léptons

Leptons are a class of elementary particles that participate in the electroweak interaction but not in the strong interaction. In the Standard Model, there are six leptons arranged in three charged members and three neutrinos: the electron (e−) and electron neutrino (νe); the muon (μ−) and muon neutrino (νμ); and the tau (τ−) and tau neutrino (ντ). All leptons are spin-1/2 fermions and are colorless.

Charged leptons carry electric charge −1, while neutrinos are electrically neutral. The charged leptons have masses that span a wide range: electron about 0.511 MeV, muon about 105.7 MeV, and tau about 1777 MeV. Neutrinos have extremely small masses, with only tiny mass differences inferred from neutrino oscillation experiments; the absolute masses are not yet precisely known.

Leptons interact via the electromagnetic and weak forces (charged leptons), and via the weak force (neutrinos). They do not participate in strong interactions. All leptons can interact through the exchange of W± and Z0 bosons, with neutrinos interacting only weakly and electromagnetically neutral.

Mass generation for leptons occurs through Yukawa couplings to the Higgs field, giving each charged lepton its mass. Neutrino masses require mechanisms beyond the original formulation of the Standard Model, and the observation of neutrino oscillations shows that flavor states mix, described by the PMNS matrix.

Leptons play a central role in particle physics and cosmology. Lepton flavor is not an exact symmetry due to neutrino mixing, and precision studies continue to test lepton universality and search for physics beyond the Standard Model.