# Parity breaking signatures from a Chern-Simons coupling during inflation: the case of non-Gaussian gravitational waves

## TL;DR

This paper investigates how a Chern-Simons coupling during inflation can produce parity-breaking signatures in gravitational waves, especially in the bispectrum, providing potential observational avenues beyond power spectrum analysis.

## Contribution

It introduces the analysis of parity-breaking signatures in the bispectrum of gravitational waves due to a Chern-Simons term, highlighting their dependence on the coupling function's second derivative.

## Key findings

- Tensor-tensor-scalar bispectra are the only unsuppressed parity-breaking signatures.
- Amplitude of these bispectra is proportional to the second derivative of the coupling function.
- Maximum signals occur in the squeezed limit.

## Abstract

Considering high-energy modifications of Einstein gravity during inflation is an interesting issue. We can constrain the strength of the new gravitational terms through observations of inflationary imprints in the actual universe. In this paper we analyze the effects on slow-roll models due to a Chern-Simons term coupled to the inflaton field through a generic coupling function $f(\phi)$. A well known result is the polarization of primordial gravitational waves (PGW) into left and right eigenstates, as a consequence of parity breaking. In such a scenario the modifications to the power spectrum of PGW are suppressed under the conditions that allow to avoid the production of ghost gravitons at a certain energy scale, the so-called Chern-Simons mass $M_{CS}$. In general it has been recently pointed out that there is very little hope to efficiently constrain chirality of PGW on the basis solely of two-point statistics from future CMB data, even in the most optimistic cases. Thus we search if significant parity breaking signatures can arise at least in the bispectrum statistics. We find that the tensor-tensor-scalar bispectra $\langle \gamma \gamma \zeta \rangle$ for each polarization state are the only ones that are not suppressed. Their amplitude, setting the level of parity breaking during inflation, is proportional to the second derivative of the coupling function $f(\phi)$ and they turn out to be maximum in the squeezed limit. We comment on the squeezed-limit consistency relation arising in the case of chiral gravitational waves, and on possible observables to constrain these signatures.