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Ultraheavy Ultrahigh-Energy Cosmic Rays
B Theodore Zhang1,2, Kohta Murase1,3,4,5, Nick Ekanger6
1Kyoto University, Yukawa Institute for Theoretical Physics, Center for Gravitational Physics and Quantum Information, Kyoto, Kyoto 606-8502, Japan.
Physical Review Letters
|May 22, 2026
Summary
Ultraheavy nuclei may explain the highest-energy cosmic rays (UHECRs). These ultraheavy cosmic rays have longer energy loss lengths, consistent with sources like collapsars and neutron star mergers, and can resolve observatory spectral tensions.
Area of Science:
- Astroparticle Physics
- Cosmic Ray Physics
Background:
- Ultrahigh-energy cosmic rays (UHECRs) pose a mystery regarding their origin and composition.
- Understanding the propagation of cosmic rays is crucial for identifying their sources.
Purpose of the Study:
- To investigate the propagation of ultraheavy (UH) nuclei as UHECRs.
- To determine if UH nuclei can explain the highest-energy cosmic rays observed.
- To constrain the contribution of UH-UHECR sources.
Main Methods:
- Simulating the propagation of UH nuclei through intergalactic space.
- Comparing model predictions with observational data from cosmic ray observatories.
- Analyzing energy loss lengths and shower maximum depths.
Main Results:
- UH nuclei exhibit significantly longer energy loss lengths than protons and intermediate-mass nuclei at energies below ~300 EeV.
- The highest-energy cosmic rays (above ~100 EeV), including the Amaterasu particle, may be UH-UHECRs.
- Current data are consistent with UHECRs originating from collapsars and neutron star mergers.
Conclusions:
- UH nuclei are viable candidates for the highest-energy cosmic rays.
- The model predicts a lower mean depth of shower maximum for UH-UHECRs beyond 100 EeV, testable by future experiments.
- Considering UH nuclei from nearby transient sources can alleviate spectral tension between major observatories.
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