Kinetic Gauge Friction in Natural Inflation

TL;DR

This paper explores a modified natural inflation model with a non-Abelian gauge sector coupled to the axion-inflaton, addressing stability issues and aligning predictions with current CMB observations.

Contribution

It introduces a non-minimal kinetic coupling in natural inflation, solves the instability problem with a Chern-Simons coupling, and demonstrates compatibility with observational data.

Findings

Achieves over 60 e-folds with sub-Planckian decay constants.

Cures perturbation instability via Chern-Simons coupling.

Results are consistent with current CMB constraints.

Abstract

We study an extension of the natural inflation model comprising a non-Abelian gauge sector coupled to the axion-inflaton kinetic term. We show how such non-minimal coupling serves as a source of friction for the rolling inflaton granting sixty or more $e$-folds of accelerated expansion for sub-Planckian values of the axion decay constant. The analysis of perturbations reveals a negative sound speed, thus signaling an instability. Implementing a Chern-Simons-type coupling between the inflaton and gauge sectors cures the instability by delivering a positive speed. We perform a numerical study of scalar and tensor perturbations for a fiducial set of parameters finding that the corresponding observables are compatible with current CMB bounds.