Deep under an Italian mountainside, a giant detector filled with tons of liquid xenon has been looking for dark matter—particles of a mysterious substance whose effects we can see in the universe, but which no one has ever directly observed. Along the way, however, the detector caught another scientific unicorn: the decay of atoms of xenon-124—the rarest process ever observed in the universe.
The results from the XENON1T experiment, co-authored by University of Chicago scientists and published April 25 in the journal Nature, document the longest half-life in the universe—and may be able to help scientists hunt for another mysterious process that is one of particle physics’ great mysteries.
Not all atoms are stable. Depending on their makeup, some will stabilize themselves by releasing subatomic particles and turning into an atom of a different element—a process called radioactive decay.
We’re much more familiar with radioactive elements like uranium and plutonium—these are the wild teenagers of radioactive elements, constantly hurling off particles. Radon-222, for example, has a half-life of just four days. Some elements, however, decay very, very slowly. Xenon-124 is one such elder statesman: Its half-life is one trillion times longer than the age of the universe, and as such, the chance of detecting its decay is very small.
“This is the longest lifetime that we have ever directly measured,” said Luca Grandi, assistant professor of physics at the University of Chicago and co-author of the study. “Its detection was possible only thanks to the tremendous effort that the collaboration put into making XENON1T an ultra-low background detector. This made the detector ideal for rare event searches such as the detection of dark matter—for which it was designed—as well as other elusive processes.”
Grandi is one of the scientists who worked on the XENON1T detector, an extremely sensitive machine tucked nearly a mile below the surface of the Gran Sasso mountains in Italy. The depth and the gigantic water pool in which the detector is immersed protect the detector from false alarms coming from cosmic rays and other phenomena as it searches for evidence of a particle called a “WIMP,” one proposed candidate for dark matter.
The XENON1T detector is filled with three tons of xenon, which is kept cooled down to minus 140 degrees Fahrenheit and constantly purified (even a few atoms peeling off the metal sides of the container could throw off the measurements). The detector, which Grandi and the UChicago team helped develop, build and operate, detects flashes of light that are produced after a particle strikes a xenon atom.
The XENON1T detector is optimized to detect very rare processes, as dark matter particles are expected to interact very rarely with ordinary matter. But it can also pick up other signals: in this case, the tracks produced as atoms of xenon-124 decay inside the detector. There are enough atoms of xenon-124 inside the detector that this was observed 126 times in the year that XENON1T was taking data.
The data helped the collaboration make the first definitive measurement of xenon-124’s half-life: 18 billion trillion years.
This decay process is called two-neutrino double electron capture. It happens when two protons in the xenon nucleus each simultaneously absorb an electron from the atomic shell and emit a neutrino—converting both protons into neutrons.
This is closely related to another process that intrigues physicists, called the double beta decay process. “If scientists observed a neutrino-less version of double beta decay, we would know that a neutrino is its own antiparticle,” Grandi said. If so, it would require physicists to revisit their picture of how the universe works—and could even open the door to some fundamental questions, like why there is more matter than anti-matter in the universe.
No one has yet been able to observe such an event, but the xenon-124 decay measurement gives scientists information about how to look for it—by nailing down the parameters of scientists’ models and reducing the chance of errors from the technique they use to search for neutrino-less double beta decays.
“Beyond constraining the nuclear models for double beta searches, this discovery tells us it might be possible to use future massive xenon detectors to search for neutrinoless double electron captures—an even rarer variant that, if detected, would also tell us the nature of neutrinos,” Grandi said.
The XENON1T detector is currently undergoing an upgrade to boost its sensitivity; it is planned to restart taking data late this year as XENONnT, with three times as much xenon and an order of magnitude more sensitivity.
The other UChicago scientists on the paper were postdoctoral researcher Jacques Pienaar; graduate students Evan Shockley, Nicholas Upole and Katrina Miller; postdoctoral researcher Christopher Tunnell (now at Rice University); and data scientist Benedikt Riedel (now at the University of Wisconsin-Madison).
Citation: “First observation of two-neutrino double electron capture in 124Xe with XENON1T.” Aprile et al, Nature, April 24, 2019.
Funding: National Science Foundation, the Swiss National Science Foundation, the German Ministry for Education and Research, the Max Planck Gesellschaft, the German Research Foundation, the Netherlands Organisation for Scientific Research, the NLeSC, the Weizmann Institute of Science, I-CORE, Pazy-Vatat, Initial Training Network Invisibles, Fundacao para a Ciencia e a Tecnologia, Region des Pays de la Loire, Knut and Alice Wallenberg Foundation, Kavli Foundation, Abeloe Graduate Fellowship and Istituto Nazionale di Fisica Nucleare.