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Tau (τ) leptons (aka tauons, tau particle) are one of the very small elementary particles. This means that they are believed to be so small that they can not be divided any more. Tau leptons can be thought of as very heavy electrons, as they are both leptons. This is because they have about 3,500 times as much mass as electrons, and about 17 times as much mass as muons. Since they only live for 2.9x10^–13 seconds, they do not have a significant role in the regular world. However, they are very important in the decay of subatomic matter.

Like the other two basic leptons, tauons have a neutrino named after them (the tau neutrino).

Tau have a charge of -1, and can be written as τ^–. Since antimatter has the opposite of charge of regular matter, anti-tauons have a charge of +1, and can be written as τ⁺. Tauons themselves are unstable, and can decay. Also, τ⁺ and τ^– can annihilate each other in a form of decay. When a single tauon decays, it is the only lepton that can decay into hadrons (things made of quarks). τ⁺ and τ^– can be formed by an electron-positron (antielectron) pair combining.^[1] The two tauons then decay into an electron and a positron or a muon and an antimuon, and four of the various neutrinos. However, a single tauon decays differently than a tauon and an antitauon.

A τ^– will quickly decay into a tau neutrino and a W boson.^[1] The W boson will exist for 3x10^–25 seconds, before it decays into an electron and an electron antineutrino, a muon and a muon antineutrino, or a down quark and an up antiquark.

1. ↑ ^{1.0 1.1} "Example e+e-→τ+τ- Events". Identifying Interesting Particle Physics Events at LEP. Manchester Particle Physics Group. Retrieved May 5, 2016.^{[permanent dead link]}