Neutrinos are "ghosts"? Why go 700 meters underground to catch them?

Recently, according to the Chinese Academy of Sciences, the world's largest single plexiglass sphere has been fully built inside the central detector of the Guangdong Jiangmen Neutrino Experiment (JUNO) located 700 meters underground and has entered the final stage of construction.

Netizens joked, "Ghosts finally have a high-sounding name - neutrinos . If neutrinos are found 700 meters underground, it means that a ghost has been caught in the underworld."

Why do people say so?

There is a saying in Taoism in my country that when a person dies, he becomes a ghost, when a ghost dies, he becomes a ghost, when a ghost dies, he becomes a ghost, when a ghost dies, he becomes a ghost, when a ghost dies, he becomes a ghost, when a ghost dies, he becomes a ghost. No wonder netizens jokingly call neutrinos "ghosts".

Yes, when you are thinking about the relationship between neutrinos and "ghosts", 300 trillion neutrinos from the sun are passing through your body every second. Neutrinos have extremely strong penetrating ability. Not only can they pass through the human body unimpeded, they can even pass through the earth and the sun. This strong penetrating ability makes neutrinos "come and go without a trace". Neutrinos have very light mass and move at a speed close to the speed of light, making them difficult to capture. They hardly interact with other matter and penetrate the earth like ghosts, carrying the "core secrets" about stars, black holes and even the entire universe, so they are called "ghost particles."

——Why is the Jiangmen Neutrino Experimental Facility built so deep?

Nuclear reactors are the most concentrated locations on Earth where neutrinos are produced. Jiangmen is 53 kilometers away from both Yangjiang and Taishan nuclear power plants. After the two reactors are fully built, their total power will rank first in the world, and double the number of experimental samples will be obtained. It can be said that this is currently the most suitable place in the world for neutrino experiments.

Since there are strong cosmic rays on the ground, they will interfere with the detection of neutrinos. The 700-meter mountain and rock layer are equivalent to filters, which can greatly reduce the intensity of cosmic rays and allow us to obtain pure neutrino signals.

What exactly are neutrinos?

In particle physics, scientists agree that there are 12 most basic particles that make up the material world, including 6 quarks (up, down, strange, charm, bottom, and top), 3 charged leptons (electron, muon, and tau), and 3 neutrinos (electron neutrino, muon neutrino, and tau neutrino).

Neutrinos are uncharged and very light, less than one millionth of an electron, and move at a speed close to the speed of light. However, light will turn due to gravity or interstellar dust during interstellar propagation, but neutrinos will not. Neutrinos hardly react with any matter and only participate in very weak interactions.

Another very important characteristic of neutrinos is their strong penetrating power. For example, the diameter of our earth is about 12,700 kilometers, and neutrinos can pass through the earth without any hindrance. They are not blocked by seawater or strata, and cannot be interfered with, intercepted or cracked. Therefore, the detection of neutrinos is very difficult. Among all elementary particles, people know the least about neutrinos, so neutrinos are also called "ghost particles."

——How are neutrinos produced?

Most particle physics and nuclear physics processes are accompanied by the production of neutrinos, such as nuclear reactor power generation (nuclear fission), solar luminescence (nuclear fusion), natural radioactivity (beta decay), supernova explosions, cosmic rays, etc.

When a supernova explodes, it generates enormous amounts of energy, and neutrinos carry away 99% of the energy released by the supernova.

Our universe was created by a big bang. Some scientists believe that at the beginning of the big bang, the material world and the antimatter world were created at the same time. So, where did the antimatter world that once existed go during the origin and evolution of the universe?

——What is the purpose of capturing neutrinos?

Our universe is filled with a large number of neutrinos, most of which are remnants of the Big Bang, about 300 per cubic centimeter. Scientists generally believe that neutrinos, which are widely present in the material world, contain important information about the antimatter world.

In fact, our Earth itself also emits neutrinos, and the composition of these neutrinos is related to structures that we cannot see now.

Neutrinos are the oldest and most primitive elementary particles that have existed since the beginning of the universe. They carry a lot of important mysterious information. Studying neutrinos is of great significance for understanding the universe and the matter in our existing world. It is also the most cutting-edge basic science in the world.

The Jiangmen Neutrino Experiment was approved in 2013, and the construction of the underground laboratory began in 2015. At the end of 2021, the underground chamber was put into use and the detector installation began. At present, the innermost organic glass ball of the detector of the Jiangmen Neutrino Experiment Center has been closed, and the outer stainless steel grid and photomultiplier tube are also being closed in an orderly manner. It is expected that all installation tasks will be completed by the end of November, and the filling of ultrapure water and liquid scintillator will be started. It will be officially put into operation and data collection in August 2025, and it is expected to operate for about 30 years.

As a major layout of my country's large-scale scientific project construction in the Guangdong-Hong Kong-Macao Greater Bay Area, the Jiangmen Neutrino Experiment has the measurement of the neutrino mass order as its primary scientific goal. At the same time, it will also conduct in-depth research on atmospheric neutrinos, solar neutrinos, terrestrial neutrinos, supernova neutrinos, etc.

——What does the Jiangmen neutrino experimental device look like?

The inner diameter of the organic glass ball of the Jiangmen Neutrino Experiment is 35.4 meters, about 12 stories high. It is made of 263 pieces of 12-centimeter-thick curved ball panels and upper and lower chimneys. The net weight of the organic glass is about 600 tons, making it the world's largest single organic glass ball.

The organic glass ball, as a container for the target liquid scintillator for detecting neutrinos, will carry 20,000 tons of liquid scintillator. At the same time, the entire ball is placed in pure water for operation. During operation, it needs to withstand a buoyancy of about 3,000 tons for a long time. The force is transmitted to the main structure of the stainless steel lattice shell through organic glass nodes, connecting rods and stainless steel nodes. Sensors are installed on the connecting rods for force monitoring. The specially designed stainless steel structure is embedded in the organic glass as an organic glass node. After repeated design optimization and hundreds of tests, it finally obtains an ultra-high load-bearing capacity. In addition, some stainless steel nodes adopt a disc spring design scheme, which effectively improves the force distribution of the organic glass nodes.

After completion, the Jiangmen Neutrino Experiment will become one of the international neutrino research centers, forming a three-way force in neutrino research together with the Kamioka Neutrino Experiment in Japan and the Deep Underground Neutrino Experiment in the United States.

Source: CCTV News, CCTV Science and Education, Institute of Theoretical Physics, Chinese Academy of Sciences