Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube
(2025) Physical Review Letters — Vol. 135 (2025)
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- 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 <math display="inline"><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>18</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math> to a level of <math display="inline"><mrow><msup><mrow><mi>E</mi></mrow><mrow><mn>2</mn></mrow></msup><msub><mrow><mi mathvariant="normal">Φ</mi></mrow><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub><mo>+</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>+</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></msub><mo>≃</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>8</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mo>-</mo><mn>2</mn></mrow></msup><mtext> </mtext><msup><mrow><mi mathvariant="normal">s</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup><mtext> </mtext><msup><mrow><mi>sr</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math>, the most stringent limit to date. Using these data, we constrain the proton fraction of ultrahigh-energy cosmic rays (UHECRs) above <math display="inline"><mrow><mo>≃</mo><mn>30</mn><mtext> </mtext><mtext> </mtext><mrow><mi>EeV</mi></mrow></mrow></math> to be <math display="inline"><mo>≲</mo><mn>70</mn><mo>%</mo></math> (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 <math display="inline"><mo>∼</mo><mn>200</mn><mtext> </mtext><mtext> </mtext><mi>PeV</mi></math> KM3NeT neutrino candidate (KM3-230213A), finding it to be <math display="inline"><mo>∼</mo><mn>2.9</mn><mi>σ</mi></math> based on a joint-livetime fit between neutrino datasets.
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de Wasseige, G., Genton, E., Kruiswijk, K., Lamoureux, M., Lazar, J., Raab, C., Myhr, P., Vereecken, M., (IceCube Collaboration)§, IceCube, & et al. (2025). Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube. Physical Review Letters, 135. https://doi.org/10.1103/PhysRevLett.135.031001 (Original work published 2025)