"The tau (τ), also called the tau lepton, tau particle or tauon, is an elementary
particle similar to the electron, with negative electric charge and aspin of 1⁄2. Together with the electron, the muon, and the three neutrinos, it is classified as a lepton. Like all elementary particles, the tau has a corresponding antiparticle of opposite charge but equal mass and spin, which in the tau's case is the antitau (also called the positive tau). Tau particles are denoted by τ− and the antitau by τ+."
"Tau leptons have a lifetime of 2.9×10−13 s and a mass of 1,777 MeV/c2 (compared to 105.7 MeV/c2 for muons and 0.511 MeV/c2 for electrons). Since their interactions are very similar to those of the electron, a tau can be thought of as a much heavier version of the electron. Because of their greater mass, tau particles do not emit as much bremsstrahlung radiation as electrons; consequently they are potentially highly penetrating, much more so than electrons. However, because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable: their penetrating power appears only at ultra high energy (above PeV energies)."