Cosmological effects of Peccei-Quinn symmetry breaking on QCD axion dark matter

Understand

We study cosmological effects of explicit Peccei-Quinn breaking on the QCD axion dark matter.

• We find that the axion abundance decreases or increases significantly depending on the initial position, even for a tiny Peccei-Quinn breaking that satisfies the experimental bound of the neutron electric dipole measurements.
• If the axion first starts to oscillate around a wrong vacuum and if it gets trapped there until the false vacuum disappears due to non-perturbative QCD effects, its abundance increases significantly and is independent of the decay constant $f_a$, as first pointed out in [JHEP 06 (2016) 150].
• Thus, the axion produced by the trapping mechanism can explain dark matter even when the decay constant is close to the lower limit due to stellar cooling arguments.