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Giant structure at high altitude transforms Chinese mountain into laboratory for extreme particles, combining thousands of detectors, purified water, and telescopes to capture rare signals from the cosmos and advance the search for the origin of the most energetic cosmic rays ever recorded.
At over 4,400 meters above sea level, on Mount Haizi, in Daocheng, Sichuan Province, the LHAASO observatory has established China in a strategic area of astrophysics by bringing together unprecedented scale, continuous operation, and sensitivity focused on the most energetic phenomena known.
According to the Chinese Academy of Sciences, the facility occupies about 1.36 square kilometers and combines ground detectors, water reservoirs, and wide-field telescopes to track rare particles and photons that reach Earth after traversing the cosmos.
Structure of the LHAASO Observatory
The complex was designed to observe the so-called atmospheric showers, cascades of particles produced when cosmic rays and ultra-high-energy gamma rays hit the atmosphere, leaving indirect traces that need to be reconstructed with great precision by instruments spread over a large area.
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This task requires an uncommon architecture even among large scientific projects, because the sought-after events are rare and blend with an immense volume of more common particles, making it essential to measure direction, energy, and composition using different techniques simultaneously.
How the LHAASO Detectors Work
The main array of the observatory is the KM2A, installed over an area of one square kilometer, with 5,216 electromagnetic particle detectors and 1,188 muon detectors, a combination used to better separate signals associated with very energetic photons from those produced by charged cosmic rays.
Next to it operates the WCDA, a water Cherenkov array with 3,120 detection cells distributed over 78,000 square meters, in addition to the WFCTA, formed by 18 telescopes, which expand the field of view and help refine the reading of observed events.
In the water system, purified water serves as a medium for recording the Cherenkov light emitted by secondary particles, and the academy itself reports that this sector uses about 360,000 tons of water and 6,240 photomultiplier tubes installed beneath the surface.
This combination was not assembled merely to increase impressive engineering numbers, but to allow composite measurements over a very wide energy range, a condition seen as central to tackling one of the most persistent problems in high-energy astrophysics.
Origin of Cosmic Rays in Focus
At the center of LHAASO’s scientific agenda is the attempt to clarify where the most energetic cosmic rays produced in the Milky Way come from and by what natural mechanisms the Universe accelerates particles to levels beyond those achieved by laboratory-built accelerators.
This effort gained additional importance because the energy distribution of cosmic rays exhibits a break known as the “knee” of the spectrum, a region where physicists are trying to understand which astrophysical sources can feed particles with energies close to the peta-electron-volt scale.
Why the Altitude of Daocheng is Strategic
The choice of Daocheng was not casual, as the average altitude of 4,410 meters reduces the thickness of the atmosphere between the detector and the particle showers, which improves the quality of the recording and increases the efficiency in identifying extreme energy events.
In addition to the height, the facility was designed to operate with a wide field of view and high duty cycle, something crucial for continuously monitoring the sky and recording both persistent sources and transient phenomena, such as explosions and short-duration energy bursts.
Discoveries Involving Black Holes and Microquasars
Among the most relevant results associated with the observatory is the identification of ultra-high-energy gamma emission linked to microquasars, systems formed by stellar-mass black holes that accumulate matter from a companion star and can launch relativistic jets into space.
This finding reinforced the hypothesis that environments associated with black hole jets also function as natural particle accelerators, bringing together two fronts that have long appeared separate in public debate: black hole physics and the origin of galactic cosmic rays.
By observing signals of this type in different systems, LHAASO has strengthened the evidence that violent processes around these objects contribute to the population of extreme particles detected on Earth, especially at energies close to the “knee” region.
Scientific Cooperation and Global Impact
The observatory underwent major construction between 2017 and 2021, entered full instrumental operation in July 2021, and then advanced to formalize its operation after the national acceptance announced in May 2023, according to official Chinese information.
Operated by the Institute of High Energy Physics of the Chinese Academy of Sciences, the project also functions under international cooperation and brings together dozens of universities and institutes, as well as around 280 scientists, which enhances its role as a shared platform for basic research.
This collaborative dimension helps explain why LHAASO appears not only as a showcase of Chinese technology but as a scientific infrastructure with global reach, capable of producing relevant data for studies on high-energy astrophysics, cosmic rays, and indirect searches for dark matter.
Why LHAASO Draws Global Attention
The rarity of photons in the peta-electron-volt range means that each detection carries disproportionate weight in advancing the field, transforming the mountain where the observatory was built into a strategic point for examining physical processes that cannot be fully reproduced on Earth.
More than just recording isolated events, the facility was designed to monitor the sky broadly, continuously, and multi-energetically, converting a high region of the Qinghai-Tibet Plateau into a natural laboratory to investigate the origin and evolution of the most violent phenomena in the Universe.
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