Dark matter detectors are essentially incredibly sensitive microphones, capable of hearing a whisper amid the roar of a rock concert. Their job is to catch the rare nudges from hypothetical dark matter particles, which are thought to occasionally collide with atomic nuclei of ordinary matter. However, in a new study, scientists have retuned these microphones to a different "sound"—the faint "rustle" of solar neutrinos. Neutrinos are ghostly particles that barely interact with matter: they pass through everything unhindered, and every second, about a hundred billion of these "ghosts" fly through your fingernail without you feeling a thing. But while ordinary neutrinos, say from nuclear reactors, are relatively "loud," those born from the Sun's thermonuclear heat are almost silent: their energy is a thousand times lower, making detection exceptionally challenging.
The detection mechanism is as follows: when a low-energy solar neutrino does happen to strike an electron in a xenon atom, it transfers a tiny fraction of its energy. This "kick" knocks out the electron, and as it moves through the liquid xenon, it leaves a microscopic trail of excited atoms. The detector picks up this tremor, like a tiny membrane quivering from a breath of air, and converts it into an electrical signal. So far, no statistically significant signal has been captured: background from natural radioactivity and other neutrino fluxes still masks the sought-after events, but it is already clear that the same instruments searching for the Universe's invisible substance can peer into the blazing heart of a star. The irony is that a machine built to hunt the most mysterious substance can now tell us how ordinary solar plasma boils, providing data for helioseismology — a method that, from the frequencies of neutrino "notes," reconstructs temperature and density in the Sun's core. The next generation of microphones, with increased xenon mass and reduced noise levels, will make this picture clear, possibly paving the way for routine monitoring of thermonuclear processes in our star.
Neutrinos escape the Sun's interior in a couple of seconds, but the light we see takes tens of thousands of years to fight its way out.