Fingerprint of low-scale leptogenesis in the primordial gravitational-wave spectrum

The dynamical generation of right-handed-neutrino (RHN) masses in the early Universe naturally entails the formation of cosmic strings that give rise to an observable signal in gravitational waves (GWs). In this paper, we show that a characteristic break in the GW spectrum would provide evidence for a nonstandard stage in the cosmological expansion history and a suppression of the RHN mass scale compared to the scale of spontaneous symmetry breaking. The detection of such a spectral feature would thus represent a unique possibility to probe the physics of RHN mass generation in regions of parameter space that allow for low-scale leptogenesis in accord with electroweak naturalness.