Searches for invisible axions

The axion, a hypothetical elementary particle, emerged from a compelling so lution to the Strong-CP problem in QCD, Subsequently, the axion was recogni zed to be a good Cold Dark Matter candidate. Although dark matter axions ha ve only feeble couplings to matter and radiation, extremely sensitive searc hes are underway around the world. We review the state of experiments searc hing for the axion through its coupling to photons. Besides answering an ou tstanding question in particle physics, the discovery of dark matter axions may provide a detailed history of the formation of our Milky Way galaxy th rough the structure of the axion signal. (C) 2000 Elsevier Science B.V. All rights reserved.