Inflationary Gravitational Wave Spectral Shapes as test for Low-Scale Leptogenesis

TL;DR

This paper investigates how low-scale non-thermal leptogenesis affects primordial gravitational wave spectra, proposing that specific spectral features could serve as observational evidence for such leptogenesis scenarios.

Contribution

It introduces a novel connection between gravitational wave spectral shapes and low-scale leptogenesis, highlighting detectable features for future GW experiments.

Findings

Damping and knee-like features in GW spectrum indicate low-scale leptogenesis.

Parameter space identified where these features are compatible with observed baryon asymmetry.

Future GW detectors like LISA and ET can potentially observe these spectral signatures.

Abstract

We study non-thermal resonant leptogenesis in a general setting where a heavy majoron $\phi$ decays to right-handed neutrinos (RHNs) whose further out-of-equilibrium decay generates the required lepton asymmetry. Domination of the energy budget of the Universe by the $\phi$ or the RHNs alters the evolution history of the primordial gravitational waves (PGW) of inflationary origin, which re-enter the horizon after inflation, modifying the spectral shape. The decays of $\phi$ and RHNs release entropy into the early Universe while nearly degenerate RHNs facilitate low and intermediate-scale leptogenesis. A characteristic damping of the GW spectrum resulting in knee-like features would provide evidence for low-scale non-thermal leptogenesis. We explore the parameter space for the lightest right-handed neutrino mass $M_1\in[10^2,10^{14}]$ GeV and washout parameter $K$ that depends on the light-heavy neutrino Yukawa couplings $\lambda$, in the weak ($K < 1$) and strong ($K > 1$) washout regimes. The resulting novel features compatible with observed baryon asymmetry are detectable by future experiments like LISA and ET. By estimating signal-to-noise ratio (SNR) for upcoming GW experiments, we investigate the effect of the majoron mass $M_\phi$ and reheating temperature $T_\phi$, which depends on the $\phi-N$ Yukawa couplings $y_N$.