Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 447
An even more ambitious goal would be to measure the cosmic ν e
background flux from all past SNe in the universe. For energies below
around 10 MeV this flux is swamped by many orders of magnitude by
that from the nuclear power plants on Earth. Above a few 10 MeV the
atmospheric neutrino flux would dominate and so there is only a small
window where the cosmic SN flux might be detectable. However, even a
moderately sized (200 tons) scintillation detector located on the moon
would have a chance of measuring this flux because these backgrounds
do not exist there (Mann and Zhang 1990).
Returning to Earth one may speculate about what could be learned
if a galactic SN were indeed observed at Superkamiokande. For the purpose
of argument a distance of 10 kpc (5 times closer than the LMC)
is assumed. (Recall that the solar system is at a distance of about
8 kpc from the galactic center.) Then one expects at Superkamiokande
about 4000 events from the reaction ν e p → ne + , compared with 270
at Kamiokande, and with 12 measured there from SN 1987A (Totsuka
1990). This would be enough to determine a statistically very significant
and very detailed “neutrino lightcurve.”
Interestingly, one expects about 13 events within the first few ms
from the prompt ν e burst. Its presence would indicate that the ν e ’s
have not oscillated, say, into ν τ ’s as would be expected for a cosmologically
interesting ν τ mass (Sect. 11.4.2), thus excluding a large range
of masses and mixing angles (Fig. 11.17). Moreover, the prompt burst
could not have been dispersed by a neutrino mass and so a m νe bound of
order 1 eV could be derived. A number of other conclusions tentatively
reached for SN 1987A could be affirmed (Sect. 13.2). The nonobservation
of the prompt burst, on the other hand, would be more difficult
to interpret as its absence could have a variety of causes ranging from
neutrino oscillations to some flaw in the standard picture of SN collapse.
The ν e lightcurve which would last, say, 10 s would not allow one to
extract interesting bounds on m νe relative to the ones already obtained
from SN 1987A and from laboratory experiments. From Eq. (11.9) one
concludes that for a distance D ≈ 10 kpc one is sensitive to masses in
the 100 eV range.
In Superkamiokande one would expect to see about 40 events for
each ν µ +ν µ and ν τ +ν τ from the elastic ν-e scattering process. Because
the final-state electrons are strongly forward peaked one can separate
them from the isotropic ν e p → ne + signal. Therefore, a ν µ or ν τ mass
would manifest itself through late forward events. Seckel, Steigman,
and Walker (1991) found that from the signal in water Cherenkov detectors
one could be sensitive to a mass down to about 75 eV. Of