A tau neutrino (ντ) is one of three types of neutrinos, the other two types being electron neutrinos and muon neutrinos. Each of the types has a characteristic particle interaction, involving tau particles, electrons, or muons, respectively. The tau neutrino was the most recently hypothesized and detected, and is the most challenging to detect. Some neutrino observatories are not very sensitive to them; neutrino observatories were generally designed to detect neutrinos from the Sun, which produces a major portion of the neutrinos in Earth's vicinity, its fusion reactions producing electron neutrinos. It has now been established that some of these metamorphose into muon neutrinos and tau neutrinos before leaving the Sun, such metamorphosis a characteristic of neutrinos that is made more probable by a high density of matter, such as in the Sun's core where they are created (Mikheyev-Smirnov-Wolfenstein effect).
Neutrinos rarely interact with (atomic) matter or electromagnetic radiation, being electrically neutral, and generally pass through the Earth. Detectors have used this fact to help distinguish neutrino interactions from cosmic rays, arranging to best sense particles from below. The detectors generally incorporate large volumes of matter, surrounded by sensors for the light (Cherenkov radiation) that interactions create, the detector's matter being selected as transparent to light, serving as a scintillator. Such detectors do produce some tau-neutrino interactions, but the interactions are relatively small, providing less input for the sensors to provide the data for deducing what occurred. The tau particle that the interaction produces generally continues in a direction close that of to the neutrino's earlier trajectory, but the tau particle soon decays back into a tau neutrino, one moving slower than the original. As a tau neutrino passes through Earth, this sequence may repeat more than once.