On November 27, 2021, a detector array spread across the Utah desert recorded a single subatomic particle carrying about 244 exa-electron volts of energy. The Telescope Array Collaboration named it the Amaterasu particle, after the Japanese sun goddess, and the measurement landed like a thunderclap: nothing in the particle’s arrival direction, a seemingly quiet patch of sky, could plausibly have launched it. Now a peer-reviewed study published in Physical Review Letters argues it has an answer. The Amaterasu particle, and others like it, may be atomic nuclei far heavier than iron, elements potentially as massive as those in the actinide range, flung across hundreds of millions of light-years by the universe’s most violent engines.
If the hypothesis holds, it would rewrite the textbook on ultra-high-energy cosmic rays and narrow the search for whatever natural accelerators are powerful enough to produce them.
A particle that shouldn’t have arrived
To appreciate why the Amaterasu particle is so puzzling, consider the energy involved. The Large Hadron Collider, the most powerful accelerator ever built, smashes protons together at a center-of-mass energy of about 13 trillion electron volts. The Amaterasu particle carried roughly 19 million times that energy in a single nucleus. It is the most energetic cosmic ray detected since the legendary “Oh-My-God” particle recorded by the Fly’s Eye detector in 1991 at an estimated 320 EeV.
Cosmic rays at these energies face a brutal commute. As they cross intergalactic space, they slam into photons left over from the Big Bang, the cosmic microwave background. Protons above about 50 EeV lose energy rapidly through this process, a threshold known as the GZK limit. That interaction caps how far an ultra-high-energy proton can travel before it bleeds away too much punch to remain in the extreme-energy club. The practical horizon for protons at Amaterasu-level energies is relatively short on cosmological scales, yet no known source sits within that horizon along the particle’s arrival direction.
Why heavy nuclei change the math
The new study, led by Penn State astrophysicist Kohta Murase, attacks the problem from the propagation side. Rather than hunting for a nearby source, Murase and colleagues asked: what kind of particle could survive a longer journey and still arrive with 200-plus EeV of energy?
Their answer, detailed in the paper first circulated as a preprint in 2024 and since accepted by Physical Review Letters, is ultraheavy nuclei, elements with atomic mass numbers well above iron’s 56. Think of the upper reaches of the periodic table: barium, platinum, uranium, and beyond. These nuclei interact with background photon fields differently than protons or mid-weight nuclei like carbon or silicon. Crucially, the team’s propagation calculations show that ultraheavy nuclei maintain longer energy-loss lengths up to roughly 300 EeV, meaning they can cross greater stretches of the cosmos before their energy degrades below the detection threshold.
“Ultra-heavy nuclei have a clear survival advantage at the highest energies,” Murase said in a Penn State release describing the work. “This naturally explains why we see particles at energies where protons should not be able to reach us from distant sources.”
That survival advantage also addresses the “empty sky” problem. If the Amaterasu particle traveled farther than any proton could, its source might lie well beyond the local cosmic neighborhood. And because heavier nuclei carry more electric charge, galactic and intergalactic magnetic fields would have deflected them more severely along the way, scrambling the arrival direction so thoroughly that it no longer points back toward the origin.
What the data actually show, and what they don’t
Two layers of evidence support the hypothesis, but a critical gap remains between them.
The first layer is observational. The Amaterasu particle’s energy of 244 ± 29 (statistical) EeV, with additional systematic uncertainties, was reported in a peer-reviewed Science paper. It is a measured event, not a model output. Its energy and arrival direction are on firm empirical ground.
The second layer is the propagation physics published in Physical Review Letters. The calculations showing that ultraheavy nuclei outlast lighter particles at extreme energies rely on well-characterized inputs: the spectrum of the cosmic microwave background, measured photodisintegration cross sections, and established nuclear physics. The modeling is peer-reviewed and internally consistent.
The gap sits between these two layers. No detector has yet identified the nuclear species of the Amaterasu particle or any other individual cosmic ray above 200 EeV. Surface arrays record the cascade of secondary particles, called an extensive air shower, that erupts when a cosmic ray strikes the atmosphere. Those showers encode composition clues: showers from heavy nuclei tend to develop higher in the atmosphere and produce more muons than proton-initiated showers. But extracting a precise nuclear identity from a single event at this energy is beyond current capability.
Broader composition trends measured by both the Telescope Array and the Pierre Auger Observatory suggest that cosmic rays grow heavier on average as energy increases, which is consistent with the ultraheavy picture. But the event rate above 100 EeV is vanishingly small, so those trends rest on limited statistics. The ultraheavy interpretation remains, for now, the best-fitting hypothesis rather than a confirmed fact.
The missing half of the puzzle
Even if ultraheavy nuclei are the right answer to “what,” the question of “where” remains wide open. The study focuses on how these particles survive their journey, not on what launches them in the first place. Accelerating a nucleus heavier than iron to 244 EeV demands an astrophysical environment of extraordinary power and the right chemical composition.
Candidate source classes discussed in the broader literature include tidal disruption events, in which a star is shredded by a supermassive black hole; relativistic jets from active galactic nuclei; and magnetars, neutron stars with extreme magnetic fields. Each has theoretical appeal, but none has been conclusively linked to the highest-energy cosmic rays. Pinning down the source class is the other half of the puzzle, and it will likely require both better composition measurements and improved arrival-direction statistics to solve.
AugerPrime and the next test
The most concrete near-term test will come from AugerPrime, the upgraded Pierre Auger Observatory in Argentina. The upgrade, which has been deploying since 2023, adds scintillator detectors and radio antennas on top of the existing water-Cherenkov stations, along with faster electronics. The new hardware is designed to separately measure the muon and electromagnetic components of each air shower, a separation that directly improves sensitivity to the mass of the incoming cosmic ray.
If AugerPrime’s first high-statistics results above 100 EeV reveal a clear rise in average nuclear mass with energy, the ultraheavy survival model gains strong observational backing. A flat or declining mass trend would challenge it, potentially forcing physicists to consider alternatives: an unidentified nearby accelerator, or perhaps new particle physics at energies no collider can probe.
Either outcome would be a landmark. As of mid-2026, the ultraheavy hypothesis is the most economical explanation for the Amaterasu particle and its rare cousins. It solves the energy-survival problem and the empty-sky problem in a single stroke, using known nuclear physics rather than exotic new theories. But nature gets the final vote, and that vote will be counted one ultra-rare particle at a time, recorded by detectors patient enough to wait for the next cosmic ray that carries the energy of a well-thrown baseball packed into a single atomic nucleus.
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*This article was researched with the help of AI, with human editors creating the final content.
