Experimental Searches for the Axion and Axion-like Particles
  Four decades after its prediction, the axion remains the most compelling
solution to the Strong-CP problem and a well-motivated dark matter candidate,
inspiring a host of elegant and ultrasensitive experiments based on
axion-photon mixing. This report reviews the experimental situation on several
fronts. The microwave cavity experiment is making excellent progress in the
search for dark matter axions in the microelectronvolt range and may be
plausibly extended up to 100 mu eV. Within the past several years however, it
has been realized that axions are pervasive throughout string theories, but
with masses that fall naturally in the nanoelectronvolt range, for which a
NMR-based search is under development. Searches for axions emitted from the
Sun's burning core, and purely laboratory experiments based on photon
regeneration have both made great strides in recent years, with ambitious
projects proposed for the coming decade. Each of these campaigns has pushed the
state of the art in technology, enabling large gains in sensitivity and mass
reach. Furthermore each modality has also been exploited to search for more
generalized axion-like particles, that will also be discussed in this report.
We are hopeful, even optimistic, that the next review of the subject will
concern the discovery of the axion, its properties, and its exploitation as a
probe of early universe cosmology and structure formation.
