Cosmic Ray Neutrino Problems
Cosmic-ray neutrinos are behaving strangely. Neutrinos are light (perhaps massless) particles that interact so weakly that enormous detectors might observe less than 100 of them in a year. Theorists estimate that among neutrinos created in air showers caused by cosmic rays smashing into Earth's atmosphere (neutrinos from the sun are another matter), muon neutrinos should outnumber electron neutrinos by a ratio of 2 to 1. Instead, recent measurements have turned up with a ratio more like 1 to 1. Such an anomalous ratio has been reported by a number of labs, including instruments in Japan, Minnesota, and in Ohio. Meanwhile, scientists at the Frejus detector under the French Alps see no anomalous effect, while yet another group working in the Gran Sasso tunnel under the Apennine Mountains in Italy, see a smaller and more ambiguous anomaly. Although short of being statistically conclusive, the data seem to indicate a shortfall of muon neutrinos. The leading theoretical explanation for this is the notion that neutrinos can transform ("oscillate") from one type to another and that muon neutrinos are being detected as electron neutrinos. (Physics Today, Oct.)
Neutrino oscillation was postulated in 1964 to explain the solar neutrino problem--ongoing experiments observe 2 to 3 times less neutrinos than theoretical models of the Sun predict. Models of nucleosynthesis have been remarkably successful in explaining stellar evolution, so alternate solutions to the neutrino problem need to be found. For more on anomalous solar phenomena, see the Ulysses article on page 3.
