Generated by the sun's nuclear processes, nearly a billion solar neutrinos--ghostly subatomic particles that can easily pass through Earth without hitting anything--shower down on each square centimeter of the planet's surface every second. Although neutrinos come in three "flavors," electron, muon, and tau, existing detectors can only see the electron variety, and they only see half as many coming from the sun as theorists had predicted. To resolve this discrepancy, physicists have proposed that half of the neutrinos switch flavors, or "oscillate," on their way from the sun's center to Earth. Other neutrino experiments have been gathering indirect evidence for oscillations by comparing the number of neutrinos from a known source with the number observed in a detector, and a new project in Japan called KamLAND could firm up the case (see main text). But SNO should provide the most direct test yet of the theory.
The key is its ability to see several varieties of solar neutrinos at once. SNO contains 1000 tons of ultrapure heavy water, water in which the hydrogen atoms have been replaced with deuterons, whose nuclei have a proton and a neutron. When an electron neutrino collides with a heavy water molecule, it can split apart the neutron and the proton and eject an electron. Other neutrino flavors split the nuclei but don't scatter electrons. By counting both neutrons and electrons, SNO should be able to measure both the total number of incoming neutrinos and the fraction of electron neutrinos, says physicist and SNO spokesperson David Wark of Oxford University in the United Kingdom. If SNO finds that the shortfall of electron neutrinos is made up in other flavors, it will provide strong support for oscillations.
"It is an extremely important experiment," agrees physicist Paul Langacker of the University of Pennsylvania, Philadelphia. "They will very likely ascertain definitively whether neutrino oscillations are taking place." Unfortunately, physicists will have to be patient: Neutrinos collide with matter so rarely that SNO will detect only some 20 neutrinos every day. As a result, says Wark, "it will be at least a year" before SNO has an answer.