Stars as Laboratories for Fundamental Physics - MPP Theory Group

560 Chapter 15
masses by a see-saw type mixing effect with the heavy states. The majoron
coupling to standard neutrinos would be extremely small in this
model, leading to no interesting consequences besides small Majorana
masses for ν e , ν µ , and ν τ .
Gelmini and Roncadelli (1981) and Georgi, Glashow, and Nussinov
(1981) suggested instead to do away with the unobserved heavy sterile
neutrinos and give a small Majorana mass directly to the sequential
neutrinos by the interaction with the new Higgs field. The intriguing
feature of this model is that it requires a very small vacuum expectation
value v, perhaps in the keV regime. As all couplings of the new Higgs
field to fermions scale with the inverse of v, the majoron would have
a rather strong coupling to neutrinos. Among many fascinating phenomenological
and astrophysical consequences (e.g. Georgi, Glashow,
and Nussinov 1981; Gelmini, Nussinov, and Roncadelli 1982) this model
predicted, however, that the new Higgs field should contribute precisely
the equivalent of two massless neutrino species to the Z ◦ decay width.
The measurements of this width at CERN and SLAC in 1989−1990,
however, correspond exactly to the known three neutrino flavors (Particle
Data Group 1994), leaving no room for this “triplet majoron model.”
Other “doublet majoron models” which would contribute one-half of an
effective neutrino species to the Z ◦ decay width are also excluded (for
references see, e.g. Berezhiani, Smirnov, and Valle 1992). It is possible,
however, to construct majoron models for Majorana neutrino masses
which retain the original idea of Chicashige, Mohapatra, and Peccei
(1981) and yet provide large majoron-neutrino couplings (e.g. Berezhiani,
Smirnov, and Valle 1992 and references therein; see also Burgess
and Cline 1994a; Kikuchi and Ma 1994, 1995).
The main motivation for going out of one’s way to construct such
models does not arise from particle theory but rather from experiments
and astrophysics. In Sect. 7.1.4 it was outlined that those nuclei
which decay predominantly by a double beta channel (emission of 2e −
and 2ν e ) can also decay in a neutrinoless mode if ν e has a Majorana
mass, allowing an emitted ν e to be effectively reabsorbed as a ν e . In
majoron models of Majorana neutrino masses there is a third decay
channel where the intermediate ν e in the 0ν mode radiates a majoron
so that effectively 2e − plus one majoron χ are emitted. The expected
sum spectrum of the electron energies would be continuous as in the
2ν mode, but with a different spectral shape. Once in a while, experiments
which search for the 0ν mode (a sharp endpoint peak of
the 2e − sum spectrum) have reported a continuous spectral signature
which allegedly could not be ascribed to the dominant 2ν mode or other