Dark Matter Detector Catches the Faintest Solar Neutrinos Ever Seen

Physicists searching for dark matter with the XENONnT detector at the Laboratori Nazionali del Gran Sasso in Italy have instead detected the lowest-energy neutrinos ever recorded from the Sun. Neutrinos are subatomic particles with almost no mass and no electric charge that rarely interact with matter; about 100 billion of them pass through a human thumbnail every second. In a paper submitted to arXiv.org on August 29, the international XENONnT team reported catching hundreds of solar neutrinos with energies as low as 17 kiloelectron volts (keV), about a tenth the energy of the weakest neutrinos detected before. The result has not yet been peer-reviewed.

The neutrinos come from the proton-proton fusion reaction that powers most of the Sun's energy output, when two protons merge to form a larger atomic nucleus. XENONnT caught them by detecting flashes of light produced when a neutrino collides with an electron inside the detector — the first time a dark matter detector has identified neutrinos this way. The detection reached a statistical significance of 5 sigma, the threshold physicists usually require to call a result definitive.

A similar but less certain hint had already turned up elsewhere. In July, the PandaX-4T detector at the China Jinping Underground Laboratory in Liangshan reported evidence of the same kind of low-energy neutrino, but only at about 2.2 sigma — not strong enough to count as a firm detection. In 2024, XENONnT and PandaX-4T had both spotted higher-energy solar neutrinos, which come from a rarer fusion reaction and are easier to detect because they strike an entire atomic nucleus rather than a single electron.

Florian Jörg, a XENONnT physicist at the University of Zurich, said a more advanced version of the detector could eventually extract detailed information about the Sun's internal composition from this class of neutrino. Kate Scholberg, a neutrino physicist at Duke University who was not involved in the study, said picking out the faint signal from background noise is like 'picking the neutrino needles out of the background haystack,' and called the team's work 'a pretty heroic job.'