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.

The ReD setup is a tank of liquid argon into which neutrons from a californium source were fired. The neutrons hit argon nuclei like billiard balls. The impact energy ranges from 2 to 10 keV — very weak, like a grain of sand landing on a table. The scientists measured how much charge is released with each hit. It turned out that at energies below 7 keV, argon gives off more charge than older models predicted.

The new data will help fine-tune detectors searching for light dark matter particles. If such a particle strikes an argon nucleus, we'll know how much charge to expect. It's like calibrating a scale before weighing an invisible load.