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arXiv · 2502.01963

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

IceCube Collaboration·R. Abbasi·M. Ackermann·J. Adams·S. K. Agarwalla·J. A. Aguilar·M. Ahlers·J. M. Alameddine·N. M. Amin·K. Andeen·C. Argüelles·Y. Ashida·S. Athanasiadou·S. N. Axani·R. Babu·X. Bai·A. Balagopal V.·M. Baricevic·S. W. Barwick·S. Bash·V. Basu·R. Bay·J. J. Beatty·J. Becker Tjus·J. Beise·C. Bellenghi·S. BenZvi·D. Berley·E. Bernardini·D. Z. Besson·E. Blaufuss·L. Bloom·S. Blot·F. Bontempo·J. Y. Book Motzkin·C. Boscolo Meneguolo·S. Böser·O. Botner·J. Böttcher·J. Braun·B. Brinson·Z. Brisson-Tsavoussis·J. Brostean-Kaiser·L. Brusa·R. T. Burley·D. Butterfield·M. A. Campana·I. Caracas·K. Carloni·J. Carpio·S. Chattopadhyay·N. Chau·Z. Chen·D. Chirkin·S. Choi·B. A. Clark·A. Coleman·P. Coleman·G. H. Collin·A. Connolly·J. M. Conrad·R. Corley·D. F. Cowen·C. De Clercq·J. J. DeLaunay·D. Delgado·S. Deng·A. Desai·P. Desiati·K. D. de Vries·G. de Wasseige·T. DeYoung·A. Diaz·J. C. Díaz-Vélez·P. Dierichs·M. Dittmer·A. Domi·L. Draper·H. Dujmovic·D. Durnford·K. Dutta·M. A. DuVernois·T. Ehrhardt·L. Eidenschink·A. Eimer·P. Eller·E. Ellinger·S. El Mentawi·D. Elsässer·R. Engel·H. Erpenbeck·W. Esmail·J. Evans·P. A. Evenson·K. L. Fan·K. Fang·K. Farrag·A. R. Fazely·A. Fedynitch·N. Feigl
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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 $10 \, \mathrm{PeV}$ constrains the all-flavor neutrino flux at $10^{18} \, \mathrm{eV}$ to a level of $E^2 \Phi_{\nu_e + \nu_\mu + \nu_\tau} \simeq 10^{-8} \, \mathrm{GeV} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1} \, \mathrm{sr}^{-1}$, the most stringent limit to date. Using these data, we constrain the proton fraction of ultra-high-energy cosmic rays (UHECRs) above $\simeq 30 \, \mathrm{EeV}$ to be $\lesssim 70\,$% (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 UHECRs 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 $\sim 200 \, \mathrm{PeV}$ KM3NeT neutrino candidate (KM3-230213A), finding it to be $\sim 2.9 \sigma$ based on a joint-livetime fit between neutrino datasets.

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IceCube Collaboration, R. Abbasi, M. Ackermann, J. Adams, S. K. Agarwalla, J. A. Aguilar, M. Ahlers, J. M. Alameddine, N. M. Amin, K. Andeen, C. Argüelles, Y. Ashida, S. Athanasiadou, S. N. Axani, R. Babu, X. Bai, A. Balagopal V., M. Baricevic, S. W. Barwick, S. Bash, V. Basu, R. Bay, J. J. Beatty, J. Becker Tjus, J. Beise, C. Bellenghi, S. BenZvi, D. Berley, E. Bernardini, D. Z. Besson, E. Blaufuss, L. Bloom, S. Blot, F. Bontempo, J. Y. Book Motzkin, C. Boscolo Meneguolo, S. Böser, O. Botner, J. Böttcher, J. Braun, B. Brinson, Z. Brisson-Tsavoussis, J. Brostean-Kaiser, L. Brusa, R. T. Burley, D. Butterfield, M. A. Campana, I. Caracas, K. Carloni, J. Carpio, S. Chattopadhyay, N. Chau, Z. Chen, D. Chirkin, S. Choi, B. A. Clark, A. Coleman, P. Coleman, G. H. Collin, A. Connolly, J. M. Conrad, R. Corley, D. F. Cowen, C. De Clercq, J. J. DeLaunay, D. Delgado, S. Deng, A. Desai, P. Desiati, K. D. de Vries, G. de Wasseige, T. DeYoung, A. Diaz, J. C. Díaz-Vélez, P. Dierichs, M. Dittmer, A. Domi, L. Draper, H. Dujmovic, D. Durnford, K. Dutta, M. A. DuVernois, T. Ehrhardt, L. Eidenschink, A. Eimer, P. Eller, E. Ellinger, S. El Mentawi, D. Elsässer, R. Engel, H. Erpenbeck, W. Esmail, J. Evans, P. A. Evenson, K. L. Fan, K. Fang, K. Farrag, A. R. Fazely, A. Fedynitch, N. Feigl. 2025-02-04. Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube. https://doi.org/10.1103/physrevlett.135.031001

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