Extra-Dimensional Axion Expectations

TL;DR

The paper argues that axions arising from higher-dimensional gauge fields naturally address the axion quality problem and that the decay constant f encodes the fundamental UV cutoff of the theory. It provides general formalism for how extra-dimensional axions originate from p-form fields, how their masses arise nonperturbatively, and how axion strings form with core tensions tied to f and the instanton action S_inst. A central claim is that the light axion and its string excitations imply a UV cutoff of order Λ_QG ≲ O(1) f/g, and that the fundamental string scale is typically within about two orders of magnitude of f in explicit string constructions, with the tension of the minimal axion string scaling as ${\cal T}_{(2)} \sim 2\pi S_{\mathrm{inst}} f^2$. The work also discusses cosmological implications (no PQ phase transition) and the interplay with chiral fermions, including how monodromy masses are constrained and how GS dynamics can preserve a light axion. Together, these results bridge UV quantum gravity considerations with low-energy axion phenomenology, offering a framework to infer Planck-/string-scale physics from axion measurements.

Abstract

Axions arising as modes of higher-dimensional gauge fields are known to offer a compelling solution to the axion quality problem and to naturally arise in string theory. In this context, it is interesting to ask how we would interpret an experimental measurement of the axion decay constant $f$. I give several arguments for, as well as concrete examples in string theory of, the existence in such a model of an axion string with tension of order $2πS_\mathrm{inst} f^2$, where $S_\mathrm{inst}$ is the instanton action. Furthermore, in models of this type axion strings are typically fundamental objects (rather than solitons), whose tension is at or above the fundamental cutoff of the theory. As a result, I argue that for an extra-dimensional QCD axion, it is likely that the fundamental cutoff scale lies at most two orders of magnitude above $f$. In addition to these core arguments, this paper begins with a self-contained introduction to the physics of extra-dimensional axions and ends with some comments on axion physics in relation to chiral fermions.

Figures (1)

• Figure 1: Loop corrections to the axion propagator from the sum over loops of gluon Kaluza-Klein modes.