Every second, tens of billions of neutrinos produced by nuclear fusion in the Sun pass through every square centimeter of the Earth – and through our bodies – almost without interacting with anything. 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.
Today, during a seminar hosted by the Laboratori Nazionali del Gran Sasso (LNGS) of Italy’s National Institute for Nuclear Physics, the XENON Collaboration announced 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.
The international XENON Collaboration has been searching for dark matter since 2006. It includes scientists from 30 institutions across the world, including the team of Prof. Ranny Budnik of the Particle Physics and Astrophysics Department at the Weizmann Institute of Science.
XENONnT was originally designed for the direct search for particle dark matter in our galaxy. At its heart is a dual-phase xenon Time Projection Chamber containing 5.9 tons of ultra-pure liquid xenon, installed at LNGS, 1,400 meters beneath the Gran Sasso massif. The detector is surrounded by water Cherenkov detectors that identify and reject cosmic-ray muons and neutrons, and it is capable of reconstructing the tiny flashes of light and ionization signals produced when a particle interacts with the xenon target.
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.
“”The physics of rare events is increasingly becoming for us a scientific target in its own right, with fascinating results like the ones we are presenting now and more still to come”
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.
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The XENON experiments became bigger and bigger: First, XENON100 operated with roughly 160 kg of xenon from 2008 until 2016; then XENON1T ran with a total of 3.2 tons from 2015 until 2018; finally, XENONnT started its operation with a total of about 8.6 tons of xenon across all its systems in 2020.
“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.”
Says the Weizmann Institute’s Budnik, who, alongside his research role, serves on the collaboration’s editorial board: “This measurement is, on the one hand, an opportunity that arose from the exceptional sensitivity and extremely low backgrounds we achieved in our search for dark matter. At the same time, however, the physics of rare events, including neutrinos, is increasingly becoming for us a scientific target in its own right, with fascinating results like the ones we are presenting now and more still to come.
“We have entered an era in which we can see things that were previously beyond our reach, and it is always fascinating to discover what is waiting for us there – whether it is what we expected to find or, even more intriguingly, something we did not expect at all.”
Prof. Ranny Budnik’s research is supported by the Krenter-Perinot Center for High-Energy Particle Physics.