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An underground detector has caught the elusive particles from the Sun and set a new limit on dark matter.
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Deep beneath the Apennine Mountains, in a tank holding three tons of liquid xenon, something gives a barely perceptible shudder every few days. It's a solar neutrino, a ghostly particle born in the Sun's thermonuclear furnace, that has traveled 150 million kilometers through empty space and solid rock to finally collide with a xenon nucleus. The XENONnT detector, like a patient pearl diver, spent seven years collecting such rare flashes of light and has gathered enough to confidently declare: the signal is real.
Neutrinos barely interact with matter — trillions of these particles pass through your body every second without touching a single atom. But occasionally, very rarely, a neutrino strikes a xenon nucleus dead on, and the nucleus recoils like a billiard ball hit by a cue. The flash of light from that recoil is the detector's prize. Over seven years of observation, physicists counted about 11 such events from neutrinos born in the decay of boron-8 in the Sun's interior. This allowed them to measure the solar neutrino flux: it matches previous estimates, but now with better precision.
Solar neutrinos are not just messengers from the star's core; they also form a background that hinders the search for dark matter. When detector sensitivity reaches a certain threshold, the neutrino signal becomes indistinguishable from hypothetical dark matter particles — the 'neutrino fog' sets in.
This fog has become the main limitation in the hunt for light dark matter. The researchers increased the observation time by 93% compared to the previous search, but the sensitivity to hypothetical particles with a mass of 5 GeV (about five protons) improved by only 10%. Further data accumulation will hardly help: the neutrino background cannot be outshouted. However, the same data allowed measuring the weak interaction mixing angle at ultra-low energies — a quantity that tests the Standard Model of particle physics. So far, everything agrees with theory, but now we have a ruler to measure deviations.
Thus, an underground detector built to search for dark matter has also become a precise tool for studying the Sun and fundamental forces. Every captured ghost is both a message from the star's core and a strict exam for our understanding of the Universe.
Over the seven-year experiment, about $10^{28}$ solar neutrinos passed through the XENONnT detector, but only about 11 of them left a noticeable trace: that's how antisocial these particles are.
In Arthur C. Clarke's novel 'A Meeting with Medusa', neutrinos are used for communication through the Earth's crust — much like in the real experiment, where particles pass through mountains without noticing them.