Stars as Laboratories for Fundamental Physics - MPP Theory Group

576 Chapter 16
state general constraints as one may easily postulate, for example, that
the decays do not involve final-state ν e ’s, that even “wrong-helicity”
Dirac neutrinos are trapped in a SN core by novel interactions, or that
heavy Majorana ν τ ’s have only negligible mixings with ν e . Surely other
loopholes could be found.
16.3.3 Electromagnetic Properties
a) Spin and Spin-Flavor Oscillations
If one contemplates neutrino interactions beyond the Minimally Extended
Standard Model, neutrino dipole and transition moments no
longer need to be small. In particular, they do not need to be proportional
to the neutrino masses; one example are left-right symmetric
models where even massless neutrinos would have large dipole moments
(Sect. 7.3.1). Dipole and transition moments can lead to spin or spinflavor
oscillations in external magnetic fields, they allow for spin-flip
scattering on charged particles, for the plasmon decay γ → νν in stars,
and for radiative decays ν → ν ′ γ.
One motivation for studying neutrino dipole moments is the apparent
flux variability of the solar neutrino signal in the Homestake
detector. It anticorrelates with solar magnetic activity too closely to
blame it comfortably on a statistical fluke (Sect. 10.4.3). The only
physical explanation put forth to date is that of Voloshin, Vysotskiĭ,
and Okun of a partial depletion of left-handed (measurable) neutrinos
by spin or spin-flavor oscillations. Unfortunately, the required value for
µ ν B (neutrino dipole moment µ ν , magnetic field B in the solar convection
zone) exceeds by about two orders of magnitude what is allowed by
typical models of the solar magnetic field and by limits on µ ν . Therefore,
this scenario appears to be in big trouble. Still, if one ignores the
µ ν limits or speculates about large convection-zone magnetic fields one
may fit all currently available solar neutrino data (Sect. 10.7).
Spin or spin-flavor oscillations can be very important in and near
the cores of supernovae where fields of order 10 12 G exist, and perhaps
pockets with much larger fields. If neutrinos are Dirac particles so that
their spin-flipped (right-handed) states are sterile, the combination of
spin-flip scattering on charged particles and the magnetic spin oscillation
in large-scale magnetic fields can lead to nonlocal modes of energy
transfer where energy can be deposited in one region that was depleted
from a distant other region. Thus, energy transfer could no longer be
treated with simple differential equations which involve local gradients