XENONnT: First Measurement of Low-Energy Solar Neutrinos Scattering off Electrons

September 2, 2026
The XENON collaboration
Kobe University Graduate School of Science
Institute of Science Tokyo
The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI)
The University of Tokyo Institute for Cosmic Ray Research (ICRR)
Nagoya University Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI)
Nagoya University Institute for Space-Earth Environmental Research (ISEE)

Scientists from the international XENON collaboration, an international experimental group including the Kobe University Graduate School of Science, the Institute of Science Tokyo, The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), The University of Tokyo Institute for Cosmic Ray Research (ICRR), Nagoya University Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), and Nagoya University Institute for Space-Earth Environmental Research (ISEE), announced on August 31 during a seminar hosted by the INFN Laboratori Nazionali del Gran Sasso (LNGS), Italy, the first observation of low-energy solar neutrinos scattering off electrons in the XENONnT detector.

The measurement extends the frontier of direct neutrino observations down to neutrino energies of about 17 keV, the lowest neutrino energy threshold ever achieved. The detected signal is dominated by pp neutrinos, produced in the proton-proton fusion reactions that power the Sun and account for the vast majority of its neutrino emission.

Every second, tens of billions of neutrinos produced by nuclear fusion in the Sun pass through every square centimetre of the Earth - and through our bodies - almost without interacting. Although they are one of the most abundant particles emitted by the Sun, their extremely weak interactions make the detection of these elusive particles one of the greatest experimental challenges in particle physics.

The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) Kamioka Branch Director and Associate Professor Kai Martens is a leader of XENON in Japan and XENON Principal Investigator at Kavli IPMU.

"We built XENONnT to win the race to find find dark matter, the stuff that keeps our universe together - unfortunately no such luck yet. Fortunately though we also built this detector to be the cleanest of its kind, allowing us to reach to lower energies than our competitors. And this what allowed us to present the first actual measurement of the lowest energy solar neutrinos from a liquid xenon detector.

"With more data already taken and new data about to come in again we expect to ultimately provide the most precise measurement of our Sun's lowest energy neutrino emission. We already probed the Sun's higher energy neutrinos and are eagerly waiting for a Supernova explosion to send its neutrinos our way: With XENONnT Neutrino astrophysics has a new leg to stand on," said Martens.

Observing this feeble low-energy neutrino signal at 5σ - the statistical significance conventionally used in particle physics to claim a discovery - required not only reducing the detector backgrounds to unprecedented levels of purity but also quantifying them precisely. The dominant challenge comes from trace amounts of radioactive radon constantly released by detector materials. Over many years, the XENON Collaboration has pioneered techniques to suppress this background through extensive material screening and a dedicated online cryogenic distillation system that continuously removes radon from the xenon. The Collaboration identified and constrained every relevant background contribution at exceptionally low rates, including beta decays from lead and krypton isotopes, smaller contributions from material-induced gamma rays, as well as other subdominant components.

The result further expands the scientific reach of XENONnT. Following the recent observation of coherent elastic neutrino-nucleus scattering from higher-energy solar neutrinos, this new measurement demonstrates that the same detector can probe complementary aspects of neutrino physics, while continuing its primary search for dark matter. XENONnT is thus emerging as one of the world's most sensitive observatories for rare low-energy particle interactions.

The measurement also builds upon a long tradition of solar-neutrino research at LNGS. GALLEX/GNO provided the first measurements of the low-energy solar neutrino flux using radiochemical techniques, while Borexino pioneered the real-time spectroscopy of individual solar neutrino interactions with a neutrino energy threshold of 335 keV. XENONnT now extends this legacy, lowering the solar-neutrino energy threshold to 17 keV.

Beyond this achievement, XENONnT offers a glimpse of the future of rare-event physics. The technologies developed to build and operate one of the cleanest particle detectors are laying the foundation for the next generation of liquid-xenon observatories. The planned XLZD experiment, with a target mass an order of magnitude larger than XENONnT, aims to extend the search for dark matter to the highest sensitivities ever achieved and to measure low-energy solar neutrinos with exceptional precision, while opening new opportunities in neutrino physics and the study of other extremely rare processes.

"This observation of low-energy solar neutrinos demonstrates how advances driven by the search for dark matter are opening entirely new windows on the Universe," said Elena Aprile, Professor at Columbia University and spokesperson of the XENON Collaboration. "It shows that technologies originally developed to observe some of the rarest interactions in nature are now enabling us to explore fundamental questions well beyond their original scientific goals."

For more details, see the press release by XENON Collaboration.

Related link
XENONnT: First Measurement of Low-Energy Solar Neutrinos Scattering off Electrons (XENON Collaboration website)