Stars as Laboratories for Fundamental Physics - MPP Theory Group

178 Chapter 5
Most excitingly, the galactic axion search is going to be taken up
again with two new experimental setups. The one in Livermore (California)
has an increased detection volume and magnetic field (B 2 V =
14 T 2 m 3 ), and a refined microwave detection method (van Bibber et al.
1992, 1994). Within a running time of two or three years it will be possible
to explore the axion mass range 1.3 − 13 µeV down to a coupling
strength g aγ which is only a factor of about 2.5 shy of the “axion line”
in Fig. 5.7. If axions interact with photons somewhat stronger than
indicated by this line, or if the local dark-matter density is somewhat
larger than assumed in Fig. 5.7, one may already be able to detect
axions in this round of measurements.
The second experiment (Kyoto, Japan) will use Rydberg atoms in a
novel scheme to detect single microwave quanta (Matsuki et al. 1995).
Because of the intrinsic low noise of this detector one can go to lower
physical temperatures, thereby reducing thermal noise, and thus allowing
one to use smaller cavities. This in turn permits one to search for
larger axion masses than is possible with the Livermore-type large cavities.
Together, the two experiments can probably cover two decades of
axion masses, between about 10 −6 and 10 −4 eV.
Fig. 5.7. Results of the galactic axion search experiments of the Rochester-
Brookhaven-Fermilab (RBF) collaboration (Wuensch et al. 1989) and of
the University of Florida (UF) experiment (Hagmann et al. 1990). The
hatched areas are excluded, assuming a local dark-matter axion density of
5×10 −25 g cm −3 = 300 MeV cm −3 . The “axion line” is the relationship between
axion mass and coupling strength for ξ = 1 or E/N = 8/3 according
to Eq. (14.24).