Ultraheavy Ultrahigh-Energy Cosmic Rays

B. Theodore Zhang; Kohta Murase; Nick Ekanger; Mukul Bhattacharya; Shunsaku Horiuchi

doi:10.1103/221m-gvs3

Ultraheavy Ultrahigh-Energy Cosmic Rays

B. Theodore Zhang ^1,7,*, Kohta Murase ^2,3,4,1,†, Nick Ekanger ⁵, Mukul Bhattacharya ^2,3,4, and Shunsaku Horiuchi ^5,6,8

¹Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
²Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
³Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
⁴Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
⁵Center for Neutrino Physics, Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA
⁶Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
⁷Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁸Department of Physics, Institute of Science Tokyo, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

^*Contact author: zhangbing@ihep.ac.cn
^†Contact author: murase@psu.edu

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Phys. Rev. Lett. 136, 181002 – Published 7 May, 2026

DOI: https://doi.org/10.1103/221m-gvs3

Abstract

We investigate the propagation of ultraheavy (UH) nuclei as ultrahigh-energy cosmic rays (UHECRs). We show that their energy loss lengths at $≲ 300 EeV$ are significantly longer than those of protons and intermediate-mass nuclei, and that the highest-energy cosmic rays with energies beyond $\sim 100 EeV$ , including the Amaterasu particle, may be UH-UHECRs. For the first time, we derive constraints on the contribution of UH-UHECR sources, and find that the current data are consistent with energy generation rate densities of UHECRs from collapsars and neutron star mergers. Our model predicts that the mean value of the depth of shower maximum is lower than that for iron nuclei beyond 100 EeV, which can be tested with future composition measurements, e.g., AugerPrime and the Global Cosmic Ray Observatory. In addition, the spectral tension between the Telescope Array (TA) and the Pierre Auger Observatory can be alleviated by considering the enhanced contribution of UHECRs—including UH nuclei—from a nearby transient.

Physical Review Letters

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Ultraheavy Ultrahigh-Energy Cosmic Rays

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