For more than 60 years, physicists have chased one of the universe's most stubborn puzzles: where do the most energetic particles ever detected come from? A new study published May 7, 2026, in Physical Review Letters by a Penn State–led team now offers a new answer — and it upends a decades-old assumption about what those particles are actually made of.
The study proposes that some of the highest-energy cosmic rays ever recorded may consist of atomic nuclei heavier than iron — a category the researchers call "ultraheavy." If confirmed, the finding would not only revise what physicists think these particles are, but would dramatically narrow the list of cosmic environments capable of producing them.
"The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years, since the first example was reported," said Kohta Murase, professor of physics and of astronomy and astrophysics at Penn State's Eberly College of Science and the leader of the research team.
What Are Ultrahigh Energy Cosmic Rays?
Cosmic rays are subatomic particles — mostly protons and atomic nuclei — that travel through space at nearly the speed of light and regularly collide with Earth's atmosphere. Ultrahigh-energy cosmic rays occupy an entirely different league. With energies above 100 exa-electron volts — roughly 100 quintillion electron volts — they arrive packing about 10 million times more energy than particles produced at CERN's Large Hadron Collider, the world's most powerful particle accelerator.
The most dramatic example in recent years is the so-called "Amaterasu particle," detected by the Telescope Array experiment in Utah in May 2021 and formally reported in Science in November 2023. Named after the sun goddess in Japanese mythology, it arrived with an estimated energy of about 244 exa-electron volts — roughly the kinetic energy of a fast-moving tennis ball, concentrated in a single subatomic particle. That places it among the highest-energy cosmic rays ever observed, second only to the "Oh-My-God particle" detected in 1991.
There was one immediate problem. The inferred direction of the Amaterasu particle pointed back to a cosmic void — an almost empty region of space bordering the Milky Way called the Local Void — with no obvious source capable of generating such energy. A separate February 2026 study from the Max Planck Institute for Physics has since suggested a possible origin in M82, a nearby star-forming galaxy about 12 million light-years away. But the Amaterasu particle's fundamental identity — what kind of nucleus it actually is — remained unresolved. The new Penn State research directly addresses that gap.
What the Ultraheavy Hypothesis Changes
To test whether ultraheavy nuclei could account for the most extreme cosmic-ray events, Murase's team ran detailed computational simulations of how particles of different masses and sizes behave as they cross intergalactic distances. The central finding: ultraheavy nuclei — those heavier than iron — lose energy far more slowly than protons or intermediate-mass nuclei as they travel through the radiation environment of intergalactic space.
This matters because of a well-established constraint called the Greisen-Zatsepin-Kuzmin (GZK) limit, a theoretical upper bound on how much energy a proton can carry across the vast distances between galaxies before interactions with the cosmic microwave background radiation bleed away its energy. Heavier nuclei interact differently with that background radiation and can in principle carry far greater energies over far greater distances without being stripped down.
"Our research showed that at energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, making them better able to survive cosmic distances and reach Earth at extreme energies," Murase said.
Critically, Murase's team is not claiming that all ultrahigh-energy cosmic rays are ultraheavy. The argument is more targeted: at the very highest energies — above about 100 exa-electron volts — ultraheavy nuclei are the better physical candidate, and their presence would reshape how researchers interpret the overall cosmic-ray population.
"We are not saying that all ultrahigh-energy cosmic rays are ultraheavy nuclei. But if some of the highest-energy events are ultraheavy nuclei, that would impact how we search for their sources," Murase said.
Where Do Cosmic Rays Come From?
One of the most consequential implications of the ultraheavy hypothesis is the constraint it places on candidate source environments. Accelerating nuclei as massive as these to such extreme energies demands environments of extraordinary magnetic power and density — and that constraint is actually a useful filter.
The team's analysis points to three leading candidates: the collapse of massive stars into black holes (known as collapsars), the formation of strongly magnetized neutron stars known as magnetars, and the merger of binary neutron-star systems — events already recognized as among the most energetic in the universe and as the engines behind many gamma-ray bursts.
"The most promising sites for producing and accelerating such ultraheavy nuclei are massive star deaths involving explosive collapse into black holes or strongly magnetized neutron stars, as well as binary neutron-star mergers known to be powerful gravitational-wave emitters," Murase said. "These violent cosmic phenomena can also power gamma-ray bursts that are among the most energetic explosions in the universe."
The team also noted that a contribution from ultraheavy nuclei could help explain a separate observational puzzle: a possible asymmetry in the ultrahigh-energy cosmic-ray flux between the northern and southern hemispheres of the sky.
Cosmic Ray Composition: An Open Scientific Debate
The Penn State hypothesis enters a field already animated by measurement disagreement. The Telescope Array experiment — which detected the Amaterasu particle — and the Pierre Auger Observatory in Argentina have historically reported different pictures of cosmic ray composition at the highest energies. Telescope Array data have generally supported a proton-dominated spectrum at the top of the energy scale, while Auger data have suggested a composition that grows progressively heavier with increasing energy — consistent with nuclei rather than bare protons. The Murase team's ultraheavy model is broadly compatible with the Auger picture and adds a new hypothesis specifically for the most extreme events.
How AugerPrime Will Test This
The findings arrive at a moment when observational tools are becoming powerful enough to adjudicate this debate. The Pierre Auger Observatory is currently operating an upgraded configuration called AugerPrime, which has been collecting enhanced data since 2022 and is designed specifically to measure the mass composition of individual cosmic rays on a shower-by-shower basis — the precise capability needed to test whether particles heavier than iron are present at the highest energies.
"If ultraheavy nuclei contribute significantly at the highest energies, future data should indicate a composition heavier than iron," Murase said, adding that the proposed Global Cosmic Ray Observatory could provide additional testing capability.
The theoretical study published in Physical Review Letters was a collaboration across six institutions: Kyoto University's Center for Gravitational Physics and Quantum Information, the Penn State Institute for Gravitation and the Cosmos, the Center for Neutrino Physics at Virginia Tech, the Kavli Institute for the Physics and Mathematics of the Universe, the CAS Institute of High Energy Physics, and the Institute of Science Tokyo. Lead author B. Theodore Zhang was a postdoctoral researcher at Kyoto University's Yukawa Institute for Theoretical Physics at the time of the study.
If the ultraheavy hypothesis survives testing by AugerPrime and next-generation observatories, it would not just identify what the Amaterasu particle is — it would redirect the entire search for the universe's most extreme accelerators toward the most catastrophic events the cosmos produces.
Frequently Asked Questions
Where did the Amaterasu particle come from?
The Amaterasu particle's origin remains uncertain. When detected in 2021 by the Telescope Array in Utah, its arrival direction pointed back toward the Local Void — a nearly empty region of space bordering the Milky Way — with no obvious source. A February 2026 study from the Max Planck Institute for Physics has since proposed that the particle may have originated in M82, a star-forming galaxy about 12 million light-years away. The new Penn State research suggests that if the particle is made of an ultraheavy nucleus, its origin environment would need to be an extremely powerful event such as a collapsar or a neutron star merger.
What is the GZK limit and why does it matter for cosmic ray composition?
The Greisen-Zatsepin-Kuzmin (GZK) limit is a theoretical energy ceiling for protons traveling across intergalactic distances. Above about 50 exa-electron volts, protons lose energy rapidly by interacting with photons of the cosmic microwave background, which limits how far and how energetically they can travel. Heavier nuclei interact differently with this radiation and can carry far more energy across greater distances without being slowed down — which is why the Penn State team argues that the most extreme cosmic-ray events are better explained by ultraheavy nuclei than by protons.
What is the most energetic cosmic ray ever detected?
The most energetic cosmic ray ever recorded is the "Oh-My-God particle," detected in 1991 by the University of Utah's Fly's Eye experiment with an estimated energy of around 320 exa-electron volts. The Amaterasu particle, detected in 2021 by the Telescope Array experiment in Utah, is the second-highest-energy cosmic ray ever observed, carrying roughly 244 exa-electron volts — about 40 million times the energy of particles produced in the Large Hadron Collider.
How will scientists confirm whether ultrahigh energy cosmic rays are heavier than iron?
The Pierre Auger Observatory in Argentina is currently running an upgraded detector configuration called AugerPrime, which began full-scale data collection in 2022 and is designed to measure the mass composition of individual cosmic-ray events. If a significant fraction of the highest-energy cosmic rays are ultraheavy nuclei, AugerPrime's composition data should show results heavier than iron at extreme energies — directly testing the Penn State team's prediction.
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