Scientist Have Seen The "Rarest Event Ever Recorded"

The photomultipliers in the Xenon1T detector. XENON collaboration

Scientists have observed what is being called the "rarest event ever recorded" in the ongoing hunt for that most elusive of particles, dark matter.

They performed this incredible observation using a dark matter detector known as XENON1T, run by the XENON Collaboration project based in Italy. It recorded the radioactive decay of a xenon-124 atom, a process that takes a mind-bogglingly long time. Xenon is a noble gas. This particular type of xenon has a half-life of 18 billion trillion years. That’s more than 1 trillion times longer than the current age of the universe. The observation is reported in detail in Nature.

"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," co-author Ethan Brown, an assistant professor of physics at Rensselaer, said in a statement. "It's amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded."

You might be wondering how we were able to observe it if it is such an incredibly rare event. The reason is that a half-life is a probabilistic measure. It's the time it takes for exactly half of the "entities" in a sample to decay. So, the time it would take for half of the atoms in a radioactive substance like xenon to decay is 18 billion trillion years. It doesn’t mean that such an event only happens once in that time.

The detector hosted 3,500 kilograms (7,716 pounds) of xenon, so it had roughly 17 billion billion billion atoms (1.701×1028) in it. Of those, just a single one decayed and the setup of the experiment was able to detect it, despite not having been designed for this particular task.

The decay of a xenon atom happens through a process called two-neutrino double electron capture. Previously this has been seen in only two other elements, krypton and barium. Here, the xenon nucleus captures two electrons from its surrounding electron shell. These electrons interact with two protons, turning them into neutrons and liberating two neutrinos.

"Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell," Brown explained. "The remaining electrons collapse to the ground state, and we saw this collapse process in our detector."

This first direct detection has allowed researchers to refine the half-life of this particular xenon atom.

The XENON Collaboration includes more than 160 scientists from Europe, the US, and the Middle East. The dark matter detector is housed in the Gran Sasso National Laboratory, located deep beneath the highest peak of the Italian Apennine Mountains. Both the location and substance used are key to possibly finding dark matter, a theoretical substance that should permeate the cosmos but does not interact with light.

Researchers hope that one way of seeing it could be if it accidentally collides with a xenon atom, creating a flash of light that the detector can spot. Xenon is used because it is extremely stable (as this research shows) and by being underground the detector is shielded from cosmic rays. The whole system is currently being upgraded to become XENONnT and it will soon have 8 tons more xenon in it, where further experiments can be carried out on grander scale.  

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