Science Programme

The SNO+ Experiment is used to investigate a large suite of physics phenomena.
Find out more about our current active research interests here.

Solar Neutrino Oscillations in Matter

The SNO experiment proved that about two thirds of the electron neutrinos produced in the sun show up on earth as muon- or tau- type neutrinos. Combining the SNO result with those of other solar neutrino experiments and the KamLAND reactor anti-neutrino experiment demonstrated that this flavour change is the result of "MSW"-type matter enhanced neutrino oscillations.

Based on our current understanding of neutrino oscillations, the probability that an electron neutrino produced in the sun is detected as an electron neutrino on earth ("the survivial probability")is a function of the neutrino energy. At high energies (at least high for solar neutrinos), matter effects are important and the survival probability is uniformly low. At lower neutrino energies, however, the matter effects gradually give way to vacuum oscillations, and the survival probability increases. The survival probability is shown here with some experimental results, with the grey band being the prediction of the LMA solution in the frame of the MSW model:

Although the electron neutrino survival probability at the "matter oscillation dominated" and "vacuum oscillation dominated" extremes of the solar neutrino flux have been studied, there is interest to make more precise measurements in the "transition region" between around 1 and 4 MeV. Directly observing the rise in the survival probability in the transition region would confirm the MSW mechanism. The transition region is also the most sensitive place to look for sub-dominant effects in neutrino oscillations, such as a sterile neutrino admixture, non-standard neutrino-matter interactions, or a large value for Θ₁₃.

As shown here, we can see that the best way to probe this interesting transition region is to measure the flux of pep neutrinos and attempt to detect ⁸B solar neutrinos down to lower energies than previously studied:

Since the SNO+ detector is located 2km underground, it is shielded from the interfering effects of cosmic rays. This gives SNO+ the potential to measure the fluxes of low energy solar neutrinos. The pep neutrinos are interesting because their flux is predicted with small uncertainty. The ⁸B solar neutrinos span the transition energy region, and their survival probability changes with energy. Neutrinos are detected in a scintillator experiment through the "elastic scattering" interaction: ν_e+e^-→ν_e+e^- SNO+ will search for new physics by probing our understanding of neutrino-matter interactions in solar neutrino oscillations. This figure shows the spectra of recoil electrons that is expected to be produced by the neutrino fluxes of interest:

Solar Metallicity and Solar Models

The nuclear reactions that power the sun are based around the creation of helium nuclei from hydrogen nuclei (individual protons). In the course of this transition, several neutrinos and a large amounts of energy (which eventually becomes sunlight) are produced. In our sun, the majority of these hydrogen-helium transitions occur via the "pp" fusion cycle, show on the left side of the figure below. The solar neutrino measurements that have been made so far are based on the neutrinos produced by the reactions in the pp chain.

It is expected that a small fraction of the sun's energy comes from the "CNO" fusion cycle, shown on the right side of the figure above. The actual CNO contribution is very poorly known, as it is difficult to predict theoretically. A measurement by SNO+ of the flux of neutrinos produced in the CNO cycle would tell us the CNO contribution to solar energy generation, giving us interesting information about the inner workings of the sun. Measuring the CNO solar neutrino flux also reveals the chemical composition or "metallicity" in the core of the sun; this is important for understanding energy production and flow in solar models.

Detection of CNO solar neutrinos is particularly challenging due to their low rate and the presence of similar radioactive backgrounds.