tau neutrino

A tau neutrino (ν_τ) is one of three known flavors (types) of neutrinos, the other two being electron neutrinos and muon neutrinos. Each of the flavors is associated with 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 solar neutrinos, a major portion of the neutrinos in Earth's vicinity, the Sun's 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 (neutrino oscillation) that is made more probable by a high density of matter, such as in the Sun's core where they are created (MSW 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, i.e., through the Earth. 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 light for the sensors, thus less 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 moving slower than the original. As a tau neutrino passes through Earth, this sequence can repeat more than once (though I assume the vast majority pass through with no interaction).