Xenon Collaboration

In a subterranean laboratory, about a mile below Italy's Gran Sasso mountains, scientists are hunting for dark matter using an incredibly powerful detector full of liquid xenon. In their search for the elusive particle, they observed something else entirely.

Their dark matter detector witnessed the rarest event ever recorded: the radioactive decay of xenon-124.

The research, published in Nature on April 24, is authored by the XENON collaboration, a project consisting of over 160 scientists aiming to discover dark matter using the XENON1T detector. Their findings demonstrate xenon-124, an isotope of the element xenon used in flash lamps and ion thrusters in spacecraft, is not as stable as was once suspected.

The detector consists of a cylindrical tank filled with over 7,000 pounds of liquid xenon cooled to -139 degrees Fahrenheit (-95 degrees Celsius). It's so far underground that it blocks out any radioactive interference that could mess with dark matter measurements.

The supreme fine-tuning and clean measurements allowed by XENON1T enabled scientists to witness xenon-124 decay away at a rate that far exceeds the life of the universe. To do so, they measured the half-life of the atoms.

"Drugs are absorbed by the body with a half-life of minutes to hours, organisms reproduce with a half-life of days or years, chemical reactions happen with a half-life of seconds," explains Ethan Brown, a co-author on the study. "The half-life for this process is the slowest one ever observed, more than a trillion times longer than the entire history of the universe."

Xenon-124's half-life is 1.8 × 10^22 years (18,000,000,000,000,000,000,000), making the observation an "ultra-rare" event, according to the research collaboration.

You may be thinking "If the universe is almost 14 billion years old and this takes a trillion times longer... how can we even see it at all?" and let me tell you, that's exactly what I was thinking, too. It's not only a groundbreaking discovery, but a mind-breaking one.

The research team weren't just holding a magnifying glass over a single xenon atom, hoping to see it decay. In XENON1T there is a nearly unfathomable amount of xenon atoms, thanks to all that liquid xenon, effectively allowing the scientists to "watch" trillions of atoms. While only a small fraction of those are xenon-124, that still provides a good chance of nabbing the physics needle in a haystack. And rather than observing the atoms decaying directly, scientists look for signs of decay -- the X-rays and electrons released when xenon-124 decays.

The collaboration was able to spot 126 such processes over two years, which allowed them to calculate the mindbogglingly long half-life.

While the decay of xenon-124 is not related to the hunt for dark matter, the discovery provides a huge boost in confidence that the XENON1T detector could one day provide evidence for dark matter. Brown calls it a "huge step in validating our ability to measure rare physics.

"This shows just how well we understand our detector and makes us extremely confident in our results for the search for dark matter," he says.

XENON1T was shut down in December 2018 for a planned upgrade known as XENONnT. The new detector will be three times larger, holding even more liquid xenon, making it even more sensitive to rare events and, hopefully, the discovery of dark matter particles.