The tau neutrino was the last neutrino to be discovered. It was predicted after the discovery of the τ lepton. Like all neutrinos, it interacts very seldomly and is therefore extremely difficult to detect.
Tau neutrinos were discovered at the DONuT experiment at Fermilab in Chicago, which took data for a few months in 1997 and published its results in 2000. This is a good example of the effort that has to be put into analysing the data. It is not uncommon for data analyses to take several years, because all the details of the experiment must be carefully understood and examined to eliminate any misinterpretation. When a new particle is discovered, the standard is that the error rate is less than 1:2,000,000.
Tau neutrinos usually appear in the decay of a tauon, for example when it is produced by colliding other particles in a particle accelerator. Dedicated neutrino experiments, such as FASERnu, in which the AEC of the University of Bern is heavily involved, or DONuT, use emulsion detectors: their working principle is similar to the old analogue photographs, where a chemical emulsion changes its properties when exposed to particles like the photographic films change colour when exposed to light.
Emulsion plates are a very precise detector used in neutrino physics and are a specialty of the University of Bern. Here, the neutrino arriving from the left interacts with the detector material and produces a tauon, which decays into a charged pion and a neutrino. The neutrinos are not visible in the detector, while the track of the tauon is only a few millimetres long.
