spacetoday.net: Physicists solve solar neutrino problem

Posted: Tue, Jun 19, 2001, 2:44 PM ET (1844 GMT)

Scientists announced Monday that they had resolved a long-standing problem regarding the Sun's output of nearly massless particles known as neutrinos. New data from the Sudbury Neutrino Observatory, located in a mine two kilometers below the surface in Ontario, show that the Sun is producing the number of neutrinos predicted by existing models, but that some are in different forms than expected. Past measurements of the Sun's neutrino flux suggested the Sun was generating as few as one-third of the particles, byproducts of the fusion of hydrogen into helium, as expected, raising concerns that solar models were in error. The new result raises a challenge for theoretical physicists, who must now try to explain how electron neutrinos -- the type generated by the Sun -- can change into the mu and tau neutrinos detected at Sudbury. The new results, along with past measurements, also give physicists the opportunity to measure the mass of neutrinos. They found that the total mass of all the neutrinos in the universe is about the same as the total mass of all visible stars; not enough, they concluded, for neutrinos to make a significant contribution to the universe's dark matter complement.