Stars as Laboratories for Fundamental Physics - MPP Theory Group

430 Chapter 11
high-energy forward component; they are not fit well by the assumed
dominant detection process ν e p → ne + which is supposed to yield an
isotropic positron distribution with no directional correlation with energy.
Elastic νe scattering, however, does not fit the data well either
because it is too forward peaked, and anyhow it is disfavored by a small
cross section unless the flux of ν µ,τ or ν µ,τ was extremely high.
However, because no plausible and/or viable nonstandard cause for
the observed events has been proposed one has settled for the interpretation
of a statistical fluctuation for the apparent anomalies. After all,
it is difficult to imagine a small sample drawn from any distribution
without some “anomalies” which are easy to overinterpret. Still, if a
reasonable alternative to the standard interpretation of the signal were
to come forth this topic would have to be reconsidered.
11.4 Neutrino Oscillations
11.4.1 Overview
The expected neutrino signature from a stellar collapse and conversely
the inferred protoneutron star properties from the SN 1987A neutrino
signal both depend on the assumption that “nothing happens” to the
neutrinos on their way to us. One simple modification of the expected
signal is a dispersion of the ν e burst caused by a nonvanishing m νe in
the 10 eV range (Sect. 11.3.4). Dispersion effects could also be caused
by novel interactions with the galactic magnetic field, dark matter, the
neutrino background, or simply by decays. All of these scenarios require
relatively exotic particle-physics assumptions which can be constrained
by the SN 1987A signal (Chapter 13). The assumption of small neutrino
masses and mixings, however, fits into the standard model with
minimal extensions, and may already be implied by the solar neutrino
observations (Sect. 10.6). Therefore, it is prudent not to ignore the
possible impact of oscillations on SN neutrinos.
The most obvious consequence is that the prompt ν e burst could oscillate
into another flavor which then would be much harder to observe
because of the reduced ν-e cross section for non-ν e flavors (Fig. 11.10).
Notably, if the solar ν e flux is depleted by resonant oscillations one may
expect the same in the SN mantle and envelope where a large range of
densities and density gradients is available. It will turn out, however,
that the small-angle MSW solution to the solar neutrino problem leaves
an observable ν e burst (Sect. 11.4.2).