In a dark room, there is a tank of liquid argon. Sometimes an invisible particle flies through it, strikes an atomic nucleus, and a faint light flashes in the liquid. From this flash, physicists try to figure out what kind of guest has arrived. It could be a dark matter particle — the same mysterious substance that makes up most of the matter in the universe but has never been seen.
The DarkSide-50 detector is exactly such a tank, albeit a small one: it holds only 46 kilograms of argon. For comparison, other similar setups weigh hundreds or thousands of kilograms. Yet even with its modest size, DarkSide-50 has set one of the best limits on the mass of light dark matter particles — down to 1.2 gigaelectronvolts, roughly the mass of a proton. To achieve this, physicists needed a very precise understanding of how an argon nucleus releases energy upon collision. This process is called the ionization yield, and it was measured in four different experiments: DarkSide-50, ARIS, SCENE, and ReD.
Now, all these data have been combined into a unified model. It describes how many electrons are knocked out of an atom when a particle strikes, across a wide energy range — from 0.4 to 150 kiloelectronvolts. The model is based on a simple idea: the nucleus and the ejected electrons interact within a small "box"—the so-called Thomas-Imel model. But even within this model, there is a choice: how exactly to describe the repulsion between nuclei during a collision. The scientists tested three options and, using Bayesian model comparison, found that two of them agree poorly with the data, while the third, the Lenz-Jensen function, fits best.
The new model made DarkSide-50 even more sensitive to light dark matter particles: those lighter than 3 gigaelectronvolts. And crucially, it refined calculations for the future DarkSide-20k detector, which will be hundreds of times larger. Once it goes online, dark matter will have even fewer places to hide.
The same tank of liquid argon is now listening to the silence more attentively. And if dark matter truly exists, its whisper will grow louder.
The liquid argon in the detector is colder than the surface of Pluto: its temperature is about minus 186 degrees Celsius. At such temperatures, argon becomes transparent to its own light flashes, allowing the detector to "see" every collision.
The hunt for dark matter resembles the plot of the movie "Interstellar," where heroes search for an invisible force affecting gravity — except here, instead of a black hole, we have a liquid argon detector.