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  • Open Access

Does thermal leptogenesis in a canonical seesaw scenario rely on initial memory?

Partha Kumar Paul*, Narendra Sahu†, and Shashwat Sharma‡

  • *Contact author: ph22resch11012@iith.ac.in
  • †Contact author: nsahu@phy.iith.ac.in
  • ‡Contact author: ph23resch11016@iith.ac.in
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Phys. Rev. D 113, 095003 – Published 4 May, 2026

DOI: https://doi.org/10.1103/kybs-clk7

Abstract

It is a common lore that in thermal leptogenesis within the type-I seesaw framework and a hierarchical spectrum of heavy right-handed neutrinos (RHNs), the CP-violating, out-of-equilibrium decay of the lightest RHN (N1) is the only relevant source of the final B−L asymmetry, since any asymmetry produced by the heavier RHNs is expected to be erased by subsequent N1-mediated washout processes. In this work, we revisit this assumption by solving the density-matrix equations, including decay, inverse decay, and relevant scattering processes, and by fully accounting for flavor-projection effects induced by the Yukawa coupling structure. We show that the asymmetries generated by the heavier RHNs (N2 and N3) generally possess components that are misaligned in flavor space with respect to N1, resulting in a partially protected contribution that survives the N1 washout. Unlike the conventional picture of N2-dominated leptogenesis, this memory effect arises even when N1 remains dynamically relevant and cannot be captured within the classical Boltzmann framework. Furthermore, imposing consistency with low-energy neutrino mass and mixing data, we find that at most one RHN can lie in the weak washout regime, which naturally divides the parameter space into four distinct dynamical regimes. We systematically quantify the memory effect in each regime and demonstrate that it can significantly modify the final B−L asymmetry. We find that including projection effects can indeed extend the viable parameter space into the sensitivity range of neutrinoless double beta decay experiments.

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Phys. Rev. D 113, 095003– Published 4 May, 2026
  • Received 22 May 2025
  • Revised 9 December 2025
  • Accepted 10 April 2026
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