Stars as Laboratories for Fundamental Physics - MPP Theory Group

Radiative Particle Decays 491
still play a significant dynamical role. Recently, such mixed dark matter
scenarios have received much attention where m ν = 5 eV is a favored
value. Such low-mass particles cannot cluster on galactic scales, but
likely they would reside in clusters of galaxies. With radiative decays
ν → ν ′ γ and a total lifetime exceeding the age of the universe one then
expects clusters of galaxies to be strong sources of optical or ultraviolet
photons. Several limits are summarized in Fig. 12.22.
A case has been made that radiatively decaying neutrino dark matter
is actually required to solve certain problems, notably the ionization
of galactic hydrogen clouds (e.g. Melott and Sciama 1981; Melott,
McKay, and Ralston 1988; Sciama 1990a,b; Sciama 1993a,b, 1995).
The predictions are very specific: An energy of decay photons of E γ =
(14.4 ± 0.5) eV and thus a neutrino mass of m ν = (28.9 ± 1.1) eV
with a radiative lifetime of τ γ = (2 ± 1) × 10 23 s which translates into
µ eff = (6.3 ± 2) × 10 −15 µ B . Such a large transition moment would
require particle physics beyond the standard model.
Nominally, this possibility is already excluded by the absence of a
uv line from the cluster A665 (Davidsen et al. 1991). However, a bound
from a single source is always subject to the uncertainty of unrecognized
absorbing material in the line of sight or internal absorption. Moreover,
the dark matter in the core of this cluster may be mainly baryonic
(Sciama, Persic, and Salucci 1993; Melott et al. 1994). Bounds from the
diffuse extragalactic background light are more reliable in this regard.
While they marginally exclude Sciama’s neutrino (Overduin, Wesson,
and Bowyer 1993) it is perhaps too early to pronounce it entirely dead.
A decisive test will be performed with a future satellite experiment
where the uv line from neutrinos decaying in the solar neighborhood
definitely would have to show up if neutrinos were the bulk of the
galactic dark matter (e.g. Sciama 1993b).
12.7.2 Axions
The axion lifetime from a → 2γ is τ = 6.3×10 24 s (m a /eV) 5 /ξ 2 where ξ
is a model-dependent number of order unity (Sect. 14.3.2). Therefore,
axions with eV masses have radiative lifetimes in the neighborhood of
the above neutrino limits whence they can be constrained by similar
methods (Kephart and Weiler 1987). Moreover, such axions would contribute
substantially to the mass density of the universe because they
would have been in thermal equilibrium until relatively late. 80 Their
80 In the early universe, axions are also produced by the relaxation of the coherent
initial field configuration at the onset of the QCD phase transition. This process