Neutrinos pose a great mystery to particle physicists. Their properties appear mysterious and point to previously unknown physics beyond the established Standard Model of particle physics. After more than ten years of construction, the Jiangmen Underground Neutrino Observatory (JUNO) is now operational and can attempt to find out more about neutrinos, such as their mass hierarchy. The Technical University of Munich (TUM) is playing a key role in the international project in China.
JUNO The observatory's central detector consists of the largest acrylic glass sphere ever made by humans, with a diameter of 34.5 meters. This sphere has now been filled with 20,000 tons of a high-purity organic scintillation liquid. When elementary particles such as neutrinos interact with this liquid, weak blue flashes of light are produced, which are registered by 43,000 photosensors. JUNO is thus twenty times larger than previous detectors of this type. This will enable JUNO to investigate one of the central questions in particle physics today: the order of neutrino masses, and thus whether the third neutrino mass state is heavier than the second.
To shield it from cosmic rays, JUNO is located 700 meters underground in a pool filled with 35,000 tons of high-purity water. This also dampens the natural radioactivity of the surrounding rock. JUNO is located at equal distance from eight nuclear reactors at the power plants in Taishan and Yangjiang, which is what makes it possible to measure the neutrino mass order. To do this, the researchers detect the light that the reactor neutrinos generate when they interact in the scintillator with unprecedented precision. About 45 such events are expected per day. In this way, a detailed spectrum is expected to emerge over time, containing information about the neutrino mass hierarchy as a fine structure.
"A new era of neutrino physics"
The spokesperson for the JUNO collaboration, Prof. Yifang Wang from the Chinese Academy of Sciences, emphasizes: "Completing the filling of the JUNO detector and starting data taking marks a historic milestone. For the first time, we have in operation a detector of this scale and precision dedicated to neutrinos. JUNO will allow us to answer fundamental questions about the nature of matter and the universe."
"We are entering a new era in neutrino physics and opening the window to new discoveries wide. Over the next few years, we will finally be able to answer key questions in neutrino physics and obtain clues to new physics," says Dr. Hans Steiger, who heads TUM's contributions to JUNO. "In numerous precision experiments in the laboratory and at particle accelerators worldwide, we carried out precision measurements of the fundamental properties of this ultra-high-performance medium. The results now enable the interpretation of the measurement data from the JUNO detector."
International collaboration in basic research
JUNO is operated by the Chinese Academy of Sciences and involves more than 700 researchers from 74 institutions in 17 countries and regions. In Germany, the universities of Tübingen, Aachen, Mainz, and Hamburg, as well as the GSI Helmholtz Center in Darmstadt, are participating alongside TUM.
JUNO is designed for a scientific lifetime of up to 30 years. In the future, the detector will also offer the possibility of an upgrade to search for neutrinoless double beta decay with unprecedented sensitivity. The significantly more complex chemistry of the future liquid scintillator is currently being developed at TUM. "In the coming years, this technology is expected to open up new avenues in the search for these rare decays and offer the best sensitivity worldwide," emphasizes Steiger.
