Lepton

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Leptons are fundamental particles with half-integer spin that do not feel the strong force. There are six types in three generations: the electron (e−) and its neutrino (νe); the muon (μ−) and its neutrino (νμ); and the tau (τ−) and its neutrino (ντ). Charged leptons have electric charge −1, while neutrinos have charge 0. Leptons interact through electromagnetism, the weak force, and gravity, but not through the strong force.

Charged leptons can form ordinary matter—atoms and many other particles—while neutrinos interact very weakly and are hard to detect. Every lepton has an antiparticle, called an antilepton. In some theories, neutrinos could be their own antiparticles.

Masses vary a lot. The electron is the lightest charged lepton. Muons and taus are much heavier and decay into lighter particles, usually along with neutrinos. Neutrinos are extremely light; experiments show they have mass and can change from one flavor to another, a phenomenon known as neutrino oscillation.

In the Standard Model, leptons come in left-handed doublets that interact with the weak force, while right-handed charged leptons do not. The Higgs field gives mass to charged leptons, but neutrinos get their mass from physics beyond the simplest model.

Lepton numbers (Le, Lμ, Lτ) help track flavors and are conserved in most processes, though neutrino oscillations violate flavor conservation. Lepton universality means all lepton flavors couple equally to gauge bosons; most tests support this, but some measurements hint at small deviations.

A brief history: the electron was discovered in 1897; Pauli proposed the neutrino in 1930 and it was observed in 1956; the muon appeared in 1936; the tau in the 1970s, with its neutrino observed in 2000. Leptons are a central part of how the universe’s matter is built and are essential to particle physics.