leptons

Leptons are a family of fundamental fermions that do not participate in the strong interaction. They have half-integer spin and come in two classes: charged leptons and neutrinos. Each generation contains a charged lepton with electric charge −1 and a corresponding neutrino with charge 0: the electron and electron neutrino (e, νe), the muon and muon neutrino (μ, νμ), and the tau and tau neutrino (τ, ντ).

Charged leptons interact electromagnetically and via the weak force, while neutrinos interact only through the weak interaction and gravity. Neutrinos are exceedingly light and interact very weakly, making them difficult to detect. Charged leptons participate in weak processes mediated by W± bosons and also couple to photons, whereas neutrinos interact primarily through Z0 exchange in neutral-current processes.

Leptons are organized into three generations, with masses increasing from electron to muon to tau. The tau lepton is much heavier and decays rapidly to lighter leptons and hadrons. Neutrinos have extremely small masses and exhibit flavor mixing, described by the PMNS matrix, which leads to neutrino oscillations as they propagate.

In the Standard Model, lepton numbers for each generation (Le, Lμ, Lτ) are conserved in perturbative processes. Leptons play a central role in weak interactions, beta decay, and many precision tests of the electroweak sector, as well as in neutrino physics and astrophysical phenomena. Evidence for neutrino masses and mixing indicates physics beyond the simplest formulation of the model.