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

Zafri A. Borboruah1,*, Anish Ghoshal2,†, Lekhika Malhotra1,‡, and Urjit Yajnik3,1,§

  • *Contact author: zafri123@iitb.ac.in
  • †Contact author: anish.ghoshal@fuw.edu.pl
  • ‡Contact author: lekhika.malhotra@iitb.ac.in
  • §Contact author: yajnik@iitb.ac.in
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Phys. Rev. D 112, 095017 – Published 14 November, 2025

DOI: https://doi.org/10.1103/hbd6-4qp7

Abstract

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∈[102,1014]  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.

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Phys. Rev. D 112, 095017– Published 14 November, 2025
  • Received 4 March 2025
  • Accepted 30 September 2025
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