A mile beneath the surface at the Sanford Underground Research Facility in South Dakota, a single unknown particle separated from light, air and cosmic rays collided with the nucleus of a xenon atom, causing electrons to be knocked free in a flash of light — and confounding scientists.
This subatomic interaction captured as part of the LZ Project was so inexplicable that researchers at the Berkeley and Livermore National Laboratories, which both analyzed the data for the experiment, believe the leading explanation is that it was the first recorded observation of dark matter, the mysterious substance that has eluded the greatest minds in physics for over 50 years.
Benjamin Safdi, an award-winning UC Berkeley physicist, said the significance of the discovery could be enormous.
“You have to go back a long time in history to find something comparable. We know it exists, but we have no idea what it is,” Safdi said. “Whatever it ends up being, it will hint at a deeper structure of nature that is totally unknown.”
But before concluding that dark matter has been found, scientists have an obligation to rule out any and all other explanations for the anomaly, said Lawrence Livermore National Laboratory scientist Jingke Xu.
“It’s like mining for gold. The majority of the things you see are rock, things you are not interested in,” Xu said. “Because when you anticipate something, it’s kind of like if you have a hammer, you see everything like a nail. And depending on the analyzers’ expectation, they may kind of change the analysis in a certain way to go in the direction of their hope or their wish.”
Dark matter’s history has always been shrouded in mystery. Swiss-American astronomer Fritz Zwicky was observing the Coma cluster of galaxies from Pasadena in 1933 when he noticed something unusual. The cluster contained just 1% of the mass needed to keep the galaxies trapped in its gravitational pull. There had to be something else.
Four decades passed before two more American astronomers, Vera Rubin and W. Kenty Ford, gave it a name. Rubin calculated the rotational rates of individual stars in galaxies and determined there must be an unseen force — or rather, unseen matter — keeping them together. It eventually became known as dark matter, which is believed to make up 85% of the mass in the universe.
The particle physics community’s interest was piqued. Not only because dark matter explained why galaxy clusters did not fling apart, but also because dark matter did not fit into the “standard model” of particle physics, said Aaron Manalaysay, a Lawrence Berkeley National Laboratory scientist working on the LZ Project.
Considered the “most successful theory” in physics history, Manalaysay said, the standard model’s accuracy and success in predicting subatomic particles before they’re discovered, such as the Higgs Boson, makes it a paramount framework in the field today. But it has not been able to account for dark matter.
“(Dark matter) is most of the mass of the universe, but we also know that it is not made up of anything — at least any substance that we currently know of,” said Manalaysay, “and so this points to a huge gap at the fundamental level of our understanding of the universe. And (this particle interaction) is a big, glaring piece of evidence for that.”
The Department of Energy — in collaboration with the SLAC National Accelerator Laboratory in Menlo Park, Lawrence Livermore Labs and Berkeley National Lab — launched the LZ Experiment at the Sanford Underground Research Facility in 2021.
To block cosmic rays that would otherwise contaminate the results of an experiment, researchers traveled 1 mile underground into a former Black Hills Forest gold mine to erect a huge vat containing 10 metric tons of xenon and surrounded it with ultra-sensitive light detectors, according to Xu.
Researchers posited that dark matter could strike the nucleus of a xenon atom, producing a tiny flash of light and freeing electrons. Those electrons are drawn upward into a layer of xenon gas, where they produce a second light signal. and emit two bursts of light that would be captured by light detectors.
In reviewing 220 days of data, scientists observed just such an anomaly: A single xenon nucleus “recoiled” in a way that could not readily be explained by known particles that the light detectors would have filtered out.
This month, LZ Project researchers presented their findings at the Particle Astrophysics Conference in Japan to considerable fanfare, and word quickly spread to the world’s top physicists. At UC Berkeley, Safdi’s reaction was two-fold.
“First, I got really excited, because it’s fantastic news whenever one of the most serious dark matter experiments in the world comes up with an anomaly that they can’t readily explain, but for dark matter,” Safdi said. “On the other hand, my other reaction naturally was skepticism.”
Physicists assess findings using a statistical measure called sigma, which indicates how far a result departs from what would be expected from known background processes. The LZ finding reached 2.6 sigma, according to Manalaysay, well below the five-sigma threshold conventionally required to claim a discovery.
Yet what made the LZ Project’s finding so compelling to physicists, Safdi said, is that it points to one of the oldest theories about dark matter: the WIMP.
The leading candidate for what dark matter is is a weakly interacting massive particle, or WIMP as it’s known among physicists. This hypothetical subatomic particle would upend the standard model of physics and affirm the super symmetry model in which every ordinary particle in the universe has a corresponding “superpartner.”
“The holy grail of particle physics is to search and find things that disagree with the standard model. And dark matter is one of those, and that motivates everyone,” Manalaysay said. “We know its explanation has to be something to do with particle physics at the fundamental scales, so a solution to dark matter kind of ties together the universe at its largest scales and its smallest scales in a very interesting way.”
Two foreign laboratories are working on similar programs to reproduce the results of the LZ Experiment at the Sanford Underground Research Facility, including PandaX in China and XENONnT in Italy. Safdi said it could take as little as a few months or as much as several years to affirm the results of the LZ Experiment.
If those facilities validate the findings at Sanford, it would not only signal the “tip of the iceberg” in understanding dark matter, it would signal a new era of discovery in physics. Discovering dark matter would rank in the same echelon of discovery as Albert Einstein’s Theory of Relativity, completely transforming our understanding of the laws of nature, said Safdi.
“Dark matter should be the last resort, a conclusion which is forced upon us after ruling out everything else, because it would be such a monumental event,” Safdi said. “What’s at stake here is the possibility of assigning a name to dark matter, assigning a mass, listing the various ways that it interacts with ordinary matter, and even the origin story for how dark matter came to be in our universe.”
