Sensor array for the LUX-ZEPLIN dark matter experiment in South Dakota
Matthew Kapust/Sanford Underground Research Facility
The latest search for dark matter has so far been fruitless, but the good news is that it has allowed physicists to place the toughest constraints yet on the properties of this mysterious substance. New measurements from the LUX-ZEPLIN (LZ) experiment in South Dakota mean that we are closer than ever to finding dark matter particles, or that we have refuted the most likely explanation.
Dark matter is invisible to the naked eye because it does not interact significantly with normal matter or light. We only know that dark matter exists through its gravitational effects, which tell us that it makes up more than 80 percent of all matter. The leading explanation for dark matter has been that it is composed of weakly interacting massive particles (WIMPs), but searches for these fundamental entities have yet to turn up anything.
The LUX-ZEPLIN dark matter detector, made of seven tonnes of liquid xenon buried 1.5 kilometres underground, is the most sensitive to date, but has yet to find any WIMPs after 280 days of searching. “We’re number one in the world at not finding dark matter,” says Chamkaur Gagh, a spokesman for the LZ at University College London.
While this result may seem disappointing, it allows physicists to place tight constraints on the nature of dark matter, narrowing the range of properties it could have. The constraints are nearly five times tighter than the best previous estimates, significantly narrowing the possibility of WIMPs. The research was presented on August 26 at two physics conferences, TeV Particle Astrophysics in the US and LIDINE in Brazil.
“It’s like they say there’s a magic fish in the ocean, but we don’t know where it is,” Gag says. “You go in and swim around, you get out, you snorkel around, and you still don’t find it, so you use a submarine.” If the magic fish is a WIMP, he says, researchers have explored about 75 percent of the oceans and still haven’t found it.
“This is the next big step, and just one in a long series of advances,” says Dan Hooper of the Fermi National Accelerator Laboratory in Illinois, who was not involved in the work. “It’s probably fair to say that at any one of these advances we can’t expect to see anything. But if we take enough of these steps, it seems quite likely that we’ll see something.”
At this point, many of the initially popular ideas for possible types of WIMPs have been ruled out. There are still a few left, but LZ isn’t done yet. It plans to observe for a total of 1,000 days before wrapping up in 2028. “If LZ doesn’t detect a WIMP, and our next-generation detector, XLZD, doesn’t detect a WIMP, then WIMPs are over,” Gag says. The XLZD project is still in the planning stages.
If WIMPs don’t constitute dark matter, it would be a major paradigm shift, but physicists aren’t ready to give up on finding it entirely. “If you have 20 suspects in a murder investigation, and you find out that 10 of them have alibis, you don’t think, ‘Oh, maybe it wasn’t a murder.’ You just get a better idea of who the right suspects are,” Hooper says. “Once you take some of the suspects off the list, the investigation becomes narrower and more focused, and that’s what progress looks like in this field.”
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