The Quietest Jar of Liquid at the Bottom of a Mountain
In central Italy, 1,400 meters below the Gran Sasso mountain, sits a vat of 5.9 tonnes of ultrapure liquid xenon. Cosmic rays are suppressed a millionfold by the rock overhead; krypton contamination is below 38 ppq (parts per quadrillion); radon radioactivity measures 0.9 µBq/kg—a number that is telling, because it exactly matches the expected interaction rate from solar neutrinos. In other words, this detector is so quiet that its residual background is on par with the signal coming from the Sun.
This vat of liquid xenon was originally built as a trap for dark matter.
On August 31, the XENON collaboration announced something else at a seminar at the Gran Sasso laboratory: using this dark matter detector, they achieved the first direct detection of the lowest-energy solar neutrinos—elastic scattering of pp neutrinos off electrons—at 5.0σ significance. The paper was simultaneously posted to arXiv (2608.29450).
The Sun's Lowest Voice
99% of the Sun's power comes from proton-proton fusion: two protons squeeze together into a deuterium nucleus, ejecting a positron and a neutrino. These pp neutrinos, with energies no higher than 0.42 MeV, are the bass section of the solar neutrino chorus. They are extremely shy—60 billion of them pass through an area the size of your fingernail every second, and nearly all of them ignore you completely.
Because of this shyness, no one had ever directly measured this energy band before. The previous record belonged to Borexino: a neutrino energy threshold of 335 keV for real-time spectroscopy. XENONnT has smashed that down to about 17 keV—an order-of-magnitude leap. The paper's abstract states it plainly: "establishing the lowest energy threshold for any neutrino detection to date."
(A rigorous note: both 17 keV and 335 keV refer to neutrino energies; the experiment actually analyzed electron recoils of 1–140 keV. A 17 keV neutrino can transfer at most 0.56 keV to an electron—so what this measurement really showcases is sub-keV-level sensitivity. For a textbook reference point: 17 keV is roughly 8,500 times the energy of a visible-light photon, or about one-third of a medical X-ray.)
The Data Itself
The exposure was 2.46 tonne·years (two data-taking periods: 96.5 days in 2021, and 117.8 days across 2022–2023). In the signal window, pp neutrinos accounted for 89% of the solar neutrino components. The measured pp neutrino flux was (10.2 ± 2.0) × 10¹⁰ /cm²/s, higher than Borexino's (6.1 ± 0.5) × 10¹⁰, but statistically compatible at 1.9σ—the standard solar model predicts 5.98 × 10¹⁰, so everything is still on the negotiating table.
There's also a competitive subplot: a paper "addendum" notes that China's PandaX-4T team pursued the same physics target but reached only 2.2σ. XENONnT got to 5.0σ.
The Ironic Part: Glare
The real drama of this story lies in the identity reversal. This machine was built to catch dark matter, yet keV-scale electron recoils are both the fingerprint of light dark matter and the footprint of low-energy neutrinos—the two look identical inside a detector. Earlier this February, the collaboration officially announced they had "entered the neutrino fog": ⁸B neutrinos had begun washing out the detector the way sunlight floods a darkroom (their words: picking up a "glare" of neutrinos from the core of the Sun). That search excluded dark matter cross-sections above 2.5 × 10⁻⁴⁵ cm² at 6 GeV/c² and earned a PRL annual highlight.
Spokesperson Elena Aprile (Columbia University) put it better than this article can: "Progress driven by the search for dark matter is opening entirely new windows to the Universe. The technology developed to observe nature's rarest interactions now allows us to explore fundamental questions far beyond our original science goals." Ranny Budnik of the Weizmann Institute added an almost theatrical remark: "We are entering an era where we can see things that were previously out of reach—and the most fascinating things are always the ones we didn't expect."
An ear built for the most silent particles in the universe has first heard the heartbeat of our own star. The next generation, XLZD (60 tonnes of active mass, roughly ten times the current detector), will listen to both sides at once: dark matter, and a sub-percent-precision checkup of the solar core.
Sources
- Official announcement: https://xenonexperiment.org/xenonnt-first-measurement-of-low-energy-solar-neutrinos-scattering-off-electrons/
- Paper PDF: https://xenonexperiment.org/wp-content/uploads/2026/08/XENONnT_Solar_pp_compressed.pdf (arXiv:2608.29450)
- Weizmann press release: https://wis-wander.weizmann.ac.il/space-physics/new-window-universe-solar-neutrinos-measured-record-low-energies
- Neutrino fog: https://xenonexperiment.org/weve-reached-the-neutrino-fog/
- Borexino reference data: https://arxiv.org/abs/1707.09279