Cosmic rays are energetic particles that originate from outer space and bombard the Earth’s atmosphere. Most of them are protons or atomic nuclei, but some are electrons or photons. They have a wide range of energies, from a few million electronvolts (eV) to more than 10 20 eV. The origin and nature of these ultra-high-energy cosmic rays are still a mystery to scientists.
The Telescope Array Experiment
The Telescope Array is a large-scale experiment that aims to study the origin and properties of ultra-high-energy cosmic rays. It consists of 507 surface detectors that cover an area of 700 km 2 in the Utah desert, and three fluorescence detectors that observe the night sky. The surface detectors measure the energy and arrival direction of the cosmic rays, while the fluorescence detectors record the light emitted by the air showers produced by the cosmic rays.
The Telescope Array has been operating since 2008 and has detected more than 30 cosmic rays with energies above 10 19 eV. However, none of them have reached the level of the Oh-My-God particle, which was detected by the Fly’s Eye experiment in 1991 and had an energy of 3.2 x 10 20 eV. The Oh-My-God particle is the highest-energy cosmic ray ever observed and its origin is still unknown.
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The Amaterasu Particle: cosmic ray
On May 27, 2021, the Telescope Array detected a cosmic ray with an energy of 2.4 x 10 20 eV, the second-highest ever observed. This particle, dubbed the Amaterasu particle, triggered 23 surface detectors and created a huge air shower that spanned 48 km 2 . Its arrival direction appeared to be from the Local Void, an empty region of space near the Milky Way galaxy.
The Amaterasu particle poses several puzzles for astrophysicists. First, its energy is beyond the theoretical limit predicted by the Greisen-Zatsepin-Kuzmin (GZK) effect, which states that cosmic rays with energies above 5 x 10 19 eV should lose energy by interacting with the cosmic microwave background radiation, the relic of the Big Bang. Second, its source is unclear, as there are no known astrophysical objects that can produce such high-energy particles in the Local Void. Third, its propagation is mysterious, as it should be affected by the magnetic fields in the intergalactic and galactic space, which would deflect its trajectory and reduce its energy.
The Implications and Future Prospects
The Amaterasu particle, along with the Oh-My-God particle and other ultra-high-energy cosmic rays, suggests that there are new phenomena in the universe that are beyond the current understanding of physics. These phenomena could be related to exotic objects, such as supermassive black holes, active galactic nuclei, gamma-ray bursts, or dark matter. Alternatively, they could indicate new physics, such as extra dimensions, quantum gravity, or supersymmetry.
To unravel the mystery of these cosmic rays, more observations and analyses are needed. The Telescope Array is planning to expand its area and sensitivity by adding more detectors and upgrading its electronics. The goal is to increase the exposure and statistics of ultra-high-energy cosmic rays and to identify their sources and mechanisms. The Telescope Array is also collaborating with other experiments around the world, such as the Pierre Auger Observatory in Argentina and the JEM-EUSO mission in space, to share data and expertise.
In summary, the Amaterasu particle is a rare and remarkable discovery that challenges the existing knowledge of particle physics and cosmology. It opens new windows to explore the extreme and unknown aspects of the universe.
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