Stars as Laboratories for Fundamental Physics - MPP Theory Group

448 Chapter 11
course, masses so large and larger are excluded from cosmology—such
neutrinos would have to be unstable. In order to obey the cosmological
limits neutrinos emitted at a distance of 10 kpc would decay before
reaching Earth if their mass exceeds a few 10 keV. Thus, the observation
of a galactic SN would allow one, at best, to probe the mass
window 100 eV < ∼ m < ν ∼ 30 keV. A similar conclusion was reached by
Acker, Pakvasa, and Raghavan (1990) who considered the signature in
the proposed BOREX detector.
It may be possible, however, to probe a somewhat smaller mass
for, say, the ν τ down to the cosmologically interesting range of 30 eV
if one takes advantage of all aspects of the observed neutrino signal
(Krauss et al. 1992). Because the late part of the neutrino lightcurve
is expected to be similar for ν e and the other flavors one can hope
to extract the behavior of the source from the ν e signal. Then one
would be more sensitive to modifications of the ν τ lightcurve caused by
dispersion effects. However, in order to identify the ν τ ’s one would have
to use energy cuts (ν e ’s and ν τ ’s have different spectra!) in addition to
angular cuts. In the analysis of Krauss et al. (1992) the possibility of
ν e ↔ ν τ oscillations was not included which would weaken the range of
accessible ν τ masses because of the modified energy spectra. Of course,
the r-process nucleosynthesis argument of Sect. 11.4.5 would indicate
that MSW oscillations did not take place late even if they took place
early and rendered the prompt ν e burst unobservable.
Still, a ν τ mass relevant for the dark matter content of the universe
as well as for scenarios of structure formation may be much lower than
30 eV. Therefore, on the basis of current analyses the prospect of being
able to recognize a cosmologically relevant ν µ or ν τ in the neutrino
signal of a galactic SN appears relatively dim.
A far more positive view was taken by Cline et al. (1994) who argued
that masses down to 15 eV may be accessible by the simultaneous
operation of Superkamiokande and their previously proposed (Cline
et al. 1990) Supernova Burst Observatory (SNBO) which is based on
neutral-current reactions alone. This would obviate the need to separate
the charged-current ν e n → pe + detection at Superkamiokande
from the neutral-current reaction νe → eν by angular and energy cuts.
One could use the Superkamiokande ν e signal to monitor the SN neutrino
lightcurve, notably its sharp onset, and relate it to the onset of
the neutral-current events at SNBO which would be “washed out” for
a cosmologically interesting mass of, say, the ν τ . One must hope that
SNBO will become a real project in the near future.