Demonstrates viable axion decay constants in a 5D warped GUT, indicating enhanced gauge coupling unification.
A bstract 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 10 9 GeV ≲ f a ≲ 10 12 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.
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Choi et al. (2026) studied this question.
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