An extra-dimensional axion in a 5D warped orbifold GUT

We study the QCD axion arising from the 5th component of a bulk U(1) gauge field in a five-dimensional warped grand unified theory, and determine the viable range of the axion decay constant f_a. Unlike flat extra dimensions, where gauge couplings run quickly above the Kaluza-Klein (KK) scale, the logarithmic running in warped geometries permits substantially smaller f_a while preserving perturbative gauge coupling unification. However, bulk tree-level contributions to the gauge coupling — interpreted holographically as CFT renormalization — place a lower bound on f_a. We find that the conventional QCD axion window 10⁹ GeV ≲ f_a ≲ 10¹² GeV is readily compatible without losing perturbativity, provided the AdS curvature is near the Planck scale. Thus, the 5D warped orbifold GUT naturally accommodates a high-quality QCD axion in a grand unified theory that provides an effective description of string-theoretic warped flux compactifications, admitting complementary geometric and holographic descriptions of the axion.