Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube
Date
2025
Authors
Abbasi, R.
Ackermann, M.
Adams, J.
Agarwalla, S.K.
Aguilar, J.A.
Ahlers, M.
Alameddine, J.M.
Amin, N.M.
Andeen, K.
Argüelles, C.
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Journal article
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Physical Review Letters, 2025; 135(3):031001-1-031001-12
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IceCube Collaboration
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Abstract
We present a search for the diffuse extremely-high-energy neutrino flux using 12.6 years of IceCube data. The nonobservation of neutrinos with energies well above 10 PeV constrains the all-flavor neutrino flux at 1018 eV to a level of E(2)Φ(ν)(e)(+)(ν)(μ)(+)(ν)()τ ≃ 10(−8) GeV cm(−2) s(−1) sr(−1), the most stringent limit to date. Using these data, we constrain the proton fraction of ultrahigh-energy cosmic rays (UHECRs) above ≃30 EeV to be ≲70% (at 90% CL) if the cosmological evolution of the sources is comparable to or stronger than the star formation rate. This is the first result to disfavor the “proton-only” hypothesis for UHECR in this evolution regime using neutrino data. This result complements direct air-shower measurements by being insensitive to uncertainties associated with hadronic interaction models. We also evaluate the tension between IceCube’s nonobservation and the ∼200 PeV KM3NeT neutrino candidate (KM3-230213A), finding it to be ∼2.9σ based on a joint-livetime fit between neutrino datasets.
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© 2025 American Physical Society