M. Atzori Corona

In the dark chamber of the detector, argon waits. When an invisible particle (say, a dark matter candidate) flies through, it gives the argon nucleus a gentle shove. The nucleus is knocked out of place, and charges are born around it. Exactly how many charges is no trivial question: physicists use that number to tell what kind of visitor dropped by.

In a dark chamber where the temperature sits just four degrees above absolute zero, a sapphire crystal the size of a grain of rice lies waiting. When a neutrino, a ghostly particle that barely notices matter, flies through it, the crystal shudders and warms by a tiny fraction of a degree.

A detector that hunts for rare particles is like an ear trying to catch a whisper in a noisy room. The quieter the signal, the harder it is to tell apart from random noise. In the NUCLEUS experiment, such detectors, cryogenic calorimeters, are cooled to near absolute zero so they can pick up even a faint pulse of heat from a single particle.