Stars as Laboratories for Fundamental Physics - MPP Theory Group

Neutrinos: The Bottom Line 579
sition moment of 0.6×10 −14 µ B plays a significant cosmological role
(Sect. 12.7.1). While this scenario is probably excluded it highlights
the possibility of interesting cosmological effects for radiatively decaying
neutrinos in the range allowed by the globular-cluster bound.
Radiative decay limits which are based on astronomical decay paths
suffer from the uncertainty of other invisible decay channels which may
compete with the radiative mode. Limits based on the cosmic background
radiations (Fig. 12.20) imply that in a large range of cosmologically
allowed neutrino masses and lifetimes the dominant decay channel
must be nonradiative. Therefore, one should use the cosmic background
radiations to derive limits on the branching ratio. One could
then construct a contour plot of the excluded transition moments in
the m ν -τ ν plane.
From the SN 1987A radiative lifetime limits I have constructed
such a plot in Fig. 12.17. For large dipole moments in excess of,
say, 10 −10 µ B these bounds are not self-consistent because neutrinos
would be trapped too strongly by electromagnetic scatterings. However,
the laboratory limits exclude large transition moments. Moreover,
the globular-cluster bound yields more restrictive limits if m ν is
less than a few keV. However, for relatively large neutrino masses, and
for lifetimes not so short that the decays would have occurred within
the progenitor star, SN 1987A yields the most restrictive limits on transition
moments. Naturally, one must assume that ν µ or ν τ were actually
emitted with about the standard fluxes. Trapping effects by additional
new interactions could circumvent this assumption.
16.3.4 Summary
Neutrinos with nonstandard interactions may well saturate the experimental
mass limits, and may have a variety of novel properties. However,
it is nearly impossible to derive generic constraints on quantities
like magnetic transition moments without specifying an underlying
particle physics model. Many constraints, notably those related to
SN 1987A or to cosmology, can be circumvented by postulating sufficiently
bizarre neutrino properties. Therefore, it is probably more
important to know the arguments that can serve to learn something
about neutrinos in astrophysics than it is to know a list of alleged limits.
If a concrete conjecture turns up, or a specific theoretical model
needs to be constrained, one can easily go through the list of arguments
and check if they apply or not. Perhaps this book can be of help at
this task.