Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 431
Another interesting possibility is a partial swap ν e ↔ ν µ,τ and ν e ↔
ν µ,τ by oscillations. Because the energy spectrum of the non-ν e flavors is
much harder than that of ν e or ν e , a number of interesting consequences
obtain. First, the detected ν e ’s could have larger average energies than
expected. Conversely, the SN 1987A-implied emission temperature and
neutron-star binding energy could be an overestimate of the true values.
It will turn out that one seriously needs to worry about these effects,
for example, if the large-angle MSW solution or the vacuum solution
to the solar neutrino problem obtain, or if the atmospheric neutrino
anomaly is caused by oscillations (Sect. 11.4.3).
Even more importantly, a swap ν e ↔ ν µ or ν e ↔ ν τ would cause
a more efficient energy transfer from the neutrino flux to the matter
behind the stalled shock after core bounce but before the final explosion.
As enhanced neutrino heating actually appears to be required to
obtain successful and sufficiently energetic explosions, neutrino oscillations
may help to explode supernovae! This scenario works only if
the spectral swap occurs inside of the stalled shock wave. In view of
the relevant medium densities, resonant transitions obtain for neutrino
masses in the cosmologically interesting range of 10−100 eV. A mixing
angle as small as sin 2 2θ > ∼ 3×10 −8 would be enough (Sect. 11.4.4).
Hardening the ν e spectrum by a swap with ν µ or ν τ , however, can
suppress r-process nucleosynthesis just outside of the nascent neutron
star a few seconds after core bounce. Normally the ν e spectrum is
harder than the ν e spectrum, driving β equilibrium in the hot bubble
to the required neutron-rich phase. The oscillation scenario can
cause the reverse. For this effect the oscillations would need to occur
close to the protoneutron star surface and so again a relatively
large neutrino mass-square difference is required which falls into the
cosmologically interesting range. However, the required mixing angle
is larger (sin 2 2θ > ∼ 10 −5 ), leaving ample room for, say, small ν e -ν τ
mixing angles where ν τ ’s with cosmologically relevant masses could
help explode supernovae without disturbing r-process nucleosynthesis
(Sect. 11.4.5).
Finally, neutrino oscillations could allow the non-ν e flavors to participate
in β equilibrium in the inner core and thus build up their own
degenerate Fermi seas. This possibility has been studied in Sect. 9.5
where it turned out that a significant flavor conversion obtains only
for large neutrino masses (keV range and above). Such large masses
are cosmologically forbidden unless neutrinos decay fast into invisible
channels, a hypothesis that would require novel neutrino interactions
beyond masses and mixings (Sect. 12.5.2).