Superheavy particles may help explain one of the most puzzling mysteries in modern astrophysics: the origin of the most energetic particles ever detected.
Ultra-high-energy cosmic rays are particles that hit Earth from space with energies far exceeding those produced by human-made particle accelerators. Among the most unusual examples is the “Amaterasu particle,” detected by a telescope array in Utah in 2021 and named after the sun goddess in Japanese mythology. Its reported energy ranks it as one of the most powerful cosmic ray phenomena ever observed, placing it in the same rare category as the “Oh my God particle” recorded in 1991. But scientists still don’t know where it came from or even what exactly it was.
super heavy cosmic rays
A new study led by scientists at Pennsylvania State University physical review letter This suggests that some of the highest-energy cosmic rays may be nuclei heavier than iron. The nucleus is the compact center of an atom and is made up of protons and neutrons. They hold almost all of an atom’s mass but occupy only a small portion of its total volume.
The researchers calculated that these superheavy nuclei may lose energy more slowly than protons or lighter nuclei as they travel through intergalactic space. This means they can survive the journey to Earth while carrying extreme energy with them. The study, conducted with collaborators at Japan’s Yukawa Institute for Theoretical Physics, Virginia Tech and other institutions, could help scientists identify the types of cosmic objects powerful enough to launch such particles.
“Ultra-high-energy cosmic rays are only accelerated by some of the most powerful sources in the universe,” said Kota Murase, professor of physics and astronomy and astrophysics at Penn State Eberly College of Science and leader of the research team. “When we detect individual cosmic ray particles like the Amaterasu particles here on Earth, we can often use their energy, direction of arrival, and expected magnetic deflection to infer their likely cosmic ray source.”
The mystery of Amaterasu particles
The Amaterasu particle is particularly difficult to explain because its presumed direction of arrival traces back to the cosmic cavity, a region of space with no clear source capable of producing ultra-high-energy cosmic rays.
“The origin and acceleration mechanism of ultrahigh-energy cosmic rays has been one of the greatest mysteries in the field for more than 60 years, ever since the first examples were reported,” Murase said.
These rare particles can exceed 100 exaelectronvolts, or 100 quintillion electronvolts. The particles are therefore about seven orders of magnitude more energetic, or 10 million times more energetic, than those accelerated in the Large Hadron Collider, the world’s largest and most powerful particle accelerator. The Amaterasu particle is reported to have about 240 exaelectronvolts, which is about the same amount of kinetic energy in one small cosmic ray particle as a fast-moving tennis ball. This makes it one of the most energetic cosmic rays ever detected.
“These highest-energy cosmic rays are thought to come from extreme astrophysical sources, such as two neutron stars colliding or a massive star collapsing,” Murase said. “Taken together for many cosmic ray phenomena, their energy distribution, direction of arrival pattern, and statistically estimated composition provide important clues about where these particles come from and how they are accelerated.”
Extreme particle simulation
To investigate what kind of particles could reach Earth with such extraordinary energies, researchers ran detailed computer simulations. They modeled how particles of different sizes gain or lose energy as they travel through intergalactic space.
“Our study shows that at energies comparable to the Amaterasu particle, superheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, can survive cosmic distances, and reach Earth even at extreme energies,” Murase said. “We’re not saying that all very high-energy cosmic rays are superheavy nuclei. But if some of the highest-energy events are superheavy, that would have implications for how we search for their sources.”
The team’s calculations also set new limits on how much these superheavy nuclei contribute to the total amount of ultrahigh-energy cosmic rays observed.
Origin of a violent universe
“The most promising locations for the creation and acceleration of such superheavy nuclei are mass deaths of stars with explosive collapse into black holes or strongly magnetized neutron stars, and binary neutron star mergers known as powerful gravitational wave emitters,” Murase said. “These violent cosmic phenomena can also give rise to gamma-ray bursts, which are among the most energetic explosions in the universe. Contributions from these sources may also help explain possible differences seen in the northern and southern skies in ultra-high-energy cosmic ray spectra. If superheavy nuclei make a large contribution at the highest energies, future data should show compositions heavier than iron.”
Future observatories may be able to test these ideas. Murase said next-generation facilities such as the proposed Auger Prime in Argentina and the proposed Global Cosmic Ray Observatory could look for predicted signatures. Further theoretical research into cosmic explosions involving black holes and highly magnetized neutron stars could also help reveal where ultra-high-energy cosmic rays originate.
In addition to Murase, the research team also included B. Theodore Chan, a postdoctoral fellow at Kyoto University’s Yukawa Institute for Theoretical Physics and a former postdoctoral fellow at Pennsylvania State University at the time of the research. Mukul Bhattacharya, Eberly Postdoctoral Fellow at Penn State University at the time of the study. and Nick Ekanger and Shunsaku Horiuchi, who were at Virginia Tech at the time of the investigation.
