provide a particularly sensitive window into physics beyond the simplest Standard Model formulation. Measurements of neutrino masses and mixing parameters continue to constrain theoretical models of particle physics and cosmology.
Leptons play important roles in ordinary matter and technological systems. Electrons determine the structure and chemistry of atoms because their quantum states govern bonding, electrical behaviour and much of condensed matter physics. Muons are used in specialised imaging and materials studies because they can penetrate matter and provide information about magnetic and internal properties. Neutrinos are used to investigate nuclear processes, the Sun, supernovae and other astrophysical environments. Electron beams are central to many technologies, while controlled lepton beams are important in accelerator research. The practical importance of leptons therefore ranges from atomic structure to fundamental tests of particle theory.
Common misconceptions include assuming that all leptons are electrically neutral or that neutrinos are simply very light electrons. Charged leptons and neutrinos have substantially different properties, and neutrinos are distinct particles rather than weakened versions of charged leptons. Another misconception is that neutrinos have no mass; oscillation experiments establish that at least two neutrino mass eigenstates have nonzero mass. Leptons are also not interchangeable with all elementary matter particles because quarks are elementary fermions as well but carry colour charge and participate in the strong interaction. The defining classification of leptons rests on their quantum numbers and interaction properties within the Standard Model.
