Higher-Dimensional QCD without the Strong CP Problem
  QCD in a five-dimensional sliced bulk with chiral extra-quarks on the boundaries is generically free from the strong CP problem. Accidental axial symmetry is naturally present except for suppressed breaking interactions, which plays a role of the Peccei-Quinn symmetry to make the strong CP phase sufficiently small. 
K.-I. Izawa, T. Watari and T. Yanagida