The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years.
Cosmic rays are high-energy particles traveling from space. The most powerful among them—known as ultrahigh-energy cosmic rays (UHECRs) —carry energies millions of times greater than the particles accelerated inside the Large Hadron Collider (LHC).
Notable examples include the 1991 "Oh-My-God particle" and the 2021 "Amaterasu particle," which carried 40 million times the energy of LHC collisions. Their exact origins remain unknown.
A New Clue: The Heaviest Nuclei
A new study led by Penn State's Kohta Murase suggests a breakthrough: the highest-energy cosmic rays may actually be atomic nuclei heavier than iron.
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.
How They Tested This
The team performed detailed simulations tracking how cosmic rays of different masses lose energy during their journey to Earth. The results showed that ultraheavy nuclei lose energy slower than lighter particles, making them more likely to survive and reach Earth at extreme energies.
What This Means for Finding Sources
If confirmed, this discovery would help pinpoint the sources of UHECRs. The most promising candidates include:
- Massive star deaths that collapse into black holes or strongly magnetized neutron stars
- Binary neutron star mergers, which also produce gamma-ray bursts and gravitational waves
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.
A Possible Explanation for a Puzzling Observation
This hypothesis could also explain the observed differences in the UHECR spectrum between the northern and southern skies—a long-standing mystery in astrophysics.