One of the greatest puzzles in the universe is figuring out the nature of dark matter, the invisible substance that makes up most of the mass in our universe.

New results from the world’s most sensitive dark matter detector, LUX-ZEPLIN, have narrowed down possibilities for one of the leading dark matter candidates: weakly interacting massive particles, or WIMPs.

LUX-ZEPLIN, abbreviated LZ, is a collaboration of 38 institutions, including the University of Michigan.

Led by the Department of Energy’s Lawrence Berkeley National Laboratory, LZ hunts for dark matter from a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota. The experiment’s new results explore weaker dark matter interactions than ever searched before and further limit what WIMPs could be.

“These are new world-leading constraints by a sizable margin on dark matter and WIMPs,” said Chamkaur Ghag, spokesperson for LZ and a professor at University College London, or UCL.

He noted that the detector and analysis techniques are performing even better than the collaboration expected.

“If WIMPs had been within the region we searched, we’d have been able to robustly say something about them,” he said. “We know we have the sensitivity and tools to see whether they’re there as we search lower energies and accrue the bulk of this experiment’s lifetime.”

The collaboration found no evidence of WIMPs above a mass of 9 gigaelectronvolts/c2, or GeV/c2. For comparison, the mass of a single proton is slightly less than 1 GeV/c2. The experiment’s sensitivity to faint interactions helps researchers reject potential WIMP dark matter models that don’t fit the data, leaving significantly fewer places for WIMPs to hide.

“We’ve demonstrated that LZ is very much a discovery-capable machine,” said LZ physics coordinator Scott Haselschwardt, a recent Chamberlain Fellow at Berkeley Lab and now an assistant professor at U-M. “If dark matter presents itself in this range, we’ll be ready to see it.”

Even though the team did not discover a dark matter signal in its latest batch of data, there will be plenty more opportunities over the course of LZ’s lifetime.

“This result is only after 25% of our data, so we definitely need to get the other 75%,” said Gregory Rischbieter, a research fellow in the U-M Department of Physics and the LZ calibration analysis coordinator who helped develop and fine-tune the software modeling framework used for distinguishing dark matter signals from background noise.

“Although a signal is still eluding us, we have the world’s best detector for this range of dark matter. If anything, it’s more motivation to keep looking.”

Wolfgang Lorenzon, professor of physics, helped U-M join the LZ collaboration in 2015. His team was responsible for reducing radon—the largest contributor to LZ’s background—in the xenon circulation system.

“It’s detective work,” he said. “Our detector works really well—in some respects, better than we anticipated. That we haven’t seen dark matter yet isn’t because of the instrument. It’s because dark matter hasn’t revealed itself yet.”

Kaiyuan “Sky” Shi, a graduate student in physics, is also part of the current U-M LZ cohort.

The new LZ results were presented at two physics conferences Aug. 26: LIDINE 2024 in São Paulo and TeV Particle Astrophysics 2024 in Chicago, where Haselschwardt is delivering a presentation. A science paper will be published in the coming weeks.

‘Looking for buried treasure’

The results analyze 280 days’ worth of data: a new set of 220 days collected between March 2023 and April 2024 combined with 60 earlier days from LZ’s first run. The experiment plans to collect 1,000 days’ worth of data before it ends in 2028.

“If you think of the search for dark matter like looking for buried treasure, we’ve dug almost five times deeper than anyone else has in the past,” said Scott Kravitz, LZ’s deputy physics coordinator and a professor at the University of Texas. “That’s something you don’t do with a million shovels—you do it by inventing a new tool.”

LZ’s sensitivity comes from the myriad ways the detector can reduce backgrounds, the false signals that can impersonate or hide a dark matter interaction.

Deep underground, the detector is shielded from cosmic rays coming from space. To reduce natural radiation from everyday objects, LZ was built from thousands of ultraclean, low-radiation parts. The detector is built like an onion, with each layer either blocking outside radiation or tracking particle interactions to rule out dark matter mimics.

And sophisticated new analysis techniques help rule out background interactions, particularly those from the most common culprit: radon.

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Scott Haselschwardt
Wolfgang Lorensen
Gregory Rischbieter