title: Cosmologically Safe QCD Axion without Fine-Tuning.
abstract: 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 Peccei-Quinn (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 chiral symmetry breaking and the PQ symmetry is broken. In contrast to Kim's original model, our model explains the origin of the PQ symmetry thanks to the extra dimension and avoids the cosmological domain wall problem because of chiral symmetry breaking in Abelian gauge theory.
authors.affiliation: Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.|Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan.|Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany.
authors.forename: Masaki
authors.initials: M
authors.lastname: Yamada
authors.affiliation: Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors.forename: Tsutomu T
authors.initials: TT
authors.lastname: Yanagida
authors.affiliation: Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan.
authors.forename: Kazuya
authors.initials: K
authors.lastname: Yonekura