In the future one will want to verify or falsify that MSW is occurring, or choose some other possibility. Assuming that MSW is correct, one will want to find the unique solution for the parameters and tell whether the oscillations are into active or sterile neutrinos. Not only will the gallium experiments have better statistics and calibrations but, in addition, there will be a new generation of experiments. For example, the Sudbury Neutrino Observatory (SNO), which is a heavy water experiment being built by a Canadian-American collaboration in Canada, will have many new capabilities. It will be able to measure charged current reactions from deuterium,
It will also be able to measure the neutral current reaction,
and will observe electron scattering
with both charged and neutral current amplitudes. The ratio of neutral
current to charged current rates can be measured directly or by comparing
the charged current rate with the electron scattering results from SNO or
from the Super-Kamiokande experiment (a large water Cerenkov experiment
being built in Japan). If different from the weak interaction
expectations, the ratio will clearly indicate neutrino oscillations, either
vacuum or MSW. Therefore, one should be able to distinguish new neutrino
physics
from astrophysical solutions
in a way independent of astrophysical uncertainties. Assuming this is
observed, one will be able to use the neutral current events to calibrate the
initial 
flux and determine the core temperature. (Or, more
accurately, a combination of the core temperature and 
.) In
addition, the SNO experiment will be sensitive to spectral distortions.
These are expected to be significant for the non-adiabatic MSW solution,
but not for the large angle, and are essentially free of astrophysical
uncertainties. Vacuum oscillation solutions would also give
large spectral distortions. This is illustrated in Figure 13.
The Super-Kamiokande experiment will have high
statistics for electron scattering and should also be able to observe
spectrum distortions.
Figure: Expected spectrum distortions for the MSW non-adiabatic and
various vacuum oscillation experiments, with expected uncertainties. The
large angle solution exhibits no significant distortions and is similar to
an astrophysical solution. From [42,49,50].
Figure: Expected seasonal variations for various vacuum oscillation
solutions in the SNO, Super-Kamiokande, and BOREXINO experiments. From
[42,49,50].
Yet another probe that is free of astrophysical uncertainties is that the large angle solution is expected to yield a significant day/night asymmetry in the SNO and Super-Kamiokande experiments. This may even be observable in Kamiokande III. Not only are there the day/night effects expected from MSW, but one expects large seasonal variations for vacuum oscillations, as indicated in Figure 14.
Another experiment, BOREXINO in the Gran Sasso Laboratory in Italy, will be
sensitive to the 
line. This is especially interesting because there
is every indication that this is where the dominant suppression occurs.
Furthermore, other explanations of the solar neutrino problem, such as
large magnetic moments [44], would imply interesting consequences for
the 
line.
Altogether, these experiments should be able to establish or refute the MSW
solution, determine the parameters, and probe alternatives precisely.
Whatever happens, one is still interested in the neutrinos not only for
particle physics but also for astrophysics. Fortunately, even if MSW is
going on it should be possible to establish it and constrain the parameters
from the methods described above, with little uncertainty from astrophysics
or nuclear physics. In fact, there should be enough data not only to
determine the MSW parameters but also to determine the initial pp, 
,
and 
flux components in a model independent way, much as was described
in Section 4.2 for astrophysical solutions without MSW
[10]. For this program one will need, in addition to SNO and
Super-Kamiokande, the BOREXINO measurement of the 
line. That is
because MSW can lead to an almost total suppression of the 
flux. However, BOREXINO will have a neutral current sensitivity to the
converted neutrinos at about 20% of the 
efficiency,
yielding a measure of the
initial flux. In addition to the model independent studies, one can
determine the parameters of the standard solar models, such as 
and
, simultaneously with 
and 
. The
projected sensitivity is shown in Figure 15.
Figure: Projected uncertainties in the simultaneous determination of
, 
, and the MSW parameters, assuming future experiments. From
[10].
