Axion USR Inflation

TL;DR

This paper explores a model of axion-driven inflation with an intermediate USR phase, where gauge field interactions produce distinctive features in the power spectrum and non-Gaussianities, offering new insights into early universe dynamics.

Contribution

It introduces a novel inflationary scenario with a rolling axion and gauge field interactions, leading to a unique double-peak power spectrum and complex non-Gaussian features.

Findings

Power spectrum exhibits two peaks at different scales.

Intermediate scales show strong scale dependence with index m > 4.

Non-Gaussianities have non-trivial shapes and multiple peaks.

Abstract

We consider a model of inflation in which the inflaton field is a rolling axion with a potential which is flat enough to support an intermediate phase of USR inflation. Because of the Chern-Simons interaction, one polarization of the gauge field experiences the tachyonic growth during the first slow-roll stage, inducing large corrections in curvature perturbations via the inverse decay effect. A non-trivial feature of our setup is that once the system enters the USR phase, the instability parameter falls off rapidly, terminating the gauge field production efficiently. Consequently, the power spectrum involves two separate peaks, the first peak is induced by the gauge field particles production while the second peak is due to standard USR mechanism. We show that the power spectrum at the intermediate scales develops strong scale-dependence $\propto k^m$ with the index $m >4$. Calculating the bispectrum, we demonstrate that non-Gaussianities with non-trivial shapes and multiple peaks are generated in this setup.