All solar neutrino experiments performed to date have detected fewer electron neutrinos than are expected from standard solar models (SSMs). There are strong hints that this is the result of neutrino flavor transformation between the production point in the sun and the terrestrial detection point. Observation of neutrino flavor transformation would be additional and compelling evidence that neutrinos have non-zero mass. Non-zero neutrino mass is evidence for physics beyond the Standard Model of fundamental particle interactions and might also have cosmological implications.
The Sudbury Neutrino Observatory (SNO) is being constructed to study solar neutrinos. The SNO experiment is unique in that it utilizes one kiloton of heavy water, D2O, as a target. This permits SNO to make separate measurements of the flux of electron neutrinos and the flux of all neutrino flavors. SNO can thereby make an SSM-independent measurement which may provide us with the first conclusive evidence of solar neutrino oscillations.