QCD axion dark matter with a small decay constant

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(10^11) 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 ~ (10^8 − 10^11) GeV. The axions may be warm enough to give deviations from cold dark matter in Large Scale Structure.