The Large High Altitude Air Shower Observatory (LHAASO) project, one of China's major national scientific and technological infrastructure facilities, has successfully passed national acceptance and formally gone into operation. LHAASO is a facility focusing on cosmic ray observation and research. It was approved by the National Development and Reform Commission (NDRC) on Dec. 31, 2015. Jointly sponsored by the Chinese Academy of Sciences (CAS) and the People’s Government of Sichuan Province, the construction project was undertaken by the Chengdu Branch of CAS and the Institute of High Energy Physics (IHEP) of CAS. The construction of the main part of the facility began in 2017 and was completed in 2021. The LHAASO project passed an acceptance inspection by five professional teams organized by CAS that covered the following areas: technical performance, construction and installation, financial matters, equipment and assets, and records.
Entrusted by the National Development and Reform Commission, CAS and Sichuan Province organized the acceptance meeting. More than 20 experts from the National Development and Reform Commission, China Research and Development Corporation, research institutes, universities and other institutions attended the meeting.
In the opinion of the acceptance committee, Chengdu Branch of Chinese Academy of Sciences, the legal entity of the project, and Institute of High Energy Physics, Chinese Academy of Sciences, the co-construction unit of the project, have completed the construction tasks approved by the National Development and Reform Commission on time, comprehensively and with high quality, and all the indicators have reached or exceeded the acceptance indicators approved by the National Development and Reform Commission. LHAASO's quarter-scale detection device was put into trial operation in April 2019, and its full-scale detection instrument was put into trial operation in July 2021. The overall performance is reliable, and it has long-term stable scientific operation capability.
Taking unique advantage of the high altitude at the roof of the world, LHAASO has become the most sensitive ultra-high-energy gamma ray detection instrument in the world, the most sensitive high-energy gamma ray wide-field-of-view survey telescope system in the world, and the compound stereoscopic cosmic ray measuring system in the “knee region” with the widest energy coverage in the world. The completion and operation of LHAASO has made it one of the three major international experimental facilities for particle astrophysics, which is of great significance in promoting the realization of major original breakthroughs in this field and driving the development of frontier cross-related disciplines and international cooperation. The acceptance committee agrees that the project has passed the national acceptance.
LHAASO is located 4,410 meters above sea level (ASL) on Mt. Haizi in Daocheng County, Sichuan Province, and covers about 1.36 km^2. It consists of three arrays: a shower particle detector array composed of 5,216 electromagnetic particle detectors and 1,188 ground-installed muon detectors covering 1 km^2; a water Cherenkov detector array covering 78,000 m^2; and a telescope array composed of 18 wide-field-of-view Cherenkov telescopes. Using these three arrays and four detector technologies, LHAASO will be able to measure gamma rays and cosmic rays generated by high-energy celestial objects omnidirectionally with multiple variables, easuring system in the "knee region" with the widest energy coverage in the world.
Through independent innovation and international cooperation, the LHAASO project team has made breakthroughs in several key technologies: 1) Novel silicon photo PMTs were adopted in the wide-field-of-view Cherenkov telescopes on a large scale. This completely changed the traditional observation mode whereby such telescopes could not work during the lunar night. As a result, the effective observation time was doubled. 2) Based on "White Rabbit" technology, a large-area, multi-node, high-precision clock synchronization technology adapted to wild field conditions at high altitudes above 4,000 meters was developed. This technology improves long-distance synchronization accuracy five-fold to 0.2 nanoseconds, thus reaching the world’s leading level. 3) A 20-inch ultra-large photomultiplier tube was developed in China. It improved the temporal response three-fold and broke a technological bottleneck, greatly expanding the observation capacity by lowering the detection threshold from 300 billion to around 70 billion electron volts. 4) Significant progress on big data acquisition technology has been achieved, enabling "triggerless" data acquisition and "zero dead time" observation of cosmic ray events with a data transmission rate up to 4 GB/s；A special noise filtering technology and lossless compression algorithm has been developed, enabling real-time data transmission from Mt. Haizi to IHEP.
During the construction of LHAASO, the trial operation of quarter-scale and full-scale arrays was carried out to continuously observe high-energy cosmic rays. Thanks to its high detection sensitivity, LHAASO has made significant scientific breakthroughs during its initial operation. For example, LHAASO discovered many ultra-high-energy cosmic accelerator candidates in the Milky Way and detected the highest energy photon ever recorded, thus launching an era of "ultra-high-energy gamma astronomy". LHAASO also accurately measured the luminosity of the "standard candle" in the ultra-high-energy band and discovered gamma rays with energies over 1 quadrillion electron volts, which challenges the theoretical limit. Observations were made during the construction of LHAASO, and scientific results continued to be produced. According to a survey, about 215 journal papers and 156 conference papers on the LHAASO project have been published so far.
The construction unit of LHAASO project takes full advantage of the institutionalized research of the Chinese Academy of Sciences, carries out observation and theoretical research by relying on this facility, and sharing both domestically and internationally. At present, 28 astrophysics research institutions have become members of the international collaboration of LHAASO. The collaboration utilizes the observation data of LHAASO to conduct particle astrophysics research, as well as basic research in many fields such as cosmology, astronomy, and particle physics.
After the national acceptance, LHAASO will become an international cosmic ray research center with China as the main hub and participation from multiple countries. By taking advantage of the observation of high-altitude gamma astronomy and cosmic ray, LHAASO will become a unique, comprehensive, and open scientific research platform.
