China's deep underground neutrino detector aims to unlock neutrino mass hierarchy

A $300 million neutrino detector buried about 2,300 feet (700 meters) underground in southern China is designed to peel back the layers of the universe by studying ghost particles called neutrinos.

The Jiangmen Underground Neutrino Observatory, located in Kaiping in Guangdong province, uses an acrylic sphere filled with a liquid that emits light when neutrinos pass through, housed inside a cylinder that holds 45,000 tonnes of pure water. It sits beneath a granite hill to shield it from cosmic rays and radiation that could interfere with measurements, and the detector will be fed by neutrinos produced at two nearby nuclear power stations in the region.

Construction of the facility took more than nine years. The depth and shielding are intended to preserve the delicate signals emitted by neutrinos as they pass through the liquid, producing flashes of light that can be detected and counted. The team expects the detector to register roughly 50 light flashes per day as neutrinos collide with protons inside the target material, a rate that will build toward the statistical significance needed for analysis. It is also projected to require about six years to generate the 100,000 flashes that scientists say will be necessary for robust measurements.

The project is part of a broader international effort to map the neutrino mass hierarchy, the ordering of the three neutrino masses. Chinese scientists, led by Wang Yifang of the Chinese Academy of Sciences, say determining this hierarchy will refine models of particle physics, neutrinos, and cosmology. They say they expect to know the hierarchy of the neutrino mass, and that the result could help shape models of particle physics, neutrinos, and cosmology.

Two other large neutrino experiments are under construction elsewhere: Japan's Hyper-Kamiokande and the Deep Underground Neutrino Experiment (DUNE) in the United States. When they come online around 2027 and 2031 respectively, researchers plan to compare results across different detector designs to strengthen conclusions about neutrino behavior.

Neutrinos are among the most abundant particles in the universe and travel through ordinary matter without interacting with it most of the time. They originate from the Big Bang and are produced in stars and in high-energy collisions; trillions pass through the human body every second. Studying these elusive particles could shed light on why matter dominates over antimatter in the cosmos and how the four fundamental forces of nature relate to one another. Scholberg, a physicist at Duke University not involved with the project, said the endeavor is highly challenging but potentially transformative. It is a very daring project, she noted. Researchers emphasize that neutrinos are difficult to study precisely because they rarely interact with matter, but their role in the evolution of the universe remains a central question for modern physics.