© CAMBRIDGE UNIVERSITY PRESS 1999

1.5.2.1 Axions

Peccei-Quinn symmetry, with its associated particle the axion, remains the best solution known to the strong CP problem. A second-generation experiment is currently underway at Los Alamos National Lab. (LLNL) (Hagmann et al. 1996) with sufficient sensitivity to have a chance of detecting the axions that might make up part of the dark matter in the halo of our galaxy, if the axion mass lies in the range 2-20 µeV. However, it now appears that most of the axions would have been emitted from axionic strings (Battye & Shellard 1994, 1997) and from the collapse of axionic domain walls (Nagasawa & Kawasaki 1994), rather than arising as an axion condensate as envisioned in the original cosmological axion scenario. This implies that if the axion is the cold dark matter particle, the axion mass is probably ~ 0.1 meV, above the range of the LLNL experiment. While current experiments looking for either axion or supersymmetric WIMP cold dark matter have a chance of making discoveries, neither type is yet sufficiently sensitive to cover the full parameter space and thereby definitively rule out either theory if they do not detect anything. But in both cases this may be feasible in principle with more advanced experiments that may be possible in a few years.