Breaking the theoretical limit again! Scientists detect the second most energetic cosmic ray ever

On November 24, 2023, in a study published in the journal Science, researchers announced that they had detected a cosmic ray event with energy that was beyond imagination. The researchers named this cosmic ray " Amaterasu particle " (Amaterasu is the sun goddess in Japanese mythology - Amaterasu). But puzzlingly, no known source can produce particles with such high energy.

Beyond theoretical limits

Cosmic rays are subatomic particles from outside the Earth's atmosphere, such as protons (which are the nuclei of hydrogen atoms), which propagate through space at speeds close to the speed of light.

When discussing the energy of cosmic rays, the most basic unit we use is the electron volt (eV), which refers to the energy gained by an electron when it passes through a potential difference of 1 volt. Cosmic rays have a wide range of energies, and the most common are those with the lowest energies, such as those from the sun. When the energy of a cosmic ray exceeds 1 × 10^18eV, it can be called an ultra-high-energy cosmic ray , which is 1 million times higher than the highest energy that can be achieved by artificial particle accelerators. The origin of ultra-high-energy cosmic rays is generally believed to be related to some of the most extreme phenomena in the universe, such as gamma-ray bursts or relativistic jets around black holes.

So how high can the energy of cosmic rays be? In the 1960s, three scientists proposed a theoretical limit for the energy of cosmic rays: the energy of cosmic rays emitted from 300 million light-years away from the Earth should not exceed 5 × 10^19eV, a value known as the GZK limit (Greisen-Zatsepin-Kuzmin limit). If this limit is exceeded, cosmic rays will interact with the cosmic microwave background as they travel through space, thus continuously losing energy as they travel.

Cosmic Microwave Background: About 380,000 years after the Big Bang, the universe had cooled enough for electrons and atomic nuclei to form stable atoms. This means that photons in the universe no longer scatter with electrons and can propagate freely. These photons still permeate the universe, but their wavelengths have been stretched to the microwave band as the universe expands. (Photo/ESA/Planck Collaboration)

However, in 1991, astronomers detected cosmic rays with energies as high as 3.2 × 10^20eV, which obviously exceeded the theoretical limit, meaning that theoretically it should not come from other distant galaxies, but no celestial body in the Milky Way is capable of producing such high-energy particles. Subsequently, this shocking high-energy particle was named the "Oh-My-God particle."

If you want to produce such high energy in an artificial accelerator, the size of the accelerator needs to be comparable to the orbit of Mercury. Now, the energy of the newly detected Amaterasu particle also exceeds the GZK limit, which is about 2.4 x 10^20eV . This is the highest energy cosmic ray detected after the Oh-My-God particle in more than 30 years. So, how did the researchers capture the Amaterasu particle?

Unique telescope array

When cosmic ray particles from outer space reach the Earth, they hit the Earth's upper atmosphere and collide with the nuclei of oxygen and nitrogen in the atmosphere, triggering a cascade of secondary particles. These so-called "atmospheric showers" contain billions of secondary particles. When they are scattered on the Earth, the surface area covered is huge. An atmospheric shower caused by 1020eV cosmic rays can cover an area as wide as 16 square kilometers on the surface.
Streams of high-energy particles from space pass through the Earth's atmosphere.

Photo: Osaka Metropolitan University/L-INSIGHT, Kyoto University/Ryuunosuke Takeshige

Therefore, we need a detector that covers a large area. The Telescope Array , located outside Delta in the western desert of Utah, is such an experiment. It consists of 507 ground detectors arranged in a square grid, covering an area of ​​700 square kilometers on the surface.

The telescope array is located at an altitude of about 1,200 meters, the best altitude for maximizing the detection of secondary particles. Its location has two advantages: dry air, which is crucial because humidity absorbs the ultraviolet light needed for detection; and an area with excellent dark skies, which is also essential because light pollution interferes with the detection of cosmic rays.

On May 27, 2021, the telescope array detected the Amaterasu particle. The air shower triggered 23 detectors in the northwest region of the telescope array, covering an area of ​​48 square kilometers. By studying the particles detected in the air shower, scientists can reconstruct the energy, mass and arrival direction of the original cosmic ray.

Cosmic rays with lower energy will zigzag under the influence of magnetic fields; however, since ultra-high energy cosmic rays have high kinetic energy, they are less affected by magnetic fields. (Photo/Osaka Metropolitan University/Kyoto University/Ryuunosuke Takeshige)
We know that cosmic rays are charged particles, so on their way to Earth, they will be deflected by the Milky Way and extragalactic magnetic fields. Their propagation paths are a bit like the balls in a pinball machine, zigzagging in the magnetic field. Those cosmic rays with lower energy are more strongly affected by the magnetic field, so their trajectories are almost untraceable. But for "Oh-My-God" and "Amaterasu", they are less affected by the magnetic field, so they will travel relatively smoothly in space. Therefore, following the direction of the cosmic rays, astronomers should be able to easily trace back to their origins.

However, when researchers tried to analyze Amaterasu's origin along its trajectory, they came up empty-handed. Because they calculated that Amaterasu's source seemed to be a region similar to a giant void with almost no galaxies . In other words, they did not find any celestial events that were powerful enough to produce such high energies. This makes these particles particularly mysterious.

Unknown physics?

What's going on? One possible explanation is that the models researchers use to predict how magnetic fields affect the paths of cosmic rays are incorrect and may need some adjustments. Another possibility, which sounds more exciting, is that ultra-high-energy cosmic rays are actually produced by unknown physical processes , allowing them to travel much farther than previously thought.

Next, researchers are upgrading the telescope array to make it four times more sensitive than before. Once completed, 500 new scintillator detectors will capture cosmic rays over an area of ​​2,900 square kilometers. Greater coverage means researchers will have the opportunity to capture more rare ultra-high-energy cosmic rays, thereby tracing their origins more precisely.

Reference Links:

An extremely energetic cosmic ray observed by a surface detector array
https://doi.org/10.1126%2Fscience.abo5095

This article is a work supported by Science Popularization China Starry Sky Project

Team: Principle

Reviewer: Luo Huiqian, Researcher, Institute of Physics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.