The "sea bells" on the bottom of the South China Sea are actually used to capture cosmic ghost messengers!

For hundreds of years, scientists have used telescopes to capture cosmic photons for astronomical observations. Today, they have a new option. Neutrinos have a ghostly penetrating power and can easily escape the extreme, dense universe and celestial environment without changing direction, helping scientists to reveal the mechanisms behind violent celestial processes and solve the mysteries of the universe.

At the Shanghai Jiao Tong University's South China Sea Neutrino Telescope "Sea Bell Project" achievement press conference held on October 10, the Tsung-Dao Lee Institute officially released the blueprint for the "Sea Bell Project". The project is led by Jing Yipeng, an academician of the Chinese Academy of Sciences, and Xu Donglian, a Tsung-Dao Lee scholar, as the chief scientist. The project will explore the construction of China's first deep-sea neutrino telescope and explore the extreme universe by capturing high-energy (sub-TeV to PeV) astronomical neutrinos.

It is reported that the Hailing team has completed its first sea trial mission, measured and verified the feasibility of the candidate sea area as the site for the neutrino telescope, and completed the conceptual design of the "Hailing" neutrino telescope. The relevant paper was published in "Nature Astronomy" on October 9.

What are neutrinos?

In the late 1920s, when scientists were studying beta decay (i.e., the nucleus radiates electrons and transforms into another nucleus), they discovered that part of the energy disappeared in this process. This puzzled scientists: Does the law of conservation of energy still apply in subatomic processes? In 1930, Hungarian physicist Pauli, who was only 30 years old at the time, firmly believed in the law of conservation of energy and predicted with extraordinary intuition: In this process, there must be a new particle that is uncharged, extremely small in mass, and interacts with matter so weakly that it cannot be detected. It is this particle that takes away that part of the energy. He called this unknown particle "small neutron", which is now called "neutrino".

In 1942, American physicist Allen indirectly confirmed the existence of neutrinos for the first time through experiments according to the method proposed by Chinese physicist Wang Ganchang.

Since the interaction between neutrinos and matter is very weak, it is very difficult to detect neutrinos directly. Even Pauli himself believed that neutrinos might never be detected. However, difficulties cannot hinder the progress of science. 26 years after Pauli proposed the neutrino hypothesis, Professor Reinis of the University of California, USA, put 400 liters of cadmium acetate aqueous solution as the target liquid into a newly commissioned nuclear reactor (as a neutrino source), and measured 2.8 neutrinos per hour, which is completely consistent with his theoretical prediction. Reinis was also awarded the 1995 Nobel Prize in Physics for this.

Modern cosmological research tells us that the upper limit of neutrino types is 3, that is, there are 3 types of neutrinos. In addition to the electron neutrinos discovered above, there are also muon neutrinos (discovered in 1962) and tau neutrinos (discovered in 1975), and each type of neutrino has the same antineutrino.

Whether neutrinos have mass or not is the most eye-catching topic in this research field. Before the 1970s, people generally believed that the mass of neutrinos was zero. In 1980, the Institute of Theoretical and Experimental Physics of the Soviet Union announced that after 10 years of testing, the mass of neutrinos was between 17 and 40 electron volts, which shocked the global physics community. Since then, many famous laboratories in the world have adopted different methods to measure and verify this result. Experts from the Chinese Academy of Atomic Energy also carried out this research in the mid-1980s and achieved certain results.

On October 6, 2015, the Nobel Prize in Physics was awarded to Japanese scientist Takaaki Kajita and Canadian scientist Arthur McDonald in recognition of their research on discovering that neutrinos have mass through neutrino oscillation.

Readers may ask, the interaction between neutrinos and matter is very weak and elusive, so what is the point of studying it?

What is the significance of studying neutrinos?

Of course, one neutrino is insignificant, but in our universe, there are a lot of neutrinos. They fill every corner of the universe, with an average of about 300 per cubic centimeter, about the same as photons, billions of times more than all other particles! Therefore, neutrinos as a whole play a decisive role in the universe.

In addition, neutrinos have another ability. They can travel freely inside planets, so they can bring us information about the interior of the sun and planets. Scientists also imagine using this characteristic of neutrinos to perform earth tomography scans, allowing the mysteries buried deep in the earth to be revealed at a glance; they also imagine allowing neutrinos to penetrate the earth to transmit information, so that long-distance communications can avoid going around in circles through satellites and ground stations. Obviously, once the confusing neutrinos are fully understood by people, they will have extremely wide applications.

How have neutrinos been detected in the past?

The biggest feature of neutrinos is that they hardly react with any matter. We cannot feel their existence, and it is extremely difficult to detect them scientifically. Therefore, the discovery and research process of neutrinos is full of the hard work of several generations of scientific researchers.

China has made important contributions in this field. In 2012, the Daya Bay Neutrino Laboratory announced the discovery of a new neutrino oscillation mode, which became an important milestone in neutrino research. In December 2020, the Daya Bay Reactor Neutrino Experiment successfully completed its scientific mission and was officially decommissioned. At the same time, another "national heavy weapon" began to be built, that is, the Jiangmen Neutrino Experiment Detector.

The Jiangmen Neutrino Experiment (JUNO) is located in Kaiping, Jiangmen, Guangdong Province. It is a large scientific facility jointly built by the Chinese Academy of Sciences and Guangdong Province. Its main scientific goals are to determine the neutrino mass order and accurately measure the neutrino mixing parameters, and to conduct a number of other cutting-edge scientific research. The project is expected to be completed and put into operation in 2024, and will become one of the centers of international neutrino research.

The Jiangmen Neutrino Experiment began construction in 2015 and was built on Dashi Mountain in the Jinji Town and Chishui Town of Kaiping City, Jiangmen City, Guangdong Province. At present, most of the infrastructure construction has been completed, whether it is the underground experimental hall or related water, electricity and other equipment, all have been completed and installed in place, and the detector installation has now begun.

One way to detect neutrinos is to capture the signals they produce using liquid scintillator detectors.

Researchers inject a transparent special liquid, liquid scintillator ("liquid scintillator" for short), into a plexiglass ball. When neutrinos pass through the ball, there is a certain probability that they will react with the dense hydrogen nuclei in the liquid. Each reaction produces a positron and a neutron. The positron annihilates and releases a fast signal, while the neutron is absorbed by other hydrogen nuclei after repeated collisions and releases a slow signal. The two flashes, one after the other, reveal the whereabouts of the neutrino.

"The bigger the detector, the more signals it can capture, the more data it can collect, and the more it can see things that others can't see.

How does the Hailing Project "capture" neutrinos?

It is understood that after sea trials, the first pathfinder team of the "Sea Bell Project" discovered a deep-sea plain with a water depth of about 3.5 kilometers in the northern South China Sea. The seabed is flat, the flow rate is slow within a few hundred meters above the seabed, and the radioactivity measured in seawater is consistent with the public data of ordinary seawater.

The Sea Bell telescope will use the entire Earth as a shield to receive neutrinos that penetrate from the opposite side of the Earth. "Because it is located near the equator, the Sea Bell telescope can detect neutrinos in the entire sky at 360 degrees through the rotation of the Earth, achieving all-around observation of neutrinos in different directions, complementing the Antarctic Ice Cube and other neutrino telescopes in the Northern Hemisphere."

The Pathfinder team also measured the optical properties of seawater at a depth of about 3,420 meters. The results showed that its average absorption and scattering lengths were approximately 27 meters and 63 meters, respectively. Clear seawater can more clearly "record" the traces of the reaction between neutrinos and seawater.

The first phase of the "Hailing" project was launched at the end of 2022. It plans to build a series of 10 telescopes in the selected sea area and connect them to an island base in the South China Sea through a long-distance submarine cable. It is expected to complete the construction of the world's first near-equatorial small neutrino telescope in 2026, conduct searches for celestial sources inside and outside the Milky Way, and complete the full-chain technical verification of the construction of a large array; the ultimate goal is to build the ultimate large array.

The "Sea Bell" detector array will consist of 1,200 vertical cables, each about 700 meters long, with a cable spacing of 70 to 100 meters, and will "grow" vertically on the seabed like seaweed. These cables carry a total of 24,000 high-resolution optical detection balls. The entire array has a diameter of about 4 kilometers and a total area of ​​about 12 square kilometers. The volume of seawater that can monitor high-energy neutrino reactions is about 7.5 cubic kilometers, and the design life is 20 years.

Scientists predict that within a year of its completion, the Sea Bell detector array will be able to discover a stable neutrino source in the barred spiral galaxy in Cetus, and will be able to discover neutrino bursts from supermassive black holes similar to those that IceCube has only preliminarily observed using a decade of data. The Sea Bell telescope will become the most advanced neutrino telescope in the world around 2030.

Source: Science Popularization China Comprehensive Science and Technology Daily