The tau neutrino is the neutral, nearly massless partner of the tau lepton, and is the cousin of the electron neutrino and muon neutrino. It is the last neutrino to be directly detected, although its existence was not in doubt due to a wealth of indirect evidence.

An experiment at Fermi National Accelerator Laborator, or Fermilab, called DONUT (Direct Observation of Nu Tau), reported their results on 21 July 2000. They found four events showing a neutrino interacting with an atomic nucleus to produce a tau lepton. Since electron and muon neutrinos produce longer lived electrons and muons, the presence of the tau is a signature for a tau neutrino.

It is interesting that these four events came from a data sample of six million recorded interactions. Demanding that various conditions be satisfied in a candidate event, they applied cuts on the data, producing 1000 candidate events. Finally four events offer definitive evidence for the observation.

The delicate experiment made use of advanced emulsion technology to track and tag the tau lepton, which again is the signature for having a tau neutrino in the initial state.

Source: Fermilab press release