Dark matter hunt heats up with first result from world’s biggest detector

Physicists working with the world’s biggest dark-matter detector—a behemoth in the United States known as LUX-ZEPLIN (LZ)—released their first results today. They see no sign of what they’re searching for, so-called weakly interacting massive particles or WIMPs. Still, the result is sparking interest among particle physicists, as the nearly 4-decade-long search for WIMPs approaches a climax. The U.S. detector is turning on at the same time as similar detectors in Italy and China, and together they likely represent the next to last generation of WIMP detectors, if not the last.

The new finding comes from 65 days worth of data taken over 4 months starting in December 2021, says Richard Gaitskell, a physicist and LZ member from Brown University. That’s a tiny fraction of the 1000 days of data LZ researchers aim to take over the next 3 to 5 years, he says, but enough to prove the detector is working well and is now the most sensitive in the world. “We would be in a position to see an excess [of events] if there was one,” Gaitskell says, “but there isn’t one.”

Dark matter is thought to account for 85% of all matter. Astronomical observations show, for example, that the stars in a typical galaxy swirl so fast that their collective gravity isn’t enough to keep them from flying into space. So physicists assume that some sort of invisible dark matter—presumably, a new particle—provides the extra gravity needed to rein in the stars.

Since the 1980s, many physicists have thought dark matter consists of WIMPs, which would interact with ordinary matter just through gravity and the weak nuclear force. WIMPs would have emerged naturally after the big bang and should linger in sufficient numbers to account for dark matter, provided they are about 100 times as massive as a proton. They would permeate the galaxy and even pass through us, but occasionally one ought to crash into an atomic nucleus. So to search for WIMPs physicists need only look for recoiling nuclei in detectors deep underground, where they’re shielded from other types of radiation that can also cause recoil events.

For 20 years, scientists have developed ever-bigger detectors consisting of tanks of liquid xenon lined on top and bottom with light-detecting phototubes. When a WIMP hits a nucleus, the recoiling nucleus produces a detectable flash of light. Also, electrons liberated by the speeding nucleus are tugged by an electric field toward the top of the tank, producing a second flash. Comparing the size and timing of the flashes, researchers can distinguish recoiling nuclei from, say, recoiling electrons, which can be generated by gamma rays hitting the detector. The xenon itself helps shield the heart of the tank, greatly reducing background radiation there.

LZ’s central tank contains 7 metric tons of liquid xenon hunkering 1480 meters down in the Sanford Underground Research Facility, in an abandoned gold mine near Lead, South Dakota. LZ researchers do see 335 nuclear recoil events in their detector, Hugh Lippincott, a physicist at the University of California, Santa Barbara, and spokesperson for the 287-member LZ team, reported today in an online seminar. However, that number roughly equals the background events expected from inevitable traces of radioactive isotopes such as lead-214 in the xenon and other sources, Lippincott reported, so LZ cannot claim it has detected WIMPs.

But the null result still has value. Physicists can’t precisely predict the mass of the WIMP or exactly how strongly it should interact with ordinary matter. LZ researchers put the strictest limits so far on the strength of those interactions for WIMP masses between about 10 and 10,000 times that of a proton. LZ’s new limits edge past those published in December 2021 by a team using PandaX-4T, a 3.7-ton liquid xenon detector located in China’s Jinping Underground Laboratory.

Given LZ’s modest amount of data and PandaX’s previous experimental limit, the null result is hardly surprising, says Dan Hooper, a theorist at Fermi National Accelerator Laboratory. Still, Hooper says he’s excited to see results from LZ, PandaX-4T, and a third experiment, XENONnT, a 5.9-ton detector in Italy’s subterranean Gran Sasso National Laboratory. It’s the fourth iteration of the XENON collaboration, and it is expected to release its first results later this year. Competition will push all teams to work harder, Hooper says. “The physicist-capitalist in me thinks this is a good thing.”

Some of the enthusiasm for WIMPs as dark matter candidates has waned in recent years, not only because searches have come up empty so far, but also because the world’s biggest atom-smasher, Europe’s Large Hadron Collider, has yet to blast out anything that looks like a WIMP. However, physicists are only now starting to probe the heart of the possible ranges of mass and interaction strength for WIMPs, contends Rafael Lang, a physicist and XENON team member at Purdue University. “In other words, half of the [possibilities] that you were excited about a decade or two ago are still alive and well.”

WIMP hunters are already sketching out the ultimate liquid-xenon detector, an 80-ton giant. “That’s what the technology can do,” Gaitskell says. Such a detector would push the liquid xenon approach to its limit, because at that size, it would be sensitive enough to begin detecting a flood of particles called neutrinos from the Sun. Those unavoidable events would be indistinguishable from WIMP collisions, making it more or less pointless to build a liquid xenon detector any bigger. Lang says the LZ and XENON teams have already begun working together on a concept. “I am thrilled that our collaborations have joined forces.”