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Maximal Parameter Space of Sterile Neutrino Dark Matter with Lepton Asymmetries

Kensuke Akita1,*, Koichi Hamaguchi1,†, and Maksym Ovchynnikov2,‡

  • 1Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
  • 2Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland

  • *Contact author: kensuke@hep-th.phys.s.u-tokyo.ac.jp
  • †Contact author: hama@hep-th.phys.s.u-tokyo.ac.jp
  • ‡Contact author: maksym.ovchynnikov@cern.ch
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Phys. Rev. Lett. 136, 111001 – Published 20 March, 2026

DOI: https://doi.org/10.1103/d2qg-clrz

Abstract

We delineate the maximal parameter space of sterile neutrino dark matter in the presence of lepton-flavor asymmetries. We focus on large flavor asymmetries with vanishing total lepton asymmetry, which are washed out by neutrino oscillations at MeV temperatures and hence are consistent with big bang nucleosynthesis and cosmic microwave background constraints. We derive a semiclassical Boltzmann equation for sterile neutrinos applicable in this regime and validate it against quantum kinetic equations. For sterile neutrino masses up to 60 keV, the viable range of mixing angles extends by up to 2 orders of magnitude, with broad prospects for tests in forthcoming x-ray, cosmic microwave background, and structure formation observations. We also release a public framework to compute the production of sterile neutrinos, and in particular their momentum distribution, enabling dedicated structure formation analyses.

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Phys. Rev. Lett. 136, 111001– Published 20 March, 2026
  • Received 31 July 2025
  • Revised 20 October 2025
  • Accepted 13 February 2026
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