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 $f_a\sim \mathcal{O}({10}^{11})\mathrm{GeV}$ (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 $f_a\sim ({10}^8–{10}^{11})\mathrm{GeV}$. The axions may be warm enough to give deviations from cold dark matter in large scale structure.

Figure 1

Constraints on the quartic theory in the parameter space of the saxion initial amplitude and vacuum mass, assuming saxion oscillations begin during a radiation-dominated era. The upper (lower) panel assumes an era of saxion domination does not (does) exist. The solid lines of the two panels coincide; the observed dark matter abundance arises on (above) these lines for the upper (lower) panel.

Figure 2

The same as Fig. 1 but for the quadratic theory.