Researchers have shown that the world’s most sensitive dark matter detectors can be repurposed to hunt for a previously undetectable class of neutrinos from the Sun: thermal solar neutrinos with energies in the kiloelectronvolt range. While no detection has been made yet, the work establishes a new experimental pathway to probe the deepest, most elusive processes inside our star.
The study, led by Carlos A. Arguelles of Harvard University and published on the arXiv preprint server on June 14, demonstrates that large-volume dark matter direct detection experiments such as XENONnT, LZ, and PandaX are sensitive to the faint electron ionization signals that would be produced by keV-scale solar neutrinos. These neutrinos, generated by electroweak processes in the solar plasma, represent the lowest-energy neutrino flux from the Sun and have never been directly observed.
### A New Use for Dark Matter Experiments
Dark matter detectors like XENONnT, located at Italy’s Gran Sasso National Laboratory, are designed to catch the rare interactions of hypothetical weakly interacting massive particles (WIMPs) with xenon atoms. But their ultra-low background environments and exquisite sensitivity to small energy deposits also make them natural neutrino observatories.
The team used S2-only data from XENONnT — signals from secondary scintillation produced when ionized electrons drift upward in a liquid xenon time projection chamber — to search for the ionization signatures of incoming thermal solar neutrinos. The analysis placed an upper limit on the flux of these neutrinos: no more than about 120 million times the Standard Model predicted value. Combined searches from XENONnT, LZ, and PandaX produced slightly weaker constraints.
While a factor of 10^8 above the predicted flux seems like a weak limit, it represents the first experimental constraint ever placed on thermal solar neutrinos, which no previous detector had any sensitivity to. The result is analogous to the early days of solar neutrino physics in the 1960s, when the first upper limits were orders of magnitude above theoretical predictions but eventually led to the discovery of neutrino oscillations and a Nobel Prize.
### The Future: XLZD
The next-generation XLZD experiment, which will combine the resources of XENON, LZ, and DARWIN collaborations into a single 60-to-100-tonne liquid xenon detector, could improve these limits by orders of magnitude, potentially making a first detection possible within the next decade.
“While still far from a detection, this result establishes low-threshold direct detection experiments as a viable probe of the lowest-energy neutrino sources in astrophysics, with important implications for stellar physics and beyond,” the authors write.
### Why Thermal Solar Neutrinos Matter
Thermal solar neutrinos are produced throughout the solar interior by processes such as neutrino pair bremsstrahlung and plasmon decay in the hot, dense plasma. Unlike the higher-energy neutrinos from nuclear fusion reactions in the Sun’s core — pp-chain and CNO-cycle neutrinos, which have been studied for decades — thermal neutrinos carry information about the Sun’s temperature profile and thermodynamic state, not just its nuclear reactions.
Detecting them would provide an independent probe of the Sun’s internal structure, complementing helioseismology and standard solar model calculations. It could also open a window to beyond-Standard-Model physics, as any deviation from the predicted flux could indicate new neutrino interactions or properties.
The 15-page paper, with 13 figures, is cross-listed in high-energy physics, cosmology, and solar astrophysics. The authors include collaborators from Harvard, the University of Virginia, Washington University in St. Louis, and the University of Amsterdam.