How do you observe a process that takes more than one trillion times longer than the age of the universe? The XENON Collaboration research team did it with an instrument built to find the most elusive particle in the universe — dark matter. In a paper published in the journal Nature, researchers announced that they have observed the radioactive decay of xenon-124, which has a half-life of 18 sextillion years.

“We actually saw this decay happen. It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it,” says Ethan Brown, assistant professor of physics and co-author of the study.

The XENON Collaboration runs XENON1T, a 1,300-kilogram vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in deep water 1,500 meters beneath the Gran Sasso mountains of Italy. The researchers search for dark matter (which is five times more abundant than ordinary matter, but seldom interacts with ordinary matter) by recording tiny flashes of light created when particles interact with xenon inside the detector. And while XENON1T was built to capture the interaction between a dark matter particle and the nucleus of a xenon atom, the detector actually picks up signals from any interactions with the xenon.

The evidence for xenon decay was produced as a proton inside the nucleus of a xenon atom converted into a neutron. In most elements subject to decay, that happens when one electron is pulled into the nucleus. But two protons in a xenon atom must simultaneously absorb two electrons to convert into two neutrons, an event called “double-electron capture.”

Double-electron capture only happens when two of the electrons are right next to the nucleus at just the right time, Brown says.