> The tau neutrino was discovered without any prior reason to believe it existed. We knew of neutrinos and they seemed to fill the only related gaps we had. It

> The tau neutrino was discovered without any prior reason to believe it existed. We knew of neutrinos and they seemed to fill the only related gaps we had. It wasn't until the tau neutrino was experimentally detected that we even realized there was a gap for it. The tau neutrino was discovered in 2000. The discovery of the tau lepton 25 years prior heavily implied the existence of an associated neutrino.