Scientists have long suspected that a see-through substance known as dark matter suffuses the cosmos, keeping the fabric of our universe from tearing. But what exactly dark matter is made of remains a mystery. Physicists have built massive underground detectors that are primed to spot rare collisions between normal matter and dark matter if the latter takes the form of theoretical particles called WIMPs: weakly interacting massive particles. Yet after decades in operation, these increasingly sensitive detectors have not picked up a single dark matter signal, leading physicists to toss out many different versions of WIMP theories.
Last week researchers announced they had dramatically slashed the remaining possibilities the model offers. WIMPs could have masses ranging anywhere from one to 100,000 giga-electron-volts divided by the speed of light squared (GeV/c2), a unit of mass roughly equal to that of a proton. (Despite their name, WIMPs are incredibly small compared with a grain of rice or a bacterial cell.) The new study, which came out of the LUX-ZEPLIN (LZ) experiment at the Sanford Underground Research Facility in South Dakota and was presented at conferences in Chicago and São Paulo, Brazil, found that if they exist, WIMPs should have a mass below 9 GeV/c2—otherwise they would have shown up by now. Of course, dark matter may not be made up of WIMPs at all, and if it is, there are lots of versions still in play. But finding them looks harder and harder. At lower mass ranges, particles called neutrinos that sail through normal matter can knock into detectors, imitating and overwhelming dark matter signals.
“For reasons we don’t know yet, nature has picked a pretty challenging combination of parameters,” says Richard Gaitskell, a principal investigator for LZ and an astrophysicist at Brown University.
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Gaitskell says that he, like other members of the LZ team, would have loved it if the project found dark matter, but had no reason to believe that it had to be on the heavier side of the WIMP spectrum. To nail down a more precise theory of the nature of dark matter, physicists need to test a wide range of theoretical models, he emphasizes. Rather than stew in frustration, he sees the latest null result as an intrinsic part of the research process. “If you assume you’re going to get a positive result, then I’m afraid, statistically, you’re going to be disappointed,” he says, with a laugh.
And even a lack of a signal is a scientific triumph when it manages to exclude such a wide range of dark matter candidates, says Scott Kravitz, deputy physics coordinator for LZ and a particle physicist at the University of Texas at Austin. “I was thrilled that we had as good sensitivity to dark matter as we did,” he explains, highlighting the 10-ton size of the team’s detector and its keen ability to limit the background noise that can interfere with potential dark matter signals.
The work “sets the best limits of any experiment even for dark matter scenarios that aren’t classic WIMPs and weren’t part of their original motivation,” notes Tracy Slatyer, a theoretical physicist at the Massachusetts Institute of Technology, who was not involved in the study. “It’s a beautiful result.”
Yet at the same time, it is a result that puts pressure on ongoing and future direct detection efforts. If the next generation of devices fails to spot dark matter, Kravitz says, researchers will likely need to switch approaches and repurpose existing detectors for other functions.
Experiments such as LZ use giant tanks of cold, liquid xenon as detectors. Collisions between otherwise-inert xenon nuclei and stray particles produce small bursts of light and energy—signals scientists can examine to hunt for dark matter. The instruments lie more than a kilometer underground to avoid interference from cosmic rays and other sources of energy. But tiny, ghostly neutrino particles pay no mind to these barriers and unfortunately present near-perfect matches for certain kinds of low-mass dark matter signals. Two other direct detection experiments, XENONnT in L’Aquila, Italy, and PandaX-4T in Sichuan, China, announced earlier this summer that they’d spotted these pesky particles, albeit not at the field’s stringent statistical thresholds for “evidence” or “discovery.”
In theory, xenon-based detectors could make out subtle distinctions between neutrino signals and dark matter signals if they grew 100 times larger or if experiments stretched 100 times longer, says Ciaran O’Hare, a dark matter researcher at the University of Sydney. Doing so, however, would demand an unfeasible commitment of money and labor. As a result, it would make more sense for physicists to use their finite time and funding on other approaches, he and Kravitz argue. But what’s known as the “neutrino fog”—the point at which neutrino signals make dark matter impossible to see—won’t stifle research for at least another 10 to 15 years, O’Hare estimates.
Dark matter researchers also search for WIMPs using telescopes that probe for signs that the particles are smashing into each other in space and “self-annihilating”—a process predicted by theory that could release light. This type of “indirect detection” typically complements direct detection efforts, so it would not replace xenon-based instruments. As a member of an international collaboration called CYGNUS, O’Hare hopes to help build a new kind of direct-detection instrument that can identify the celestial sources of incoming particles and thereby distinguish between solar neutrinos and potential dark matter candidates. But he also imagines attention might shift further to a different theory on the nature of dark matter: the possibility that it’s made of axions. These hypothetical particles are even lighter than the tiniest WIMP and could possibly be detected via oscillations in their waveforms. (Like all particles, axions exist as both particles and waves.)
And there’s a slight chance that the dark matter candidates that LZ ruled out are much more weakly interacting than most physicists believe, meaning that longer-term studies of that mass range could still reveal dark matter signals. “We’ve explored a large swath of the most well-motivated theories but not 90 percent,” Kravitz cautions.
Members of PandaX, the competing direct detection effort behind the PandaX-4T experiment, told Scientific American they looked forward to learning more about LZ’s work in its upcoming paper and testing the null finding in the project’s own experiments. Even if the 9-GeV/c2 cutoff holds, here’s still plenty of room for detectors to explore, the team emphasized.
LZ’s ongoing research could also corroborate its initial findings. The null result emerged from 280 days of data collection. The experiment, however, is slated to run for a total of 1,000 days before it ends in 2028, so it can acquire another 720 days’ worth of information.
The 9-GeV/c2 limit represents a threshold for data analysis, not for data collection, so the LZ team also has another mound of data related to lower-mass signals to sort through in future work. “The detector certainly behaved itself in a way that could make that data very exciting,” Gaitskell says.