Dark matter detector captures the weakest neutrinos ever detected

A detector built to search for dark matter has unexpectedly become a tool for studying another elusive particle. The XENON collaboration has announced that its XENONnT experiment has, for the first time, detected the scattering of low-energy solar neutrinos off electrons.

The detector is located about 1,400 meters underground beneath Gran Sasso mountain in Italy. Researchers were able to detect neutrinos with energies of around 17 keV — the lowest energy ever recorded in an experiment of this kind, according to Interactions.

The signal has a statistical significance of 5 sigma, meaning the probability that it is a random fluctuation is extremely small. The signal is dominated by so-called pp neutrinos, particles produced by proton-proton reactions inside the Sun. These reactions power our star, and pp neutrinos account for most of the solar neutrino flux.

A dark matter detector becomes a neutrino observatory

XENONnT was originally built for an entirely different purpose: searching for WIMPs, or weakly interacting massive particles. For years, WIMPs were considered one of the leading candidates for dark matter, which is estimated to make up about 85% of all matter in the universe.

At the heart of the experiment is a huge chamber containing 5.9 tonnes of ultra-pure liquid xenon. When a particle interacts with the xenon, it produces faint flashes of light and ionization signals. Scientists can use these signals to determine what happened inside the detector.

To detect extremely rare events, the researchers had to reduce background radiation from radioactive contaminants, particularly radon, to almost negligible levels. They did this through rigorous screening of the materials used in the detector and with a specialized cryogenic distillation system.

As a result, XENONnT became sensitive enough to detect interactions involving solar neutrinos that no other detector had previously been able to observe.

“This is an important milestone toward studying the physics of low-energy neutrinos,” said Masatoshi Kobayashi of Nagoya University, who led the data analysis.

The more sensitive the detector, the closer we get to the “neutrino fog”

The new result builds on another important XENONnT achievement. In 2024, the experiment became the first to detect coherent elastic scattering of solar neutrinos off atomic nuclei. The latest finding gives scientists another way to observe neutrinos and further demonstrates just how sensitive the detector has become.

But that sensitivity comes with a downside.

As detectors become better at picking up faint signals, solar neutrinos will become increasingly prominent in their data. That is a problem for dark matter searches because neutrinos can produce signals that look similar to the interactions researchers are trying to find.

Scientists refer to this fundamental limit as the “neutrino fog.” At some point, the neutrino background will become so significant that distinguishing it from a potential dark matter signal will be extremely difficult.

“This result shows that we are indeed approaching the point where neutrinos become a significant background,” Kobayashi told Scientific American.

Next-generation detectors will search for both dark matter and neutrinos

The experience gained with XENONnT is already helping shape the next major experiment, XLZD. The planned next-generation liquid-xenon detector will be roughly an order of magnitude larger.

The larger detector should improve the chances of detecting rare dark matter interactions while also allowing scientists to study solar neutrinos with much greater precision.

“This is compelling evidence of how mature the technology has become,” said Luca Grandi, a professor at the University of Chicago and a member of the XENON collaboration.

The hunt for dark matter is therefore gradually becoming a hunt for neutrinos as well. And as these experiments become more sensitive, physicists are getting closer to a fundamental limit — a point where a faint signal of new physics could effectively disappear into the stream of particles constantly arriving from the Sun.