After Martin Perl and colleagues at SLAC National Accelerator Laboratory discovered the charged tau lepton, they hypothesized the existence of the tau neutrino. The tau is the heaviest member of a family of three electron-related particles. The electron and muon, as well as electron and muon neutrinos, were already known. The tau neutrino, according to the parallel structure, should be added to the neutrino family now that the tau has been proven. It wasn’t until the DONUT experiment at Fermilab in 2000 that this particle was detected experimentally.