Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube

Document Type

Article

Publication Date

7-2025

Publisher

American Physical Society

Source Publication

Physical Review Letters

Source ISSN

0031-9007

Original Item ID

DOI: 10.1103/PhysRevLett.135.031001

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 𝐸2⁢Φ𝜈𝑒+𝜈𝜇+𝜈𝜏≃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  ∼2.9⁢𝜎 based on a joint-livetime fit between neutrino datasets.

Comments

Physical Review Letters, Vol. 135, No. 3 (July 2025): 031001. DOI.

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