Scientists Propose Ultraheavy Nuclei as Explanation for Amaterasu Particle Mystery

Scientists at Penn State University, working with collaborators at institutions including the Yukawa Institute for Theoretical Physics in Japan and Virginia Tech, have proposed a new explanation for the origin of ultrahigh-energy cosmic rays, including the famous Amaterasu particle detected by the Telescope Array in Utah in 2021. Using computer simulations, the research team modeled how particles of different compositions lose energy while traveling through intergalactic space and found that ultraheavy atomic nuclei—heavier than iron—lose energy more slowly than protons or lighter nuclei. This property would allow ultraheavy nuclei to survive the journey across cosmic distances while retaining extreme amounts of energy. The Amaterasu particle, with an energy of approximately 240 exa-electron volts (about 10 million times more energetic than particles in the Large Hadron Collider), has been particularly difficult to explain because its arrival direction points to a cosmic void with no obvious source. The research, published in Physical Review Letters, suggests that identifying the composition of ultrahigh-energy cosmic rays could help scientists pinpoint the extreme astrophysical sources—such as colliding neutron stars or collapsing massive stars—responsible for accelerating these particles.