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Widening the Axion Window via Kinetic and Stückelberg Mixings

Gary Shiu, Wieland Staessens, Fang Ye

TL;DR

The paper addresses the limited axion decay-constant window by showing that kinetic and Stückelberg mixings in multi-axion systems can yield an effectively large light axion decay constant f_xi even when individual entries are sub-Planckian. Using a minimal two-axion model with a Stückelberg U(1) and a non-Abelian instanton sector, they derive f_xi and identify regions in moduli space where the decay constant becomes super-Planckian, while also outlining how to lower it to the axion dark-matter window. They discuss a string-theory embedding in Type II orientifolds, where D-brane data and moduli isotropy realize the required axion metric G_ij and Stückelberg couplings, with explicit Type IIA examples. The results show that mixing effects decouple the decay-constant enhancement from the low-energy spectrum, offering a more economical route than traditional N-flation or alignment scenarios and enabling both high-scale inflation and DM phenomenology within a unified framework.

Abstract

We point out that kinetic and Stückelberg mixings that are generically present in the low energy effective action of axions can significantly widen the window of axion decay constants. We show that an effective super-Planckian decay constant can be obtained even when the axion kinetic matrix has only sub-Planckian entries. Our minimal model involves only two axions, a Stückelberg U(1) and a modest rank instanton generating non-Abelian group. Below the mass of the Stückelberg U(1), there is only a single axion with a non-perturbatively generated potential. In contrast to previous approaches, the enhancement of the axion decay constant is not tied to the number of degrees of freedom introduced. We also discuss how kinetic mixings can lower the decay constant to the desired axion dark matter window. String theory embeddings of this scenario and their phenomenological features are briefly discussed.

Widening the Axion Window via Kinetic and Stückelberg Mixings

TL;DR

The paper addresses the limited axion decay-constant window by showing that kinetic and Stückelberg mixings in multi-axion systems can yield an effectively large light axion decay constant f_xi even when individual entries are sub-Planckian. Using a minimal two-axion model with a Stückelberg U(1) and a non-Abelian instanton sector, they derive f_xi and identify regions in moduli space where the decay constant becomes super-Planckian, while also outlining how to lower it to the axion dark-matter window. They discuss a string-theory embedding in Type II orientifolds, where D-brane data and moduli isotropy realize the required axion metric G_ij and Stückelberg couplings, with explicit Type IIA examples. The results show that mixing effects decouple the decay-constant enhancement from the low-energy spectrum, offering a more economical route than traditional N-flation or alignment scenarios and enabling both high-scale inflation and DM phenomenology within a unified framework.

Abstract

We point out that kinetic and Stückelberg mixings that are generically present in the low energy effective action of axions can significantly widen the window of axion decay constants. We show that an effective super-Planckian decay constant can be obtained even when the axion kinetic matrix has only sub-Planckian entries. Our minimal model involves only two axions, a Stückelberg U(1) and a modest rank instanton generating non-Abelian group. Below the mass of the Stückelberg U(1), there is only a single axion with a non-perturbatively generated potential. In contrast to previous approaches, the enhancement of the axion decay constant is not tied to the number of degrees of freedom introduced. We also discuss how kinetic mixings can lower the decay constant to the desired axion dark matter window. String theory embeddings of this scenario and their phenomenological features are briefly discussed.

Paper Structure

This paper contains 5 sections, 15 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Kinetic and Stückelberg mixings
  3. String theory implementation
  4. Conclusions
  5. Acknowledgments

Figures (1)

  • Figure 1: Contour plots of decay constant $f_{\xi} (\theta, \varepsilon)$ for $2r_1=2r_2=2k^1=k^2$ (left) and $r_1=2r_2=k^1=2k^2$ (right). The $f_{\xi}$-values range from small (purple) to large (red) following the rainbow contour colors. Unphysical regions with complex $f_{\xi}$ are located in the black band.