Stars as Laboratories for Fundamental Physics - MPP Theory Group

Axions 541
Fig. 14.5. Astrophysical and cosmological bounds on “invisible axions.” One
globular-cluster limit (white exclusion bar) is based on the axion-electron
coupling and thus applies only if axions are of the DFSZ type (cos 2 β = 1
was used). For the coupling to photons ξ = 1 was assumed. Slanted ends of
exclusion bars indicate an estimated uncertainty of the bounds. The anticipated
range of sensitivity of the Livermore and Kyoto search experiments
are also indicated (Sect. 5.3).
limits which I quote from my contribution to the axion session of the
XVth Moriond Workshop Dark Matter in Cosmology, Clocks, and Tests
of Fundamental Laws, (Villars-sur-Ollon, Switzerland, January 21–28,
1995). The proceedings of this meeting will provide many up-to-date
accounts of different aspects of the axion saga.
If axions were sufficiently strongly interacting (f < a ∼ 10 8 GeV) they
would have come into thermal equilibrium before the QCD phase transition
and so we would have a background sea of invisible axions in
analogy to the one expected for neutrinos (Turner 1987). This parameter
range is excluded by the astrophysical arguments summarized
in Fig. 14.5. Hence axions must be so weakly interacting that they
have never come into thermal equilibrium. Still, the well-known mis-