Plateau observatory ready to advance knowledge of universe

Having passed appraisal, facility to help shed light on high-energy cosmic rays

China's Large High-Altitude Air Shower Observatory, one of the world's most advanced cosmic ray observatories, passed national appraisal on Wednesday. The facility will help scientists uncover the origins of high-energy cosmic rays and expand humanity's knowledge of the universe.

Cosmic rays are high-energy particles consisting mainly of protons and atomic nuclei that travel through space at nearly the speed of light. Since their discovery in 1912, such rays have fascinated scientists because their electric charge, energy, speed and other properties can contain critical scientific information about their origins and that of the universe.

The facility, also known as LHAASO, sits at an altitude of 4,410 meters on Haizi Mountain in Daocheng county, Sichuan province.

The observatory is comprised of 5,216 electromagnetic particle detectors, 1,188 muon detectors, a 78,000-square-meter Cherenkov water detector array and 18 wide-field-of-view Cherenkov telescopes. Construction began in 2017 and was completed in 2021.

A muon is a type of elementary particle that serves as one of the basic building blocks of the universe. The water detector array is used to detect Cherenkov radiation, which is the form of energy that one can perceive as blue light emitted when charged particles move faster than light in a specific medium, such as water.

Cao Zhen, chief scientist of LHAASO and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said the facility is one of China's key national science and technology infrastructures and one of the most advanced observatories in the world for researching high-energy cosmic rays.

"It represents the cutting-edge of China's science infrastructure engineering, and we hope it can make groundbreaking discoveries and contribute to human civilization's understanding of the universe," he said.

"Studying cosmic rays is vital for understanding the universe's origins, the evolution of stellar bodies, solar activities and other major scientific questions. They are like the couriers of the universe."

By researching cosmic rays, scientists have discovered numerous fundamental particles, including muons and positrons — the antiparticle of electrons with a positive charge. It led to the creation of particle physics, and five Nobel Prizes were given to scientists working in this field.

However, Cao said that despite over a century of research, scientists still know little about the origins of cosmic rays and how some of them can accelerate to such mind-blowing energy levels.

Some cosmic rays that hit Earth had energies thousands of times greater than those produced by the largest human-made particle accelerator, the Large Hadron Collider in Geneva, Switzerland. Scientists have hypothesized certain celestial phenomena, such as black holes or supernovas, as origins, but no one has confirmed this conclusively.

One of the reasons is that these cosmic particles carried electric charges and were susceptible to alteration by magnetic forces as they passed through space. So when they finally reached Earth, scientists had difficulty tracing their source, Cao said.

Another reason is that the higher the energy, the rarer and more challenging the cosmic ray is to detect. Moreover, the Earth's atmosphere can absorb most cosmic rays upon entering.

The ideal way to catch high-energy cosmic rays is in outer space, but creating a massive detector in space is unfeasible, so scientists opted for the next best option: mountain tops.

He Huihai, chief technician of LHAASO, said when an ultrahighenergy particle hits the atmosphere, it can produce an extensive cascade of ionized particles that fall to the ground in a phenomenon called cosmic ray air shower.

"By capturing and studying these secondary particles, we can piece together information about the energy and direction of the original particle," he said. "This is why we need to pack so many different types of detectors across a large area so that we can collect as much data from the air shower as we can before the secondary particles decay to much lower energies."

He said the location, size and design of LHAASO give it an "unprecedented ability to detect ultrahigh-energy cosmic rays and will open a new window for scientists to study the most energetic particles in the universe."

Even when LHAASO was partially operational in 2019, it made major discoveries that were later published in top scientific journals such as Nature and Science, He said.

In May 2021, the journal Nature reported that LHAASO had found 12 sources of ultrahigh-energy photons and detected a photon with a record-breaking energy level of 1.4 peta-electron volts, or 1.4 million billion electron volts.

The photon packed an energy level 200 times higher than what can be achieved by the Large Hadron Collider. It is the highest recorded energy photon to reach Earth to date.

In July 2021, with the help of LHAASO, an international team of scientists said they had found proof that the Crab Nebula is housing an ultrapowerful particle accelerator that can accelerate an electron to 2.3 peta-electron volts, according to a study published in the journal Science.

Dong Yuhui, deputy director of the Institute of High Energy Physics of the Chinese Academy of Sciences, said LHAASO has cemented China's position as one of the global leaders in high-energy astrophysics.

"It will facilitate China's development in particle physics, leading to more original breakthroughs and spearheading interdisciplinary and international research," he said.