Stars as Laboratories for Fundamental Physics - MPP Theory Group

578 Chapter 16
apply to a “heavy” ν τ so that one is confronted with a nontrivial allowed
region in µ ντ -m ντ space where even MeV masses become cosmologically
allowed because of the dipole-induced annihilation process
ν τ ν τ → e + e − (µ ντ in the ballpark of 10 −7 µ B ). Of course, such large
dipole moments must be caused by a fairly nontrivial arrangement of
intermediate charged states and so one may wonder if a large ν τ magnetic
moment could be realistically the only manifestation of these new
particles and/or interactions.
Still, a large ν τ dipole moment and the correspondingly large annihilation
cross section in the early universe is one possibility to tolerate
a large ν τ mass without the need for fast decays—such a particle
could be entirely stable. Another similar possibility is that ν τ has a
“huge millicharge” in the neighborhood of 10 −5 −10 −3 e. Such a scheme
would require the violation of charge conservation as the possibility of
neutrino charges within a simple extension of the Standard Model discussed
above always gives charges to two neutrino flavors; for ν e or ν µ
the required value is not tolerable (Sect. 15.8).
For the issues of stellar evolution, the only conceivable consequence
of such large ν τ electromagnetic interaction cross sections would be
a reduced contribution to the energy transfer in SNe because of the
reduced mean free path. In the study discussed in Sect. 13.6 one should
have included the possibility of only two effective flavors! Still, there
is little doubt that large ν τ cross sections could be accommodated in
what one knows about SNe today.
d) Astrophysical Bounds on Dipole and Transition Moments
For all neutrinos with a mass below a few keV a very restrictive limit
on dipole or transition magnetic or electric moments arises from the absence
of anomalous neutrino emission from the cores of evolved globularcluster
stars, notably of red-giant cores just before helium ignition
(Sect. 6.5.6). One finds a limit µ < ν ∼ 3×10 −12 µ B which applies to Dirac
and Majorana neutrinos, and which does not allow for a destructive
interference between electric and magnetic amplitudes.
Neutrino transition moments would reveal themselves by radiative
decays. Because the decay rate involves a phase-space factor m 3 ν this
method is suitable only for large masses. In the cosmologically allowed
range with m < ν ∼ 30 eV, the only radiative limit which can compete
with the globular-cluster bound is from the cosmic diffuse background
radiations (Fig. 12.21). In fact, Sciama has proposed a scheme where a
28.9 eV neutrino with a radiative decay time corresponding to a tran-