The three types of neutrinos change into each other over time , so an electron neutrino could turn into a tau neutrino and then back again .
When the third type of lepton , the tau , was discovered in 1975 at the Stanford Linear Accelerator Center , it too was expected to have an associated neutrino ( the tau neutrino ) . First evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay leading to the discovery of the neutrino .