Construction on the first half of the 1-km² Extensive Air Shower (EAS) array (KM2A) located at the Large High Altitude Air Shower Observatory (LHAASO) was recently completed. The array of detectors is now in operation for science.

The array will be a great tool for observing the highest energy electromagnetic radiation from deep in the universe. In fact, it has already observed the highest energy gamma rays ever detected.

In 2019, the construction of LHAASO array make significant progresses. KM2A construction began in April 2019 at the LHAASO site, at 4410 m a.s.l. on Haizi Mountain in Daocheng, Sichuan. By the end of September, one-quarter of the array – consisting of 1314 electromagnetic particle detectors (EDs) and 307 muon detectors (MDs) – had been already stably collecting cosmic ray data. Now, 2390 EDs and 578 MDs in total have been connected in the data acquisition system. On Dec. 27, official data-collecting operations began for the newly installed detectors. The acquisition of data from some cosmic ray EAS events spanning more than 1 km2 marks the successful completion of the detector deployment in 2019.

The R&D for both EDs and MDs was conducted by Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) with close cooperation with institutions including Tsinghua University, Shandong University, Hebei Normal University and the University of Science and Technology of China of CAS, among others. For the past year, all participants worked extremely hard and supported each other. They eventually overcame difficulties such as too many thunderstorms in summer and extremely cold weather in winter. All engineering tasks have been done with high quality in time.

LHAASO is one of large projects of national science infrastructure construction the 12^th Five-year Plan that were approved by the central government in 2013. Its main scientific goal is to explore the origin of high-energy cosmic rays, known as one of the 'mysteries of the century'. With the completion of the first half of KM2A and the start of data acquisition and corresponding analyses, LHAASO has now taken an important step towards its ultimate goal.

First Half of LHAASO KM2A Now in Operation (Image by IHEP)