The mass of neutrinos is one of the biggest mysteries in physics. Now the new neutrino observatory JUNO in China has delivered its first measurement data – they raise hope that the mystery will soon be solved. The results from the first 60 days of measurement are already more precise than any previously achieved with other detectors, as the JUNO collaboration reports in “Nature”. The observatory could soon reveal more about how the masses of the three types of neutrinos differ.
Billions of neutrinos flow through our bodies every second. But because these elementary particles hardly interact with matter, we don’t notice this. But they play a crucial role in our Standard Model and many fundamental questions in cosmology and physics – from the imbalance between antimatter and matter to “new physics” beyond the Standard Model to the still unknown particles of dark matter.
The problem of neutrino mass(es)
But in order to clarify the role of neutrinos in these questions, physicists must first solve the biggest mystery of these elementary particles: their mass. For a long time, neutrinos were considered completely massless, but neutrino oscillation taught physicists otherwise: the ability of neutrinos to transform from one of the three types to another in flight requires that electron, tau and muon neutrinos have slightly different masses. When and how often a neutrino converts from one type to one of the other two depends on this mass ratio.
However, so far there are only approximate values for the neutrino mass and these do not provide sufficient information about the individual types of neutrinos. Neutrino detectors and experiments have so far only been able to determine that there are two mass gaps of different sizes between the three types of neutrinos: If you look at the square of their masses, one of these gaps is around 30 times larger than the other.
The problem: It is still unclear which types of neutrino lie between these two mass distances of different sizes. This means it is still unclear which type of neutrino is the heaviest and which is the lightest.
Nuclear reactors, 20,000 tons of detector fluid and 45,000 photodetectors
A new neutrino detector could now solve this mystery. Initial measurements from a neutrino detector in China that was only inaugurated in August 2025 have produced promising results. The Jiangmen Underground Neutrino Observatory (JUNO) is specifically designed to measure the oscillation of neutrinos and the mass fractions of their different varieties. The neutrino observatory is located under a 650-meter-thick rock cover that protects it from interference.
The heart of the JUNO system is a 35-meter-large detector sphere that is filled with around 20,000 tons of a liquid scintillator – a detector liquid made from organic hydrocarbons. When a neutrino flies through this scintillator fluid, it creates a trail of light through collisions and decay products. This is detected by around 45,000 photosensors. The aim of this detector is to capture and analyze the oscillation of antineutrinos created by nuclear fission in two nuclear power plants located around 53 kilometers away.
“At this distance, the interference between the oscillation frequencies produces a maximum spectral expansion of the reactor neutrino spectrum,” explain the physicists of the JUNO collaboration. “The oscillation parameters can be precisely extracted from this.” JUNO is the first facility that can measure the exact values of both mass gaps simultaneously.
“New era of neutrino measurements”
The first measurement results are now available. The JUNO collaboration team led by Liangjian Wen from the Institute for High Energy Physics in Beijing evaluated the data from the first almost 60 days of measurement. During this time, the JUNO facility detected almost 2,380 antineutrinos from beta decay in the nuclear reactors. Using the energy spectrum of these particles, the physicists were able to determine two important parameters of the neutrino oscillation with high precision and simultaneously for the first time.
“This gives us the uncertainties in the mass difference Δm221 reduced to 1.6 percent,” writes the team. So far, the measurement uncertainty of the best measurement data was 2.5 percent. These first measurement results already confirm that the JUNO system fulfills the hopes placed on it. “Our results confirm that the performance of the detector, the calibration strategies and the analysis pipeline work as expected,” state the researchers in the JUNO collaboration.
“This first result from JUNO marks the dawn of a new era in which neutrino oscillation can be precisely measured,” write US physicists Patricia Vahle and Zoya Vallari, who were not involved in the study, in an accompanying commentary in Nature. “Understanding the behavior of neutrinos is crucial to fully describe matter and forces even at the smallest scales. The JUNO observatory promises us important insights into the properties of these mysterious elementary particles.”
This is how it continues
There are therefore good prospects that the JUNO neutrino observatory will solve the puzzle of neutrino oscillation and neutrino mass distribution in the next few years. Then physicists could finally clarify what mass order the three types of neutrino have and what mass gap lies between which types. The short measurement period was not yet sufficient to determine the second mass gap Δm231 to be narrowed down more precisely, as the JUNO collaboration explains.
The precision of this value should improve through further measurements and an expansion of the JUNO observatory. For this purpose, an additional detector, the Taishan Antineutrino Observatory (TAO), will be set up around 30 meters from the reactor core of a nuclear power plant. This makes it possible to determine the amount of electron antineutrinos produced in the reactor immediately after they are created. By comparing them with the values at the JUNO detector 53 kilometers away, the neutrino oscillation can then be recorded even more precisely.
Source: Liangjian Wen, Lei Liu (Institute of High Energy Physics, Beijing) et al., Nature, 2026; doi: 10.1038/s41586-026-10538-z