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  • Open Access

Cosmic horizon of neutrinos

James Fardeen1,*, Stefano Profumo1,2,†, and M. Grant Roberts1,2,‡

  • 1Department of Physics, 1156 High Street, University of California Santa Cruz, Santa Cruz, California 95064, USA
  • 2Santa Cruz Institute for Particle Physics, 1156 High Street, Santa Cruz, California 95064, USA

  • *Contact author: jfardeen@ucsc.edu
  • †Contact author: profumo@ucsc.edu
  • ‡Contact author: migrober@ucsc.edu
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Phys. Rev. D 113, 063054 – Published 25 March, 2026

DOI: https://doi.org/10.1103/56hz-5dfm

Abstract

The persistent discrepancy between the experimental measurement and the Standard Model (SM) prediction of the muon’s anomalous magnetic moment (g−2)μ remains one of the most intriguing hints of physics beyond the SM. A well-motivated explanation involves a light Z′ gauge boson associated with a broken U(1)Lμ−Lτ symmetry. Such a boson not only resolves the (g−2)μ anomaly, but also induces resonant interactions between high-energy cosmic neutrinos and the cosmic neutrino background (CνB), potentially shaping the observable neutrino flux at Earth. In this work, we explore the implications of such interactions for the cosmic propagation of high-energy neutrinos. We compute the optical depth for neutrino attenuation via Z′-mediated scattering, accounting for neutrino masses, hierarchies, and thermal distributions. We delineate the regions in (mZ′,mν) space where the optical depth exceeds unity, defining a “neutrino cosmic horizon” beyond which high-energy neutrinos are significantly attenuated. We confront these results with the parameter space required to simultaneously explain the muon g−2 anomaly and ease the Hubble tension via an additional contribution to the effective number of relativistic degrees of freedom, ΔNeff≃0.2–0.5. Our analysis reveals a consistent region in parameter space where all three phenomena [(g−2)μ, Neff, and high-energy neutrino attenuation] can be explained by the same light mediator. These findings motivate future searches for spectral features in IceCube and its next-generation successors as indirect probes of new physics in the neutrino sector.

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Phys. Rev. D 113, 063054– Published 25 March, 2026
  • Received 30 July 2025
  • Accepted 19 February 2026
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