China's first deep-sea neutrino telescope construction "Seabell Plan" ushered in new progress

Pictured: The "Hailing Project" pathfinder project team aims to explore the construction of China's first deep-sea neutrino telescope Xinmin Evening News reporter Yi Rong photographed

Xinmin Evening News (reporter Yi Rong) Cosmic rays in interstellar propagation by the magnetic field of the direction of deflection, its source is still a mystery. Once the neutrinos produced by cosmic ray reactions are detected, they can be traced back to the source, so the centennial mystery of the origin of cosmic rays can be solved by detecting the neutrino source of high-energy celestial bodies. For this puzzle, China has begun to look up from the deep sea to the universe, planning to build the first deep-sea neutrino telescope in the South China Sea.

The reporter learned from the Li Zhengdao Research Institute of Shanghai Jiao Tong University today that recently, the "Hailing Plan" pathfinder project team aimed at exploring the construction of China's first deep-sea neutrino telescope completed various scheduled sea trial tasks and arrived in Shanghai safely, laying a solid foundation for follow-up advancement. The "Hailing Project" explores the extreme universe by capturing neutrinos in high-energy celestial bodies, builds a complete multi-messenger astronomical network in China, and promotes cutting-edge cross-cutting research in particle physics, astrophysics, geophysics, marine geography, marine life, etc., and has the great potential to breed a number of original scientific discoveries.

At present, Europe and the United States are actively preparing to build a second-generation neutrino telescope with a greatly optimized performance target of 10 times sensitivity, which is expected to be built around 2030, when the field of neutrino astronomy may achieve a major breakthrough. The "Hailing Project" is led by the Li Zhengdao Research Institute of Shanghai Jiao Tong University, the project team leader is Jing Yipeng, an academician of the Chinese Academy of Sciences, the chief scientist is Xu Donglian, a scholar of Li Zhengdao, and invites academicians and top scientists of the two academies with rich experience in various fields of science and engineering as project consultants, bringing together the cutting-edge research advantages and strong engineering capabilities of Shanghai Jiao Tong University in physics, astronomy, marine engineering, marine science, materials, electronic information and other disciplines, and bringing together the multi-stranded scientific research forces of other sister universities and research institutes.

The idea of neutrino astronomy originated in 1960 when Markov proposed to build an array of Cherenkov light detection elements in the deep sea or in the lake. The Antares/KM3NeT projects in the Mediterranean and the Baikal/GVD projects in Lake Baikal are both being planned, but it is difficult to build neutrino telescopes in seawater, and the most well-known neutrino telescope in the world, IceCube, was built in 2010 in 2500 meters deep Antarctic ice, making it the world's largest neutrino detector.

In 2013, Ice Cube first detected a diffuse stream of high-energy neutrinos from beyond the earth, knocking on the door to high-energy neutrino astronomy. However, this neutrino stream shows no signs of agglomeration or explicitly points back to any known celestial source, suggesting that there are no celestial sources of high-energy neutrinos strongly radiating in the universe near Earth. To effectively find celestial sources of high-energy neutrinos, it is still necessary to improve the detection sensitivity of the next generation of neutrino telescopes.

As the earth rotates, the "sea bell" located in the South China Sea will have the advantage of "time and place", and after completion, it will be able to "see" the galactic center and truly achieve full sensitive sky survey.

Xu Donglian introduced that the "Sea Bell" cable places anti-high voltage optical ball cabins to maintain work in the deep sea and high water pressure environment; each ball cabin covers dozens of photodetectors, eventually forming a huge grid point array. At present, the team has achieved breakthroughs in many fields, including a new optical module, which can increase the capacity of the current equipment by 50-100%, and the preliminary simulation results have been presented at the International Cosmic Ray Conference in July this year. "The volume of the 'Ice Cube' is 1 cubic kilometer, and the 'Sea Bell' we will build will be 10 times larger than the Ice Cube, depending on the 'monster' in the universe - ultra-high-energy neutrinos." Look at the 'monster' in the universe – the ultra-high-energy neutrino. Compared with the probe built by Jiangmen, the "sea bell" is aimed at neutrinos with higher energy levels. Xu Donglian made a very vivid analogy, "The Jiangmen detector needs to compile a large net with a fine grid so that it can catch krill in the sea; and the 'sea bell' needs to weave a special net to catch whales." ”

Looking up at the universe in the deep ocean can shield you from the distractions of the atmosphere. But to build huge detectors in the deep sea, it is also necessary to find clean, less luminous plankton, wide and flat seabed, and smooth ocean currents. In order to install the "wayfinding" cable to detect the characteristics of seawater optics, deep ocean currents, seabed geography and other characteristics, the voyage was led by Xu Donglian, a scholar of Li Zhengdao of Shanghai Jiaotong University, and Tian Xinliang, a scholar of marine engineering, as the leader, and more than 30 researchers and technicians from Shanghai Jiao Tong University, Peking University, Tsinghua University, University of Science and Technology of China, the Second Institute of Oceanography of the Ministry of Natural Resources and other institutions participated together.

It is understood that the "Sea Bell Pathfinder" sea test team deployed several sets of self-developed experimental instruments in the predetermined sea area, collected in situ precious data of more than 1TB of 3500 meters of sea depth, and also scanned and tested the relevant properties of all-water deep seawater. After preliminary analysis, the feasibility of pre-selected sea area as a candidate site for neutrino telescopes is verified. In addition, the team also successfully deployed a set of submarine markers that can monitor the submarine flow field, biological activity, sediment and test telescope components for a long time, providing a basis for the design and long-term operation and maintenance of subsequent telescope arrays.

Tian Xinliang introduced that the "sea bell" will occupy the equivalent of one million Beijing water cube water body level, will become the largest man-made marine structure. "We face complex problems across disciplines, but we bring together multidisciplinary teams and the creativity and imagination of scientists is limitless," he said. ”

[Further reading]

Neutrinos are one of the basic units that make up the universe and the most numerous particles in the universe. It is not charged and interacts with matter extremely weakly, like a ghost, and extremely difficult to catch. Neutrinos were first predicted theoretically in 1930, but were not experimentally observed until 1956. They are full of mystery, and scientists' research on their properties has repeatedly refreshed our understanding of basic physical laws and won four Nobel Prizes. At present, there are three types of neutrinos known to exist, electron neutrinos, Miao neutrinos and Tao neutrinos; due to quantum effects, they can be converted to each other in the process of space-time propagation, similar to the face-changing performance of Sichuan opera, which can change their appearance at the moment of space-time transformation, which is the famous neutrino oscillation phenomenon. By building different detectors to study the oscillatory behavior of neutrinos, humans can partially glimpse the basic laws of matter formation in the universe, but neutrinos themselves still have many unsolved mysteries, such as the absolute mass of neutrinos is geometric, whether they are antiparticles of their own, and so on. A deeper exploration of neutrinos could once again subvert our understanding of fundamental physical laws.

There are many sources of neutrinos in the universe, such as the Big Bang, supernova explosions, binary neutron star mergers, black hole explosions and other extreme astrophysical processes; if dark matter is elementary particles, neutrinos may also be produced through mutual annihilation or spontaneous decay. Because of its ghostly nature, neutrinos have strong penetration, can easily escape the extreme, dense celestial environment, carrying the information of the violent physical processes in it, and are the ideal for studying the extreme universe. As early as 1912, physicists discovered that the Earth's atmosphere was continuously bombarded by space-rich ions (aka "cosmic rays") and produced a large amount of secondary radiation that could reach the ground, or was closely related to the origin and evolution of life on Earth.

Xu Donglian has studied and worked in the Ice Cube Cooperation Group for many years, and is a young scholar active in the field of neutrino astronomy in recent years. Through the accumulation of time in the international cooperation group, she gradually germinated a "dream" - china-led construction of neutrino telescopes in the waters of the South China Sea. In September 2018, Xu Donglian returned to China to join the Li Zhengdao Research Institute, and in November of the same year, her "Neutrino Astronomy Research" project received support from the "Overseas High-level Young Talents Special Project", mainly to carry out the site selection of neutrino telescopes and the development of prototype prototypes of detectors, and her "dream" in the international cooperation group was gradually painted as an exciting blueprint, and also attracted a large number of like-minded scientists and engineering and technical experts from Shanghai Jiaotong University and other universities and scientific research institutions. In August 2020, Xu Donglian, on behalf of the "Hailing Project" team, made an invited report at the National High-Energy Physics Development Strategy Seminar (Qingdao), and formally proposed the construction plan and action plan of the South China Sea Neutrino Telescope - "Hailing Project".