Dark Matter Hunters Observe 'Rarest Event Ever Recorded'

The XENON1T dark matter collaboration has observed the radioactive decay of xenon-124. XENON1T

Researchers have measured a process that takes more than one trillion times the age of the universe to complete, using an instrument built to search for dark matter—the most elusive particle known to man.

An international team from the XENON Collaboration announced it has observed the radioactive decay of a substance called xenon-124, a form, or isotope, of the element xenon—a colorless, dense, odorless gas found in trace amounts in Earth's atmosphere.

Experts have previously predicted that xenon-124's half-life—the time it takes, on average, for half a radioactive substance's material to decay—would be around 160 trillion years. However, no evidence of this process has appeared until now.

In a study published in the journal Nature, the scientists show that the true figure is far higher. In fact, they determined that xenon-124's half-life is a staggering 18 sextillion years, or 18,000,000,000,000,000,000,000 years—far surpassing the age of the universe at around 13.8 billion years old.

Ethan Brown, a professor of physics at Rensselaer Polytechnic Institute and co-author of the study, said that this is the "longest, slowest process that has ever been directly observed."

"We have shown that we can observe the rarest events ever recorded," Brown told Newsweek. "The key finding is that an isotope formerly thought to be completely stable has now been shown to decay on an unimaginably long timescale."

To make their observation, the researchers used an advanced detector known as XENON1T—a 2,900-pound vat of super-pure liquid xenon in a cool chamber that's submerged in water around 5,000 feet beneath Italy's Gran Sasso mountain.

"We designed the XENON1T experiment to look for dark matter, a new kind of matter that makes up 85 percent of the mass of the universe, but interacts so rarely that it's never been observed," Brown said. "This experiment is so sensitive to very rare events that we can make all kinds of other rare physics measurements. One of those is this decay of xenon-124. Although our primary goal was always the discovery of dark matter, we knew there was a good chance we could see this rare decay, so we set out to do so."

Nevertheless, the odds of observing any single decay of xenon-124 are still vanishingly small because the timescales involved are "more than a trillion times longer than the entire history of the universe," according to Brown.

"There have to be enough decays to be measured, and the slower the process the fewer decays there are," he said. "With such a long time scale, the decays happen extremely rarely, so we get around this by having a huge number of xenon-124 atoms in our detector. Even at that, it took a year's worth of looking to see a very small number of decays."

Aside from having huge numbers of xenon atoms, the researchers also needed to ensure that the detector was one of the purest places on Earth, in order to eliminate any outside interference, such as cosmic rays.

"XENON1T is a giant vat of liquid xenon surrounded by light sensors," Brown said. "When dark matter collides in the xenon, or when a radioactive decay occurs inside, we get a tiny flash of light and a little bunch of charge out of the xenon. We measure these with the light sensors and reconstruct everything we can about the original event that caused the light and charge."

"Since dark matter collisions and xenon-124 decays are so rare, we have to have the cleanest environment imaginable, so we use ultra-clean materials and operate the detector deep under the mountains to block from cosmic rays and other backgrounds," he said. "We then monitor this huge volume of xenon for as long as we can to try to see these rare events."

According to the researchers, the evidence for the decay came from the first direct observation of a physical process known as "neutrino double electron capture" in xenon-124. This is when a proton inside the nucleus of a xenon atom converts into a neutron.

For most elements that experience radioactive decay, this happens when one electron is sucked into the nucleus. But a proton in a xenon atom has to absorb two electrons to convert into a neutron—thus the name "double-electron capture."

This can only happen when two electrons are right next to the nucleus at exactly the right time—"a rare thing multiplied by another rare thing, making it ultra-rare," Brown said in a statement.

"The rare double electron capture and long half-life that we measured are exciting, but along with this we have demonstrated a huge step in rare event detection," he said. "Other rare event searches for dark matter and rare neutrino interactions will build upon the success of this measurement."