Discovery of the tau-neutrino. This reconstruction shows one of the four events that revealed the tau-neutrino for the first time in the DONUT experiment at Fermilab in 2000. A tau-neutrino has entered DONUT unseen from left & interacted to produce several charged particles (coloured tracks). The red track is a tau particle, which soon decays to another particle (green). The presence of the tau particle proved that the event was initiated by a tau-neutrino. Discovery of the tau-neutrino completed the predicted family of six elementary 'leptons': the electron, the muon, the tau, and the electron-neutrino, muon-neutrino, and tau-neutrino.