Stars as Laboratories for Fundamental Physics - MPP Theory Group

562 Chapter 15
formation and thus could be a novel ingredient for cold dark matter
cosmological models (Dodelson, Gyuk, and Turner 1994).
Even if in the long run the 2β experiments do not yield any compelling
evidence for majoron emission, one may consider a decayingneutrino
cosmology as a motivation in its own right for majoron models.
Such cosmologies may explain the discrepancy between the cosmic
density fluctuation spectrum inferred from the cosmic microwave background
and from galaxy correlations which persists in a purely cold
dark matter cosmology (Bond and Efstathiou 1991; Dodelson, Gyuk,
and Turner 1994; White, Gelmini, and Silk 1995). Another neutrinorelated
explanation of this discrepancy is a hot plus cold dark matter
cosmology which involves neutrinos with a mass of a few eV.
In summary, the simplest majoron model which implied large couplings
to neutrinos (the Gelmini-Roncadelli model) is experimentally
excluded although one can construct more complicated ones which retain
sizeable neutrino-majoron couplings and yet are compatible with
the Z ◦ decay width. The possibility of such models is entertained because
2β decay experiments may yet turn up compelling evidence for
majoron decays, and because certain cosmological models of structure
formation may be taken to suggest massive, decaying neutrinos.
15.7.2 Majorons and Stars
Majorons could also have an impact on stellar evolution. Besides interacting
with neutrinos, they typically also couple to other fermions,
allowing one to apply the astrophysical bounds on pseudoscalars derived
throughout this book to majoron models.
Because of the possibility of fast decays ν → ν ′ χ the neutrino signal
from distant sources, notably from the Sun or from SN 1987A, would
be affected. Such decays are not likely to be able to explain the solar
neutrino problem as discussed in Sect. 10.8. It remains interesting,
however, that the matter-induced ν e -ν e energy splitting allows for decays
ν e → ν e χ (Sect. 6.8). Should a solar ν e flux show up in future
measurements, it could be an indication for such decays.
As for SN neutrinos, the decay of massive ν µ ’s or ν τ ’s with finalstate
ν e ’s could modify the ν e signal observed in a detector, notably
the energy distribution and duration of the observed pulse (Soares and
Wolfenstein 1989; Simpson 1991). The SN 1987A data have not been
analyzed in detail with regard to this possibility, although the observed
signal can be accounted for without invoking such effects. It is not
clear if one could derive significant constraints on majoron models from