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Probing a New Regime of Neutrino Self-Interactions with Astrophysical Neutrinos and the Relativistic Cosmic Neutrino Background

Isaac R. Wang1,*, Xun-Jie Xu2,†, and Bei Zhou1,3,‡

  • *Contact author: isaacw@fnal.gov
  • †Contact author: xuxj@ihep.ac.cn
  • ‡Contact author: beizhou@fnal.gov
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Phys. Rev. Lett. 135, 181002 – Published 28 October, 2025

DOI: https://doi.org/10.1103/9ddp-j1z9

Abstract

Neutrino self-interactions beyond the standard model have profound implications in astrophysics and cosmology. In this Letter, we study an uncharted scenario in which one of the three neutrino species has a mass smaller than the temperature of the cosmic neutrino background. This results in a relativistic component that significantly broadens the absorption feature on the astrophysical neutrino spectra, in contrast to the sharply peaked absorption expected in the extensively studied scenarios assuming a fully nonrelativistic cosmic neutrino background. By solving the Boltzmann equations for neutrino absorption and regeneration, we demonstrate that this mechanism provides novel sensitivity to sub-keV mediator masses, well below the traditional ∼1–100  MeV range. Future observations of the diffuse supernova neutrino background with Hyper-Kamiokande could probe coupling strengths down to g∼10−8, surpassing existing constraints by orders of magnitude. These findings open new directions for discoveries and offer crucial insights into the interplay between neutrinos and the dark sector.

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Outline

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Phys. Rev. Lett. 135, 181002– Published 28 October, 2025
  • Received 23 January 2025
  • Revised 28 June 2025
  • Accepted 10 September 2025
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