Widening the Axion Window via Kinetic and St\"uckelberg Mixings

TL;DR

This paper demonstrates that kinetic and St"uckelberg mixings in axion models can significantly enlarge the axion decay constant range, enabling super-Planckian values even with only two axions, with implications for dark matter.

Contribution

It introduces a minimal two-axion model with mixings that can achieve super-Planckian decay constants without increasing degrees of freedom.

Findings

Effective super-Planckian decay constants are possible with sub-Planckian kinetic matrix entries.

Kinetic mixings can also reduce the decay constant to match axion dark matter requirements.

The model involves a St"uckelberg U(1) and a non-Abelian instanton group, with phenomenological implications.

Abstract

We point out that kinetic and St\"uckelberg 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 St\"uckelberg U(1) and a modest rank instanton generating non-Abelian group. Below the mass of the St\"uckelberg 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.