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The tau lepton was discovered by Martin Perl in 1975 at SLAC who won the Nobel Prize for this discovery in 1995.30

The 6 different leptons are the electron, the electron neutrino, the muon, the muon neutrino, and the tau and the tau neutrino. The electron, muon and tau all have an electric charge of -e, whereas the neutrinos have no charge. The leptons can be broken into 3 families, the electron and electron neutrino, the muon and the muon neutrino, and the tau and the tau neutrino. Each of the particles has an electron, muon, and tau number. The electron and electron neutrino have an electron number of +1, where as the anti-particles of the electron and electron neutrino (positron and anti-electron neutrino) has an electron number of -1. The muon and tau numbers are analogous to that of the electron number. All of the numbers are known as another quantum number called the Lepton number. In any reaction with leptons the Lepton number must be conserved in order for the reaction to be possible. Table 2 shows a summary of the leptons.

Table 2
Lepton Symbol Lepton Quantum Number Spin Quantum Number Electric Charge (e)
Electron e- 1 1/2 -1
Electron Neutrino ne 1 1/2 0
Muon u 1 1/2 -1
Muon Neutrino nu 1 1/2 0
Tau t 1 1/2 -1
Tau Neutrino nt 1 1/2 0