Cosmologically safe QCD axion without fine-tuning
  Although QCD axion models are widely studied as solutions to the strong CP
problem, they generically confront severe fine-tuning problems to guarantee the
anomalous PQ symmetry. In this letter, we propose a simple QCD axion model
without any fine-tunings. We introduce an extra dimension and a pair of extra
quarks living on two branes separately, which is also charged under a bulk
Abelian gauge symmetry. We assume a monopole condensation on our brane at an
intermediate scale, which implies that the extra quarks develop the chiral
symmetry breaking and the PQ symmetry is broken. In contrast to the original
Kim's model, our model explains the origin of the PQ symmetry thanks to the
extra dimension and avoids the cosmological domain wall problem because of the
chiral symmetry breaking in the Abelian gauge theory.
