SNO Flies: The Solar Neutrino Problem Resolved

More than a mile beneath the Canadian Shield is a detector filled with 1000 tons of pure heavy water and 8000 tons of ordinary light water. The Sudbury Neutrino Observatory, built by a Canada-US-UK collaboration, has been taking data for two years.  SNO uses heavy water in order to make an unambiguous determination of whether neutrinos emitted by the sun, created as electron neutrinos, arrive at earth in a state with a different flavor (mu, tau, or possibly sterile).  The shortfall of the number of solar neutrinos observed experimentally over the last 30 years compared to the predictions of solar models could be explained if that happened. Such a transformation can occur if physical neutrinos have rest mass and not a unique flavor.  Neutrino mass is a major issue in physics because the completely successful Standard Model does not include it, and because massive neutrinos may play a role in shaping the evolution of the universe.