New results from the Sudbury Neutrino Observatory (SNO) are beginning to pin down more precisely the parameters for mixing between different types of neutrino.
Unlike other neutrino detectors, SNO can detect neutrinos in three different ways through its use of heavy water. Only electron neutrinos give rise to charged-current reactions with deuterons in the water, while all types of active neutrino can scatter elastically off the deuterons or induce neutral current (NC) reactions. In the NC reactions the neutrino splits the deuteron into a proton and a neutron, and the gamma rays emitted when the neutron is subsequently captured by another nucleus provide the signature for the reaction.
For the earlier analyses based on events detected with pure heavy water, the SNO collaboration assumed an energy-independent survival probability for the neutrinos. While this allowed the team to say that their data show that electron neutrinos must oscillate to another type, it was not sufficient for calculating the constraints on parameters in MSW mixing. For the new measurements, the team added 2 tonnes of high-purity sodium chloride to the 1000 tonnes of heavy water in the detector. This increased the detection efficiency for NC events three-fold, due to neutron capture on chlorine-35 nuclei. The increased sensitivity allowed them to measure the total active boron-8 solar-neutrino flux, which was found to agree with standard solar-model calculations. A global analysis of solar and reactor neutrino results, including the new measurements, yields Δm2 = 7.1 + 1.2/ -0.6 x 10-5 eV2 and θ = 32.5 + 2.4/-2.3°, disfavouring maximal mixing at a confidence level equivalent to 5.4σ.