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The Mikheyev-Smirnov-Wolfenstein effect (MSW effect) is the theory of a quantum-mechanical mechanism regarding neutrino oscillation, the theory producing a particular oscillation between the three flavors of neutrino (electron neutrino, muon neutrino, and tau neutrino).
Neutrino detection eventually produced evidence confirming the effect and solving the solar neutrino problem. The problem was that theory predicted more neutrino detections from the Sun (solar neutrinos) than the neutrino observatories of the time were sensing, and the MSW effect was considered one possible explanation. Without this effect, neutrinos would oscillate according to the theory of quantum mixing, which did not match what was observed. The MSW effect is a specific change in the amount of mixing that occurs as neutrinos pass through matter, and its presumed effect on solar neutrinos would take place before the neutrinos exit the Sun. Subsequent detection experiments have found subgroup populations of neutrinos matching the predictions of the MSW effect.