Abstract:

Thirty years of solar neutrino observations have shown a deficit relative to models of the Sun's energy production and---even more suggestively---an energy dependence to that deficit. The Sudbury Neutrino Observatory (SNO) was designed to determine whether the neutrino measurements were caused by the change of the Sun's electron neutrinos into other active flavors, by directly comparing exclusive measurements of the flux of electron neutrinos with the inclusive measurement of the flux of all neutrino flavors. SNO's first results, combined with earlier measurements by the Super-Kamiokande collaboration, in fact do show a significant difference between these two measurements, providing direct evidence that neutrinos from the Sun do change flavor. SNO is now working on further measurements---such as the time dependence of the solar flux---which may help to identify neutrino oscillations as the mechanism for the flavor change.