The discovery of the tau

Martin Perl was a member of a research team at Stanford Linear Accelerator Center (SLAC) in the 1960's. Like several others, the team was experimenting to find new charged particles, including new leptons. A new electron-positron collider called SPEAR became operational at SLAC in 1973. It was a dream machine for the production of new leptons. It gave Perl the chance to hunt for leptons within a hitherto inaccessible energy region, 5,000 million electronvolts. The first sign that a new type of lepton was being produced was seen in 1974. It was designated with the Greek , the first letter of Triton, meaning 'third'. The tau ranks third among the charged leptons, after the electron and the muon. The electron belongs to the first family. The approximately 200-times-heavier muon is a member of the second family and the almost 3,500-times-heavier tau is a member of the third. The discovery of the tau was the first sign that a third family of fundamental building blocks exists.
    Physicists work from a theoretical model, the standard model, which describes how nature's smallest constituents interact. Without the third family, the model does not include CP (Charge-conjugation and Parity) violation and hence would have been incomplete. CP violation is a symmetry principle that governs, among other things, the decay of particles and the decomposition of the Universe into matter and antimatter after the Big Bang.