In A Tiny Flash 5,000 Feet Underground, Scientists May Have Finally Spotted Dark Matter

3 min readHere’s what you’ll learn when you read this story:Although scientists have indirectly observed dark matter’s influence on light from distant galaxy clusters, it’s never been directly observed. A new preliminary result from the LUX-ZEPLIN (LZ) Experiment at the Sanford Underground Research Facility (SURF) in South Dakota shows what might be a weakly interacting massive particle, or WIMP. While it’s only a one-off event and below the statistical threshold for researchers to claim detection, the results are in line from what scientists expect from a dark matter particle.Nearly 5,000 feet below the surface at the Sanford Underground Research Facility (SURF) in South Dakota sits the LUX-ZEPLIN (LZ) Experiment, a next-generation direct-detection experiment searching for one of dark matter’s leading candidates: weakly interacting massive particles, or WIMPs. The experiment relies on extremely sensitive detectors to record when a WIMP crashes into atomic nuclei in a 10-ton tank of liquid xenon.Of course, this isn’t expected to be a common interaction. As their name suggests, WIMPs are theorized to only interact with matter through gravity or the weak nuclear force. Over time, scientists have indirectly observed dark matter—for example, through gravitational lensing, which affects the light we see from far-away galaxy clusters. Direct observation of a dark matter particle, on the other hand, has never happened before.But now, that might be changing. According to a new result presented at the TeV Particle Astrophysics (TeVPA) conference in Tendo, Japan, the LZ team found an event (which occurred in 2023) that LZ spokesperson Rick Gaitskell called “absolutely fascinating.”The LZ detector picks up on two distinct signals when a collision occurs. When the xenon nucleus recoils from impact, it creates a flash of light detectable by sensors above the vat of liquid xenon. That same array also records electrons when they reach the top and create another flash of a light.Scientists have estimated how certain hypothesized versions of dark matter would strike xenon nuclei and produce these flashes while releasing electrons, and after combing through 220 days’ worth of data, they found an event that matches those mathematical expectations to a statistical significance of 2.6 sigma (five sigma is the threshold for something to be considered a direct detection). A paper detailing the detection will be uploaded to the preprint server arXiv shortly.“We’re very intrigued to see this event in the data, in the region where we expect dark matter to show up, and the competing backgrounds are very low,” Gaitskell said in a press statement. “With only one event, we don’t want to get ahead of ourselves. We are not claiming to have seen dark matter. But we have seen something interesting that we want to share with the scientific community for their input.”It’s worth noting a few things about the discovery as it currently stands. For one, this event took place in June, and summer (in the northern hemisphere) is when Earth runs headlong into the galaxy’s dark matter stream at peak velocity. For another, according to their calculations, if a WIMP did indeed cause the one-off event, it would likely have had a mass of at least 200 GeV/c2 (200 times that of a proton). And for a third, the event was not the simplest interaction that astrophysicists would have expected to see—that version would have created a cascade recoil event at lower energies before the main event. However, it’s possible that other interaction types (such as inelastic scattering and momentum-dependent elastic scattering) could explain the result.While definitely an intriguing result—and one that will certainly be pored over in the months and years to come—it remains only one interaction. For now, the researchers appear to be promoting a sense of cautious optimism.“This is the first example in any experiment I’ve worked on of an outlier that appears valid in every way,” Aaron Manalaysay, a member of the LZ institutional board and physicist at Berkeley Lab, said in a press statement. “We’re still twisting our brains trying to think if there’s a rare background mechanism we could’ve missed, but it’s thrilling to wonder if this could be the first hint of a dark-matter observation.”Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.