On the evening of March 25, 1938, Italian physicist etore mayorana boarded the ship from Palermo to Naples, but then disappeared forever at the age of 32. Where's mayorana? This is the unsolved mystery of the world, and whether the "mayorana" particle named after it, the fermion equivalent to its antiparticle, exists is an unsolved mystery in the field of physics.

Britain's "nature" website recently reported that China, the United States, Italy, Japan and other countries have built (or will build) a variety of detectors and are (or plan to) search for neutrino free pairs β Decay (nldbd) phenomenon to prove that neutrinos are mayorana particles.

Rare decay: the key to solving the big mystery

In 1935, Nobel laureate and physicist Maria gopett Meyer predicted that two neutrons in some atomic nuclei would decay into two protons at the same time, releasing two protons β Particles (electrons), this "double" β Decay should also produce two anti neutrinos. Meyer's calculation proved to be correct, but this decay is extremely rare.

Four years later, American physicist Wendell Frey pointed out that if neutrinos are their own antiparticles and their mass is not zero, then a special kind of atomic nucleus will not only produce neutrinos β Decay, positive and negative neutrinos annihilate each other, that is, nldbd phenomenon.

Since then, scientists began to look for nldbd to prove that neutrinos are mayorana particles.

Han Ke, a special researcher at the school of physics and astronomy of Shanghai Jiaotong University, told the science and Technology Daily: "mayorana particles may be the key to revealing the mystery of the asymmetry of positive and antimatter in the universe. In addition, it can be proved that, unlike all other known basic particles (such as electrons or quarks), the mass of neutrinos does not come from the Higgs boson."

Hanko explained that the standard model tells us that the big bang should have produced an equal amount of positive and antimatter, and the positive and antimatter will annihilate when they meet, but now the positive matter accounts for the vast majority in the universe, so where is the antimatter? Ordinary double β Decay produces two electrons and two anti neutrinos, which will not change the balance of positive and negative particles, but nldbd will only produce two electrons, increasing the number of positive matter in the universe, which helps to explain why the universe contains much more matter than antimatter.

Competing for the best: detectors competing for power

In order to find nldbd, the United States, Italy, Japan and China are upgrading existing detectors or planning to build new detectors.

According to Han Ke, the United States established the nldbd experiment as the most important new direction of the next generation of nuclear physics experiments as early as 2015. In 2019, the xenon enrichment Observatory (exo-200) limited the half-life of nldbd to 3.5 × In 1025, the experimental group is planning to upgrade it to a 5-ton NEXO experiment“ The Majorana experiment is also using high-purity germanium γ The spectrometer looks for the nldbd of germanium-76. The dark matter detector LZ also joined the search army. From July 13 to 16 this year, the U.S. Department of energy held an nldbd project review meeting. It is expected to select one or more nldbd experiments for support, with a total budget of $250 million.

In Italy, the low temperature underground Observatory for rare events (cuore) uses tellurium dioxide crystals to find nldbd, while Japan takes kamland Zen experiment as the core.

Han Ke pointed out: "China also has a number of detectors that are or will carry out such exploration. China Jinping underground laboratory under construction is the deepest underground laboratory in the world and an ideal place for nldbd experiment. The pandax cooperation group led by Shanghai Jiaotong University used high-pressure gas and liquid time projection room technology to find nldbd of xenon-136. At present, pandax-4t liquid xenon detector has been put into operation. About 350 kg of xenon-136 in the detector can be used to find nldbd. The "China dark matter experiment" (CDEX) led by Tsinghua University shows that the lower nldbd half-life of germanium-76 is 6.4 × In 1022, it was the first nldbd result of germanium-76 in China. Cupid China cooperation group led by Fudan University and nvdex experiment led by central China Normal University will respectively use optical thermal double readout low-temperature calorimeter technology and high-pressure gas time projection room technology to find nldbd in Jinping underground laboratory. "

"The Jiangmen neutrino experiment led by the Institute of high energy physics of the Chinese Academy of Sciences will build a 20000 ton large liquid scintillator detector in Jiangmen City, Guangdong Province. The construction of relevant infrastructure and detectors is in full swing. The cooperation team has carried out relevant pre research and considered to carry out nldbd research after the completion of its main physical objectives. " Han Ke further pointed out.