Construction has begun on one of the world’s largest and most sensitive cosmic-ray facilities. Located about 4410 m above sea level in the Haizi Mountain in Sichuan Province in southwest China, the 1.2 billion yuan ($180m) Large High Altitude Air Shower Observatory (LHAASO) will attempt to understand the origins of high-energy cosmic rays. LHAASO is set to open in 2020.

Cosmic rays are particles that originate in outer space and are accelerated to energies higher than those that can be achieved in even the largest man-made particle accelerators. Composed mainly of high-energy protons and atomic nuclei, cosmic rays create an air shower of particles such as photons and muons when they hit the atmosphere. Where cosmic rays come from, however, has remained a mystery since they were first spotted some 100 years ago.

Cosmic showers

LHAASO aims to detect cosmic rays over a wide range of energies from 1011–1018 eV using a Cherenkov water detector, covering a total area of 80 000 m2, together with 12 wide-field Cherenkov telescopes. These two types of instrument, which are above ground, will spot the Cherenkov radiation emitted when a charged particle travels through a medium faster than light can travel through that medium. LHAASO will also consist of a 1.3 km2 array of 6000 scintillation detectors that will study electrons and photons in the air showers, while an overlapping 1.3 km2 underground array of 1200 underground Cherenkov water tanks will detect muons.

LHAASO is not the only facility in the world trying to study the origin of cosmic rays. The IceCube facility at the South Pole observes high-energy neutrinos, while the Pierre Auger Observatory in Argentina explores cosmic rays with energies above 1018 eV. “LHAASO will play a complementary role with existing detectors to offer a more comprehensive picture of the cosmic-ray sky,” says Yifang Wang, head of the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences.

Challenges ahead

According to IHEP researcher Zhen Cao, who is LHAASO’s chief scientist, the construction of LHAASO will not be easy, with the Cherenkov water detectors being particularly tricky. Benedetto D’Ettorre Piazzoli, a former vice president of the National Institute of Nuclear Physics (INFN) in Italy, who has been involved in Sino-Italian collaborations in cosmic-ray research, agrees, adding that combining all of these detector types will be difficult. “The deployment, debugging, and operations management of many thousands of detectors of different types is very challenging at a level never faced before,” he says.

D’Ettorre Piazzoli is, however, confident that the facility will play an important role in cosmic-ray research. “As LHAASO is a very large installation, with a large amount of many types of detectors allowing the observation of cosmic rays and photons over a wide range of energies, it is expected to provide detailed and statistically relevant information on the transition from the galactic to the extra-galactic contribution,” he adds.

LHAASO is an international collaboration that includes scientists from China, France, Italy, Russia, Switzerland and Thailand. First mooted in 2008, the facility won approval from the National Development and Reform Commission of China in December 2015. Haizi Mountain was selected as the site due to its high elevation and good accessibility – being only 10 km away from Yading Airport – the world’s highest – and about 50 km from Daocheng County, which will be the base for the LHAASO team.