QCD Axion Dark Matter with a Small Decay Constant

Abstract

The QCD axion is a good dark matter candidate. The observed dark matter abundance can arise from misalignment or defect mechanisms, which generically require an axion decay constant fa∼O(1011) GeV (or higher). We introduce a new cosmological origin for axion dark matter, parametric resonance from oscillations of the Peccei-Quinn symmetry breaking field, that requires fa∼(108-1011) GeV. The axions may be warm enough to give deviations from cold dark matter in large scale structure.