But over the years detectors found only about half the muon neutrino predicted by theory. The apparent shortage of muon neutrinos was explained by the recent observations that led to Friday's announcement. The scientists reasoned that by traveling through the entire Earth these neutrinos had had time to oscillate, probably many times, between muon neutrinos and some other type, especially the tau neutrino, and this accounts for the deficit seen in muon neutrinos. A related problem has to do with neutrinos produced by the fusion process in the sun. This process, which merges the nuclei of hydrogen atoms to form helium nuclei and energy, produces neutrinos.
Astrophysicists believe they understand the mechanism in complete detail.
The trouble is that all the best detectors ever built find far fewer neutrinos than should be present according to understanding of the fusion reaction. Scientists believe the anomaly can be explained by the oscillation of detectable solar neutrinos into types that cannot be detected by existing instruments. But no one has proven this explanation. Investigation of the neutrino have not been limited to the Super-Kamiokande collaboration. Another approach to penetrating the neutrino secrets involves the use of particle accelerators capable of producing intense beams of neutrinos.