Stars as Laboratories for Fundamental Physics - MPP Theory Group

496 Chapter 13
a temperature of around 4 MeV. This precludes that a major swap
by oscillations with the higher-energetic ν µ or ν τ flux has taken place.
The impact of neutrino oscillations on the observable signal has been
discussed in Sect. 11.4.
Also, ν µ,τ or ν µ,τ decays with final-state ν e ’s would produce additional
higher-energy events. While the SN 1987A data are probably too
sparse to extract significant information on the presence or absence of
this effect, a future galactic SN would certainly allow one to exclude a
certain range of masses and decay times or to detect this effect (Soares
and Wolfenstein 1989).
The trapping of neutrinos in a SN core together with the condition
of β equilibrium inevitably implies that there is a large ν e chemical
potential, leading to typical ν e energies of order 200 MeV. Moreover,
the inner temperature during deleptonization reaches values of up to
40−70 MeV so that typical thermal (anti)neutrino energies of up to
100−200 MeV are available. Therefore, if neutrinos could escape directly
from the inner core they would cause high-energy events in the
detectors which have not been observed.
A mechanism to tap the inner-core heat bath directly is the production
of r.h. neutrinos by a variety of possible effects such as spinflip
scattering by a Dirac mass term or a magnetic dipole moment
(Sect. 13.8). R.h. states could not be detected directly because they are
sterile with regard to standard l.h. weak interactions, a property which
allows them to avoid the SN trapping. However, they could produce
detectable l.h. states by decays (Dodelson, Scott, and Turner 1992) or
by magnetic oscillations (Nötzold 1988; Barbieri and Mohapatra 1988).
13.2.3 Prompt ν e Burst
The prompt ν e burst can be seen in a water Cherenkov detector by the
reaction ν e e → eν e where the final-state electron essentially preserves
the direction of the incident neutrino. At Kamiokande, the directionality
of the first event 81 is consistent with the interpretation that it was
caused by this reaction. However, the expected fluence corresponds
only to a fraction of an event and so the first event may also be due
to the ν e p → ne + reaction and point coincidentally in the forward direction.
A random direction has about a 5% chance of being forward
within 25 ◦ which is approximately the uncertainty of the Kamiokande
directional event reconstruction.
81 In the first publication of the Kamiokande group (Hirata et al. 1987) the second
event was also reported forward; its most probable direction was later revised.