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A lepton is an elementary particle and a fundamental constituent of matter.[1] The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed.

There are six types of leptons, known as flavours, forming three generations.[2] The first generation is the electronic leptons, comprising the electrons (e
) and electron neutrinos (ν
); the second is the muonic leptons, comprising muons (μ
) and muon neutrinos (ν
); and the third is the tauonic leptons, comprising tauons (τ
) and tau neutrinos (ν
). Electrons have the least mass of all the charged leptons. The heavier muons and tauons will rapidly change into electrons through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and tauons can only be produced in high energy collisions (such as those involving cosmic rays and those occurring in particle accelerators).

Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction. For every lepton flavor there is a corresponding type of antiparticle, known as antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.

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