Cosmologically safe QCD axion as a present from extra dimension

Understand

We propose a QCD axion model where the origin of PQ symmetry and suppression of axion isocurvature perturbations are explained by introducing an extra dimension.

• Each extra quark-antiquark pair lives on branes separately to suppress PQ breaking operators.
• The size of the extra dimension changes after inflation due to an interaction between inflaton and a bulk scalar field, which implies that the PQ symmetry can be drastically broken during inflation to suppress undesirable axion isocurvature fluctuations.