Inferring the Composition of High-Energy Cosmic Rays
The sources and composition of ultrahigh-energy cosmic rays (UHECRs)—those with energies above 1018 eV—are largely mysterious. Now researchers from the IceCube Neutrino Observatory in Antarctica have used their lack of high-energy neutrino detections to determine that the fraction of UHECRs that are protons is less than 70%, whereas the typical assumption for decades was that these cosmic rays are nearly all protons [1]. Nailing down the composition of UHECRs is an important step toward finding their birthplaces.
UHECRs are protons and heavier nuclei, and they generate high-energy neutrinos when they interact with the photons of the cosmic microwave background during their intergalactic travels. The number of neutrinos generated depends on several factors, including the UHECR particles’ compositions and the distances that the particles travel. During 12.6 years of observation, the IceCube facility at the South Pole recorded no neutrinos with energies above 1016 eV. This nonobservation allowed the collaboration to set an upper limit on the fraction of UHECRs that are protons because a higher proton fraction would have generated more high-energy neutrinos. The upper bound of 70% is incompatible with some well-accepted UHECR theories that will now need revisions. The new neutrino results agree with recent estimates from two large cosmic-ray observatories, but they don’t rely on nuclear physics models with inherent uncertainties that are essential for cosmic-ray observations.
The KM3NeT Collaboration’s recent observation of a cosmic neutrino of 1017 eV is inconsistent with the IceCube data by a statistical measure of 3 (see Research News: An Ultrahigh Neutrino Detection Makes Waves). To understand the discrepancy, data from the next-generation facilities and new analysis techniques are needed, says IceCube Collaboration member Brian Clark of the University of Maryland, College Park.
–David Ehrenstein
David Ehrenstein is a Senior Editor for Physics Magazine.
References
- R. Abbasi et al. (IceCube Collaboration), “Search for extremely-high-energy neutrinos and first constraints on the ultrahigh-energy cosmic-ray proton fraction with IceCube,” Phys. Rev. Lett. 135, 031001 (2025).