Revisiting the Chern-Simons interaction during inflation with a non-canonical pseudo-scalar

TL;DR

This paper explores a non-canonical pseudo-scalar during inflation that enhances chiral gravitational wave production while suppressing scalar perturbations, enabling detectable tensor signals without violating non-Gaussianity constraints.

Contribution

It introduces a non-canonical kinetic term for the pseudo-scalar, allowing tensor modes to dominate over scalar modes without conflicting with observational limits.

Findings

Suppressed scalar perturbations due to reduced sound speed

Enhanced chiral gravitational wave production

Tensor modes can dominate without violating non-Gaussianity constraints

Abstract

A Chern-Simons interaction between a pseudo-scalar field and a U(1) gauge field results in the generation of a chiral gravitational wave background. The detection of this signal is contrasted by the fact that this coupling also generates primordial scalar perturbations, on which strong limits exist, particularly at CMB scales. In this study, we propose a new extension of this mechanism characterized by a non-canonical kinetic term for the pseudo-scalar. We find that a decrease of the sound speed of the pseudo-scalar field highly suppresses the sourced scalar with respect to the sourced tensor modes, thus effectively allowing for the production of a greater tensor signal. Contrary to the case of a canonical axion inflaton, it is in this case possible for the sourced tensor modes to dominate over the vacuum ones without violating the non-Gaussianity constraints from the scalar sector, which results in a nearly totally polarized tensor signal at CMB scales. We also study the extension of this mechanisms to the multiple field case, in which the axion is not the inflaton.