# Gauss-Bonnet Chern-Simons gravitational wave leptogenesis

## TL;DR

This paper explores how the inclusion of the Gauss-Bonnet term enhances lepton asymmetry generation via gravitational waves in the early universe, without affecting the CMB spectrum.

## Contribution

It introduces the Gauss-Bonnet term into gravitational leptogenesis, revealing exponential amplification of gravitational waves and increased lepton production.

## Key findings

- Lepton production can be exponentially enhanced by the Gauss-Bonnet term.
- CP-violating instability of gravitational waves occurs transiently after inflation.
- The instability does not impact the cosmic microwave background spectrum.

## Abstract

The gravitational Chern-Simons term coupled to an evolving axion is known to generate lepton number through the gravitational anomaly. We examine this leptogenesis scenario in the presence of the Gauss-Bonnet term over and above the gravitational Chern-Simons term. We find that the lepton production can be exponentially enhanced. The Gauss-Bonnet term creates CP-violating instability of gravitational waves that may appear transiently after inflation, and during the period of instability elliptically polarized gravitational waves are exponentially amplified at sub-horizon scales. This instability does not affect the spectrum of the cosmic microwave background as it occurs at much shorter length scales. In a typical scenario based on natural inflation, the observed baryon asymmetry of the Universe corresponds to the UV cutoff scale at $10^{14-16}$ GeV.