A Unique Oscillation Solution to the Solar Neutrino Problem?

Neutrino flavor oscillations are currently the most favored way to understand the larger solar B neutrino interaction rate measured by Super-Kamiokande (SK) with respect to the B rate reported by the SNO-collaboration. The SNO rate includes only e-type B neutrinos while SK measures solar neutrinos via neutrino-electron elastic scattering which has a small sensitivity to other (active) flavors as well. The SK excess is then interpreted as a hint for appearance of other active flavors in the B neutrino flux (born in the sun as purely e-type). Neutrino oscillations also explain the deficit of both rates as well as the deficits of the “Chlorine rate” (Homestake) and the “Gallium rates” (Gallex/GNO, SAGE) with respect to the Standard Solar Model (SSM). Chlorine and Gallium rates include only e-type solar neutrinos. The large neutrino flavor mixing between the second and third generation inferred from atmospheric neutrino data and the absence of an oscillation signal in the CHOOZ reactor neutrino experiment requires the neutrino flavor mixing between the first and third generation to be small. Solar neutrino oscillations can therefore be approximated by a two-neutrino description with the parameters θ (mixing angle) and ∆m (difference in mass). The mixing angle ranges between 0 and π/2, since ∆m is defined to be positive. For ∆m between ≈ 10eV and ≈ 10eV, the matter density in the sun and earth can strongly affect the oscillation probability. For θ < π/4, resonant enhancement of the oscillations (MSW effect) can occur in the sun. On the “dark side” of the parameter space (θ > π/4), an anti-resonance can suppress the oscillations. Below ≈ 10eV (quasi-vacuum/vacuum region), the oscillation probability is more affected by the oscillation phase than by matter effects.