An underground detector in China has detected the first signals from ‘ghost particles’

The JUNO Observatory has published its first major findings in the journal *Nature*. During its first 59.1 days of operation, the detector was able to measure very precisely how neutrinos change their type as they travel. This is an important step towards the experiment’s main goal — to understand which neutrinos are heavier and which are lighter.

JUNO has not yet solved the main mystery. But the initial data showed that the detector is working as physicists expected and is capable of measuring neutrinos more accurately than previous experiments.

Details

Neutrinos are tiny elementary particles. They are produced in stars, during supernova explosions, in nuclear reactors and other powerful processes. There are vast numbers of them all around us, but they are extremely difficult to detect.

The reason is simple: neutrinos hardly ever collide with anything. Huge streams of these particles pass through our bodies every second, but we do not feel them.

JUNO is located in the city of Kaiping in the Chinese province of Guangdong. The detector is situated approximately 700 metres underground. This depth is necessary to shield the facility from unwanted cosmic particles that could interfere with the measurements.

The detector itself is a huge sphere containing 20,000 tonnes of a special liquid. It is called a liquid scintillator. When a particle interacts with this liquid, a faint flash of light is produced. It is precisely these flashes that scientists are trying to detect.

In this study, JUNO was not detecting neutrinos from deep space, but antineutrinos from nearby nuclear reactors — ‘twin’ particles produced in nuclear reactors. The detector is located approximately 52.5 km from several reactor cores, and this distance was chosen deliberately: whilst the antineutrinos are travelling to JUNO, they have time to change their properties.

The simplest way to picture this is as follows: there are three ‘types’ of neutrino. During its flight, a particle can, as it were, change from one type to another. Physicists call this oscillation. These transformations allow us to understand how neutrinos are structured and how they differ from one another.

JUNO’s initial data allowed two important parameters of these oscillations to be refined simultaneously. According to Nature, the accuracy of the measurements was 1.6 times better than the combined result of previous measurements.

Why this matters

Neutrinos are important because they do not quite fit into the simple picture of particle physics. For a long time, it was thought that they had almost no mass. But experiments have shown that they do have mass, albeit a very small one.

Now physicists want to understand the order of these masses. It is known that there are three types of neutrino. But it is not yet clear how they are arranged by mass: are two particles heavier and one lighter — or the other way round?

This may sound like a narrow problem, but in fact it is linked to fundamental questions: how matter is structured, why the Universe looks the way it does, and whether there is physics beyond the current Standard Model.

JUNO has not yet answered the question about the order of the masses. But the initial results have shown that the facility is ready for this task. More data will be needed for a definitive answer.

Background

Neutrinos have been studied for decades. They interact so weakly with matter that to detect them, huge detectors have to be built underground, underwater or in ice.

The underground location helps to filter out ‘noise’ from cosmic rays. And the detector’s enormous volume is necessary because actual collisions between neutrinos and matter occur very rarely.

JUNO is one of the largest projects of its kind. It does not simply capture individual particles, but attempts to measure their behaviour with very high precision. In the coming years, its results will be compared with data from other major experiments, including Hyper-Kamiokande in Japan and DUNE in the US.

Source

Study: The JUNO Collaboration, “Measurement of reactor neutrino oscillation with the first JUNO data”, Nature, 2026.