Stars as Laboratories for Fundamental Physics - MPP Theory Group

Neutrinos: The Bottom Line 577
of temperature and lepton number. (Of course, a nonlocal energy transfer
mechanism is already thought to be important for reviving the shock
wave in the delayed-explosion scenario.) As far as I know, nothing more
quantitative than back-of-the-envelope estimates of this scenario exist
in the literature. Therefore, alleged bounds of order 10 −12 µ B (Bohr
magneton µ B = e/2m e ) on Dirac-neutrino dipole moments probably
have to be used with some reservation (Sect. 13.8.3). Conversely, such
dipole moments may actually help to explode supernovae.
With regard to the observable neutrino signal, spin and spin-flavor
oscillations both in the SN and in the galactic magnetic field may cause
vast modifications of the ν e fluxes and spectra observable in water Cherenkov
detectors if neutrinos have dipole moments in the ballpark of
10 −12 −10 −14 µ B .
Spin and spin-flavor oscillations can be very important in the early
universe where strong magnetic fields may exist, and where a population
of the r.h. degrees of freedom would accelerate the expansion
rate of the universe. These issues are being investigated in the current
literature; final conclusions do not seem to be available at the present
time. Still, it appears that this effect may well be the most significant
impact of small Dirac neutrino dipole or transition moments anywhere
in nature.
b) Laboratory Limits
Less problematic bounds on neutrino dipole moments arise from laboratory
experiments where one studies the recoil spectrum of electrons
in the reaction ν + e → e + ν ′ where ν ′ can be the same or a different
flavor (Sect. 7.5.1). A sensitivity down to, perhaps, as low as 10 −11 µ B
can be expected from a current effort involving reactor neutrinos as
a source (MUNU experiment). Current limits on dipole or transition
moments are about 2×10 −10 µ B if ν e is involved, and about 7×10 −10 µ B
if ν µ is involved. For transition moments, these limits are subject to
the assumption that there is no cancellation between a magnetic and
an electric dipole scattering amplitude.
c) Huge ν τ Dipole Moments or Millicharges
In principle, the possibility of a large diagonal moment for ν τ remains
open as it has not been possible to produce a strong ν τ source in the laboratory
so that only extremely crude limits exist on the ν τ -e-scattering
cross section. The globular-cluster µ ν bounds discussed below do not