Neutrino detector rules out cosmic ray origin theory for first time
IceCube's 12-year search found no trace of the neutrinos that should exist if cosmic rays come purely from protons. The finding challenges a leading theory about where the universe's most violent particles originate—and opens new questions for physics, astronomy, and the instruments designed to detect extreme cosmic events.
Originaltitel: Search for Extremely-High-Energy Neutrinos and First Constraints on the Ultrahigh-Energy Cosmic-Ray Proton Fraction with IceCube
<p>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 10<sup>18</sup> eV to a level of 𝐸<sup>2</sup>Φ<sub>𝜈𝑒+𝜈𝜇+𝜈𝜏</sub>≃10<sup>−8 </sup> GeV cm<sup>−2</sup> s<sup>−1</sup> sr<sup>−1</sup>, 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 PeVKM3NeT neutrino candidate (KM3-230213A), finding it to be ∼2.9𝜎 based on a joint-livetime fit between neutrino datasets.</p>