Stars as Laboratories for Fundamental Physics - MPP Theory Group

Miscellaneous Exotica 561
backgrounds, although such claims have tended to disappear with the
collection of more significant data. If interpreted in terms of an upper
limit to the majoron-ν e Yukawa coupling, current experiments give
about g < ∼ 2×10 −4 (Beck et al. 1993 and references therein). At the
present time there does not appear to exist a compelling signature for
majorons in any of the experiments, although several of them seem
to find certain spectral anomalies—see, e.g. Burgess and Cline (1993,
1994b) for an overview and references.
If the spectral anomalies were to represent the first evidence of majoron
emission in 2β decays, the Yukawa coupling would have to be near
the 10 −4 level. The neutrino-majoron coupling is given as g = m ν /v in
terms of the neutrino mass and the symmetry breaking scale v. With
m < νe ∼ 1 eV for a Majorana mass from measured limits on the 0ν decay
mode one finds the requirement v < ∼ 10 keV which is an extremely
small scale of symmetry breaking. The majoron is the “angular degree
of freedom” of a complex scalar field; the “radial degree of freedom,”
often referred to as the ρ field, has a mass typically of order v. Therefore,
a low-mass scalar particle beyond the majoron would appear in
the low-energy sector of the theory.
One severe limitation on such models is provided by big-bang nucleosynthesis
which has been widely used to set limits on additional
low-mass degrees of freedom which are thermally excited during the
epoch of nucleosynthesis; an upper limit of about 0.3 is often quoted as
the maximum allowed extra contribution in units of effective neutrino
degrees of freedom (e.g. Walker et al. 1991). On the face of it, this
limit excludes majoron models where the majoron (and possibly the ρ)
interact sufficiently strongly with neutrinos to reach thermal equilibrium.
In one recent study Chang and Choi (1994) found that one must
require g < ∼ 10 −5 for the largest majoron Yukawa coupling to any neutrino
species in order to avoid thermalization of the majorons before
nucleosynthesis.
Such constraints rely on the assumption of 3 standard light neutrino
species being in thermal equilibrium at the epoch of nucleosynthesis.
However, in majoron models the usual cosmological neutrino
mass bound does not apply because of the possibility of fast decays of
the type ν → ν ′ χ (majoron χ) which are induced by flavor off-diagonal
Yukawa couplings. Therefore, ν τ ’s may have a mass of, say, a few
MeV and may have disappeared by decays and annihilations before nucleosynthesis,
thus making room for majorons; for detailed numerical
studies see Kawasaki et al. (1994) and papers quoted there. Such heavy,
short-lived ν τ ’s may provide interesting effects on scenarios of galaxy