Future experiments  Solar Neutrinos  The solar neutrino problem

New neutrino physics?

To solve this paradox the hypothesis of new neutrino physics has been raised, and in particular the hypothesis of neutrino oscillation has been taken into consideration. The oscillation can take place in the travel between Sun and Earth (Vacuum Oscillations) or within the Solar Matter (Oscillation in matter: MSW effect).

In the frame of the assumption of only two-neutrino mixing, in the plane $\Delta m^2$ vs $sin^{2}\vartheta$, the main parameters of the oscillations, two regions are still allowed for MSW by the present experimental results: one at Small Mixing Angle (SMA) and the second one at Large Mixing Angle (LMA). These two zones are 90$\%$ C.L. regions around two points, which represent the Best Fit Values. A third zone exists, at 99$\%$ C.L. at low $\Delta m^2$ and large mixing angle.

Similarly for the Vacuum Oscillation a large region is still allowed, around a Best Fit value.

For both the oscillations: Vacuum Oscillation and MSW the ⁷Be region is crucial. For the oscillations in matter in the SMA region the ⁷Be neutrino flux is much more reduced than the ⁸B flux. This is true also in the LMA, even if the effect is not so strong.

For the oscillation in vacuum the modulation wavelength is very short in the pp neutrino region, with the consequence that the effect appears to be averaged. In the high energy range of the solar neutrino spectrum the wavelength is larger than the distance Sun-Earth and the effect cannot be detected. Finally at the ⁷Be neutrino energy, the Sun-Earth distance is just few times the wavelength $[L_{Sun-Earth}=(n+1/2)\lambda]$.

Evidence of the neutrino oscillation can be obtained:

1.

by comparing the total neutrino fluxes with what expected by the Solar Model;

2.

by studying the energy spectra for non monochromatic sources;

3.

by looking for seasonal variations which have to be different from what expected by the pure geometrical effect (eccentricity of the Earth orbit), as a consequence of the Vacuum Oscillations. Another important effect is the possible day/night flux difference due to the MSW effect due to the possibility that $\nu_\mu$ should be converted again to $\nu_e$ crossing the Earth;

4.

by detecting neutral current events which can be induced by any flavour neutrinos.