A Place in the Sun for the Neutrino Hamish Robertson, University of Washington Center for Experimental Nuclear Physics and Astrophysics

More than a mile beneath the Earth's surface in a Canadian mine is a detector filled with 1000 tons of pure heavy water and 8000 tons of ordinary water. Oblivious to all that rock, neutrinos from space can reach the detector unhindered when no other particles can. The Sudbury Neutrino Observatory, built by a Canada-US-UK collaboration, has been watching the Sun night and day for four years. The heavy water allows SNO to measure both the number and the "flavor" (electron, mu, or tau) of neutrinos emitted by the Sun as it makes its energy. SNO shows that the shortfall of the number of solar neutrinos observed in experiments over the last 30 years compared to the predictions of solar models actually is explained by neutrinos having mass and a very high degree of flavor mixing. Long suspected to be the culprit, models of how the sun works turn out to be spectacularly accurate.