Neutrino detectors are designed to spot the elusive particles produced by nuclear reactions in the sun and other stars. Such spheres are commonly filled with ordinary water, but the SNO detector--which rests in a nickel mine 2 kilometers underground--will contain heavy water, in which hydrogen atoms are replaced by deuterium. Heavy water interacts not just with solar-generated electron neutrinos but also with the other two "flavors," muon and tau. That means SNO will be able to tell if solar neutrinos have "oscillated," or transformed into other flavors, on their journey to Earth.

Oscillation could explain why other detectors see only about half the expected number of neutrinos streaming from the sun. And because oscillation is associated with having mass, it would also indicate that neutrinos are not massless, as assumed in the Standard Model. SNO is "the kind of experiment that comes along once every decade or so," says John Bahcall of the Institute for Advanced Study in Princeton, New Jersey. "I'm deliriously happy."