In a groundbreaking discovery, the Telescope Array experiment has detected the second-highest-energy cosmic ray ever observed, named the "Amaterasu particle," according to an observation published in the journal Science. Led by the University of Utah and the University of Tokyo, the international collaboration of researchers utilized the Telescope Array, consisting of 507 surface detector stations, to capture this rare event.

The Amaterasu particle, detected on May 27, 2021, possesses an astonishing energy level of 2.4 x 10^20 electron volts (eV). To put it into perspective, the energy of this single subatomic particle is equivalent to dropping a brick on your toe from waist height. The event triggered 23 detectors at the north-west region of the Telescope Array, covering an area of 48 km^2 (18.5 mi^2) outside of Delta, Utah.

Researchers have been astonished by ultra-high-energy cosmic rays like the Amaterasu particle since the first detection of the Oh-My-God particle in 1991 by the University of Utah Fly's Eye experiment. The energy levels of these cosmic rays surpass anything previously thought possible within our galaxy or from other galaxies. The origins and mechanisms behind the generation and travel of these particles remain a mystery.

"The particles are so high energy, they shouldn't be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky," said John Matthews, co-spokesperson of the Telescope Array and co-author of the study. "But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there's nothing high energy enough to have produced it. That's the mystery of this-what the heck is going on?"

The Telescope Array experiment is uncovering fascinating insights into the nature of these ultra-high-energy cosmic rays. The study suggests that these phenomena might follow particle physics unknown to science. Astrophysicists have been exploring various theories, including defects in the structure of spacetime or colliding cosmic strings. However, a conventional explanation is yet to be found.

Cosmic rays are particles that result from violent celestial events, stripped down to their subatomic structures and accelerated to nearly the speed of light. Upon reaching Earth's upper atmosphere, cosmic rays collide with oxygen and nitrogen nuclei, generating secondary particles in a cascading shower. The extensive footprint of this secondary shower necessitates the use of large surface detector arrays like the Telescope Array to capture and analyze them.

Tracing the trajectory of cosmic rays, especially ultra-high-energy ones, is challenging due to the influence of galactic and extragalactic magnetic fields. However, particles like the Amaterasu particle, with its colossal energy, can breeze through intergalactic space relatively unaffected by these fields. Only the most powerful cosmic events are believed to be capable of producing such energetic particles.

Theoretical calculations have established the Greisen-Zatsepin-Kuzmin (GZK) cutoff, which places a theoretical limit on the energy that a proton can hold while traveling over long distances. The new Amaterasu particle, with an energy of 2.4 x 10^20 eV, and the Oh-My-God particle, with an energy of 3.2 x 10^20 eV, significantly surpass this cutoff.

Researchers are examining the composition of cosmic rays, analyzing clues that could reveal their origins. The Amaterasu particle is likely a proton, which suggests an unknown particle physics phenomenon at play. Despite intense research, no definite conclusions have been reached regarding the source of these ultra-high-energy cosmic rays.

The Telescope Array, located in Utah's West Desert, has proven to be an ideal observatory for studying ultra-high-energy cosmic rays. Its elevated position allows for maximum development of secondary particles before decay. Additionally, the region's dry air and dark skies facilitate precise detection by minimizing background noise and interference.

Astrophysicists remain intrigued and puzzled by these mysterious cosmic events. The ongoing expansion of the Telescope Array, which includes the deployment of 500 new scintillator detectors, aims to capture more cosmic ray events across a larger area of 2,900 km^2 (1,100 mi^2). This expanded footprint, equivalent to the size of Rhode Island, holds the promise of shedding light on the perplexing nature of ultra-high-energy cosmic rays.

As scientists continue to unravel the secrets of the universe's most energetic particles, the discovery of the Amaterasu particle marks another significant step forward in our understanding of cosmic phenomena.

Reference:
"An extremely energetic cosmic ray observed by a surface detector array." Science (2023). DOI: 10.1126/science.abo5095.