Axion overview

Twenty years ago, in order to understand the perplexing lack of strong inte raction CP violation, Peccei and Quinn postulated the existence of a new sp ontaneously broken continuous symmetry of QCD. From this symmetry arose the axion, a new hypothetical elementary particle. Axions could be very light and possess extraordinarily feeble interactions. These light axions are goo d candidates for non-baryonic cold dark matter, but they are "invisible axi ons" by virtue of being so difficult to detect. Now, twenty years later, ax ions remain an active field of theoretical and experimental research, exemp lified by highly sensitive new searches aimed squarely at invisible axions and new insights on axion production and interactions arising from Supersym metry and topological strings. This is a review of the current status of ax ion physics reported at the recent Florida Axion Workshop held in honor of the 20th anniversary of a grand idea.