lepton

Leptons are a class of elementary particles that do not participate in the strong interaction. They are fermions with spin 1/2 and form a key part of the Standard Model. The six leptons are divided into three charged states—electron (e−), muon (μ−), and tau (τ−)—and their corresponding neutrinos—electron neutrino (νe), muon neutrino (νμ), and tau neutrino (ντ). These six members appear in three generations: (e−, νe), (μ−, νμ), and (τ−, ντ).

Charged leptons interact with electromagnetic and weak forces, while neutrinos interact only via the weak interaction (and gravity). Leptons do not carry color charge, so they do not participate in strong interactions. In the electroweak sector of the Standard Model, left-handed leptons form SU(2)L doublets with their neutrino partners, while right-handed charged leptons are SU(2)L singlets.

Masses vary significantly among leptons. The charged leptons have rest masses of about 0.511 MeV for the electron, 105.7 MeV for the muon, and 1777 MeV for the tau. Neutrinos have very small masses, with differences determined by oscillation experiments; absolute masses are not yet precisely known, but each is sub-electronvolt, and the ordering of the mass states is still unresolved.

Lepton number conservation is a feature of many processes, with separate electron, muon, and tau lepton numbers commonly used. Neutrino oscillations show flavor change, preserving total lepton number but violating individual flavor numbers. The existence of neutrinoless double beta decay would imply total lepton-number violation and Majorana neutrinos, signaling physics beyond the Standard Model; no such decay has been observed.

Leptons play a central role in particle physics, enabling precise tests of the Standard Model and serving as probes in cosmology, astrophysics, and neutrino research.