Scientists have discovered that ultraheavy atomic nuclei could explain some of the highest-energy cosmic rays ever observed. The particles may come from extreme events such as neutron-star mergers and collapsing massive stars.
Scientists may have uncovered a new clue behind the origin of the most energetic particles ever detected in the universe.
Ultrahigh-energy cosmic rays are particles from space that slam into Earth with energies far beyond anything produced by human-made particle accelerators. One of the most extreme examples is the “Amaterasu particle,” detected in Utah by the Telescope Array in 2021 and named after the Japanese sun goddess. Its energy rivals that of the famous “Oh-My-God particle” discovered in 1991, but scientists still do not know exactly what it was or where it came from.
New research from Penn State scientists, published in Physical Review Letters, suggests that some of these record-breaking cosmic rays may be made of atomic nuclei heavier than iron. Atomic nuclei are the dense centers of atoms composed of protons and neutrons, containing nearly all of an atom’s mass.
Ultraheavy Nuclei May Explain Extreme Cosmic Rays
The researchers found that these ultraheavy nuclei may lose energy more slowly than protons or lighter nuclei while traveling through intergalactic space. That could allow them to cross enormous cosmic distances and still reach Earth with exceptionally high energies. The work involved scientists from the Yukawa Institute for Theoretical Physics in Japan, Virginia Tech, and several other institutions and could help narrow the search for the cosmic environments capable of accelerating such particles.
“Ultrahigh-energy cosmic rays can only be accelerated by some of the most powerful sources in the universe,” said Kohta Murase, professor of physics and of astronomy and astrophysics in the Penn State Eberly College of Science and the leader of the research team. “When we detect individual cosmic-ray particles such as the Amaterasu particle here on Earth, we can often use their energies, arrival directions, and expected magnetic deflections to infer their possible cosmic sources.”
However, the Amaterasu particle appeared to originate from a giant cosmic void with no obvious source capable of producing ultrahigh-energy cosmic rays.
A 60-Year Mystery in Astrophysics
“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,” Murase said.
These particles carry energies above 100 exa-electron volts, or 100 quintillion electron volts. That makes them about 10 million times more energetic than particles accelerated in the Large Hadron Collider, the world’s most powerful particle accelerator. The Amaterasu particle alone reached roughly 240 exa-electron volts, carrying about the same kinetic energy as a fast-moving tennis ball concentrated into a single particle.
“These highest-energy cosmic rays are thought to come from extreme astrophysical sources, like two neutron stars colliding or a massive star collapsing,” Murase said. “For many cosmic-ray events taken together, their energy distribution, arrival-direction pattern and statistically inferred composition provide important clues about where these particles come from and how they are accelerated.”
Simulating How Particles Travel Across Space
To investigate which particles could survive the journey to Earth at such enormous energies, the researchers ran detailed computer simulations tracking how particles of different masses lose energy while moving through intergalactic space.
“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. “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.”
The study also established new limits on how much these ultraheavy nuclei may contribute to the overall population of ultrahigh-energy cosmic rays detected on Earth.
Black Holes and Neutron Stars as Likely Sources
“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. A contribution from these sources could also help explain a possible difference seen between the northern and southern skies in the ultrahigh-energy cosmic-ray spectrum. If ultraheavy nuclei contribute significantly at the highest energies, future data should indicate a composition heavier than iron.”
Murase said future observatories, including the proposed AugerPrime project in Argentina and the Global Cosmic Ray Observatory, may be able to test these predictions. Additional studies of black holes and strongly magnetized neutron stars could also help scientists better understand where these extraordinary cosmic rays originate.
Reference: “Ultraheavy Ultrahigh-Energy Cosmic Rays” by B. Theodore Zhang, Kohta Murase, Nick Ekanger, Mukul Bhattacharya and Shunsaku Horiuchi, 7 May 2026, Physical Review Letters.
DOI: 10.1103/221m-gvs3
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