The team caught a glimpse of a process that takes 18,000,000,000,000,000,000,000 years.
Scientists have observed an extremely rare particle physics
event using a detector that’s hunting for dark matter, the mysterious material
that physicists have yet to observe.
In a paper published in the journal Nature, researchers with
the XENON Collaboration said they’d observed the radioactive decay of a
substance called xenon-124, an isotope of the element xenon — a colorless and
odorless noble gas found in tiny amounts in the atmosphere. The event — a
“two-neutrino double electron capture” — has eluded scientists for decades.
It happens when “two protons in a nucleus are simultaneously
converted into neutrons by the absorption of two electrons from one of the
atomic shells and the emission of two electron neutrinos.” After this occurs,
the event shoots out a predictable cascade of X-rays and Auger electrons that
scientists look for using an ultra-sensitive detector, buried about 5,000 feet
beneath Italy’s Gran Sasso mountain where it’s shielded from cosmic rays.
“We have shown that we can observe the rarest events ever recorded,” Ethan Brown, a professor of physics at Rensselaer Polytechnic Institute and co-author of the study, told.
“The key finding is that an isotope formerly thought to be completely stable has now been shown to decay on an unimaginably long timescale.”
How long is that timescale? The team estimated that
xenon-124’s half-life is about 18 sextillion years — or 18,000,000,000,000,000,000,000
years — which is than one trillion times the age of our universe, according to
the team. It’s the slowest process ever measured directly, the team wrote in a
statement.
“It’s an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded,” Brown told.
“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.”
To get that good chance, the team had to expose their
detector to a huge amount of xenon atoms by stocking it with 3.2 tons worth of
liquid xenon.
“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.”
Although the team didn’t observe dark matter — which is the
primary purpose of the detector — the recent observations could help scientists
learn more about neutrinos, one of the least understood fundamental particles
in the universe.
“It proves that this XENON detector technology we use for
dark matter is much more versatile,” graduate student Christian Wittweg, Ph.D
student at the University of Münster in Germany, told Gizmodo. “We get all
these cool analyses… for free after having built an experiment sensitive enough
to hunt for dark matter.”
The team plans to use its newer XENONnT detector to continue
hunting for dark matter, the elusive material that’s estimated to comprise
about 26.8 percent of all the content in the universe.