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  • Open Access

Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw

Rishav Roshan1,* and Sudipta Show2,†

  • *Contact author: r.roshan@soton.ac.uk
  • †Contact author: sudiptas@iitk.ac.in
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Phys. Rev. Lett. 137, 111802 – Published 8 September, 2026

DOI: https://doi.org/10.1103/nfkx-q6z7

Abstract

While most studies of the minimal type-I seesaw neglect the heaviest decoupled right-handed neutrino, assuming negligible contributions to neutrino masses and leptogenesis, we demonstrate that its cosmological role can be significant. When long-lived, the presence of this particle can substantially modify the charged lepton Yukawa equilibration temperature in the early universe, necessitating reassessment of lepton flavor effects in thermal leptogenesis and potentially shifting flavor regime boundaries. Additionally, we identify experimental probes, including neutrino oscillation measurements and gravitational wave observations to investigate this scenario. More broadly, the resulting modification of the cosmic expansion history carries implications beyond leptogenesis, impacting dark matter production and evolution, the matter-antimatter asymmetry generation, and the primordial gravitational wave spectrum.

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Phys. Rev. Lett. 137, 111802– Published 8 September, 2026
  • Received 6 November 2025
  • Revised 19 June 2026
  • Accepted 12 August 2026
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