Stars as Laboratories for Fundamental Physics - MPP Theory Group

572 Chapter 16
The main aspect of the current situation is that there does not seem
to be a simple “astrophysical solution” to reconcile the solar source
spectrum with the measured fluxes in experiments with three different
spectral response characteristics. Even allowing for large modifications
of the p 7 Be cross section or the solar central temperature does not yield
consistency unless one stretches the experimental uncertainties of the
flux measurements beyond reasonable limits. Still, a final verdict on
the question of solar neutrino oscillations can be expected only from
the near-future experiments Superkamiokande, SNO, and BOREXINO
as discussed in Chapter 10.
16.2.3 Oscillation of Supernova Neutrinos
Naturally, neutrino oscillations would also affect the characteristics of
SN neutrinos which have been observed only from SN 1987A, although
it is not unrealistic to hope for the observation of a galactic supernova
at Superkamiokande or SNO within, say, a decade of operation. In water
Cherenkov detectors, the main signal of SN neutrinos is thought to
be from the ν e p → ne + reaction, although the angular characteristics of
the SN 1987A observations do not square well with this assumption except
that there is no convincing alternate interpretation (Sect. 11.3.5).
The MSW solution in the Sun requires a “normal” mass hierarchy with
ν e being dominated by the smaller mass eigenstate. In this case resonant
oscillations do not occur among the ν’s.
Still, for large-angle vacuum oscillations such as those corresponding
to the solar vacuum solution, the signal in the IMB and Kamiokande
detectors would have been “hardened” by the partial swap of, say, the
ν e with the ν µ spectrum. It is not entirely obvious from the current
literature if this effect is ruled in or ruled out by the SN 1987A observations
(Sect. 11.4.3).
If the mixing angle of ν e with both ν µ and ν τ is small (sin 2 2θ ∼ < 0.1)
there is no impact on the ν e SN signal. Still, the prompt ν e burst could
be affected even for small mixing angles because of the possibility of
resonant oscillations (Sect. 11.4.2) and so the observation of a future
galactic supernova could serve to measure this effect.
If one were to contemplate more general neutrino mass matrices
with an “inversion” so that ν e is not dominated by the lowest-mass
eigenstate, there could be resonant oscillations in the ν sector which
would then lead to dramatic modifications of the SN signal in a large
range of masses and mixing angles. This possibility has not been explored
much in the literature.