At the heart of the XENONnT detector, 5.9 tonnes of ultra-pure liquid xenon have enabled the first observation of low-energy solar neutrinos scattering off electrons. The XENON Collaboration reports achieving a record neutrino energy threshold of 17 keV, allowing detection of these elusive particles from the Sun’s proton-proton fusion reactions. This measurement, announced during a live seminar hosted by the INFN Laboratori Nazionali del Gran Sasso, extends the frontier of direct neutrino observations and demonstrates the detector’s capability to probe complementary aspects of neutrino physics while continuing its search for dark matter.
XENONnT Detector Achieves 17 keV Threshold for Solar Neutrino Observation
Originally constructed for dark matter research, the 5.9-tonne detector has now facilitated the detection of previously unseen low-energy solar neutrinos produced by proton-proton fusion within the Sun. Detecting these elusive particles required meticulous background reduction and precise quantification, particularly addressing trace amounts of radioactive radon released from detector materials. The XENON Collaboration developed techniques, including extensive material screening and a dedicated cryogenic distillation system, to suppress this interference and constrain every relevant background contribution to exceptionally low rates.
The resulting signal is dominated by pp neutrinos, which account for the vast majority of the Sun’s neutrino emission and provide insight into the primary energy source of our star. Achieving a statistical significance of 5σ, the standard for discovery in particle physics, validates the observation and underscores the detector’s capabilities.
This result builds upon decades of solar neutrino research conducted at the Gran Sasso facility, following earlier measurements from GALLEX/GNO and Borexino. Elena Aprile, Professor at Columbia University and spokesperson of the XENON Collaboration, said, “This observation of low-energy solar neutrinos demonstrates how advances driven by the search for dark matter are opening new windows on the Universe.” The success of XENONnT is also laying the foundation for future experiments like XLZD, which aims to increase the target mass tenfold and further refine the search for dark matter and low-energy neutrino interactions.
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.
Elena Aprile, Professor at Columbia University and spokesperson of the XENON Collaboration
Radon Suppression and Background Control Enable 5σ Neutrino Signal
The challenge of detecting low-energy solar neutrinos hinges on isolating incredibly faint signals from pervasive background radiation, a feat accomplished by the XENON Collaboration through meticulous control of detector materials and innovative purification techniques. A dedicated, online cryogenic distillation system was implemented to continuously remove radon directly from the 5.9-tonne liquid xenon volume. This rigorous approach extended beyond radon suppression, as the Collaboration identified and constrained every relevant background contribution at exceptionally low rates.
Beta decays from lead and krypton isotopes, alongside smaller contributions from material-induced gamma rays and other sources, were all carefully quantified and accounted for in the data analysis. The success in managing background noise not only enabled this specific neutrino detection but also expands the scientific capabilities of XENONnT.
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