Stars as Laboratories for Fundamental Physics - MPP Theory Group

278 Chapter 7
mal equilibrium. A reasonable cutoff by the Debye screening scale was
used by Fukugita and Yazaki (1987) who found
µ ν ∼ < 0.5×10 −10 µ B , (7.31)
about a factor of 3.5 looser than Morgan’s original constraint.
This limit can be avoided for the diagonal ν τ magnetic moment
which would contribute to the ν τ ν τ → e − e + annihilation process. If
the ν τ also had a mass in the 10 MeV regime the spin-flip excitation of
the r.h. degrees of freedom would be compensated by the annihilation
depletion of the ν τ and ν τ population before nucleosynthesis (Giudice
1990). A detailed analysis (Kawano et al. 1992) reveals that a µ ντ larger
than about 0.7×10 −8 µ B is allowed in the mass range between a few and
about 30 MeV.
Like in a SN core, there could exist large magnetic fields in the
early universe that would allow for left-right transitions by the magnetic
dipole induced spin precession (Sect. 8.4); for early discussions
of this possibility see Lynn (1981) and Shapiro and Wasserman (1981).
Arguments of this sort naturally depend on assumptions concerning the
primordial magnetic field distribution; such fields may be required as
seeds for the dynamo mechanism to create present-day galactic magnetic
fields. A quantitative kinetic understanding of the process of
populating the r.h. neutrinos requires a simultaneous treatment of the
neutrino spin-precession and scattering much along the lines of Chapter
9 where flavor oscillations are studied in an environment where neutrinos
scatter frequently. The most recent investigation of primordial
neutrino magnetic oscillations is Enqvist, Rez, and Semikoz (1995); see
their work for references to the previous literature. In certain plausible
scenarios of primordial magnetic field distributions neutrino Dirac
dipole moments as small as 10 −20 µ B seem to be in conflict with BBN.
7.5.4 Search for Radiative Neutrino Decays
The search for radiative decays of reactor, beam, solar, supernova, and
cosmic neutrinos will be discussed at length in Chapter 12. For electron
neutrinos, the effective transition moment in the sense of Eq. (7.27) will
be found to be limited by
0.9×10
⎧⎪ −1 µ B (eV/m ν ) 2 Reactors,
⎨
µ < 0.5×10 −5 µ B (eV/m ν ) 2 Sun,
ν ∼
1.5×10 −8 µ B (eV/m ν ) 2 SN 1987A,
⎪ ⎩
3×10 −10 µ B (eV/m ν ) 2.3 Cosmic background,
(7.32)