An Extra-Dimensional Axion in a 5D Warped Orbifold GUT

TL;DR

This paper explores a five-dimensional warped grand unified theory where the QCD axion emerges from a bulk gauge field component, allowing for a viable decay constant range compatible with unification and string theory insights.

Contribution

It demonstrates that a 5D warped orbifold GUT can naturally host a high-quality QCD axion within the conventional decay constant window, balancing perturbativity and gauge coupling unification.

Findings

Viable axion decay constant range $10^{9}$ to $10^{12}$ GeV in warped GUTs.

Warped geometry permits smaller $f_a$ compared to flat extra dimensions.

Lower bounds on $f_a$ set by gauge coupling contributions.

Abstract

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 $10^{9}\,\mathrm{GeV} \lesssim f_a \lesssim 10^{12}\,\mathrm{GeV}$ 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.