As we heard here from van Bibber (van Bibber 2003), axions were invented in order to conserve CP in the strong interactions: in the picturesque analogy of Sikivie (Sikivie 1996), to explain why the strong-interaction pool table is apparently horizontal. Axion-like particles may be characterized by a two-dimensional parameter space, consisting of the axion mass ma and its coupling to pairs of photons, ga. Many areas of this parameter space are excluded by laser experiments, telescopes, searches for solar axions currently being extended by the CAST experiment at CERN (Irastorza et al. 2002), astrophysical constraints (Hagiwara et al. 2002) and searches for halo axions using microwave cavities (Asztalos et al. 2001), as seen in Fig. 8. These and searches using Rydberg atoms (Yamamoto et al. 2001) have the best chances of excluding ragions of parameter space where the axion might constitute cold dark matter.
Figure 8. Exclusion domains for the halo density of different types of axion, as functions of the frequency corresponding to the possible axion mass, as obtained from microwave cavity experiments (van Bibber 2003; Asztalos et al. 2001).