Inflationary gravitational wave spectral shapes as a test for low-scale leptogenesis

We study nonthermal resonant leptogenesis in a general setting, where a heavy Majoron ϕ 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 ϕ or the RHNs alters the evolution history of the primordial gravitational waves (GWs) of inflationary origin, which re-enter the horizon after inflation, modifying the spectral shape. The decays of ϕ 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 kneelike features would provide evidence for low-scale nonthermal leptogenesis. We explore the parameter space for the lightest right-handed neutrino mass M1 ∈½102; 1014 GeV and washout parameter K that depends on the light-heavy neutrino Yukawa couplings λ, 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 Einstein Telescope. By estimating the signal-to-noise ratio for upcoming GW experiments, we investigate the effect of the Majoron mass Mϕ and reheating temperature Tϕ, which depends on the ϕ − N Yukawa couplings yN.