Powerful dark-matter hunter ends up seeing ‘rarest thing ever recorded’

25 Apr 2019

Image: © Jürgen Fälchle/Stock.adobe.com

A lab buried deep in an Italian mountain has spotted an event so rare that it is almost beyond comprehension.

Once again, the almost incomprehensible scale of the universe boggles the mind with the discovery of something that, on the face of it, almost no one should be able to see.

In the search for elusive dark matter, scientists in the underground laboratory LNGS (Laboratori Nazionali del Gran Sasso), 1,500 metres down in the Italian Gran Sasso mountains, have observed the radioactive decay of xenon-124. In doing so, they have observed one of the rarest events ever recorded as this process is 1trn times longer than the age of the universe, currently a little under 14bn years old.

The incredible discovery was made using something called the Xenon1T detector, a 1,300kg vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in water at the heart of the mountain lab. The powerful equipment is used in the search for dark matter by recording tiny flashes of light created when particles interact with xenon inside the detector.

While built to sniff out 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.

Publishing their findings in Nature, the international researchers said evidence of the decay of xenon-124 was seen when 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. However, in an event called ‘double-electron capture’, a proton in a xenon atom must absorb two electrons to convert into a neutron.

According to a co-author of the study, Ethan Brown of the Rensselaer Polytechnic Institute in the US, this is “a rare thing multiplied by another rare thing, making it ultra-rare”.

He added: “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.

“It’s … amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.”

Colm Gorey was a senior journalist with Silicon Republic