[Link]Researchers sit between two outer layers of LZ during construction. Scott Haselschwardt of U-M is the fourth person from the right. The clear inner tank was later filled with a special liquid scintillator; photomultipliers on the outer wall collect light from background particle interactions. Image credit: Matthew Kapust/Sanford Underground Research Facility

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.

[Link]An array of photomultiplier tubes that are designed to detect signals from particle interactions occurring within LZ's liquid xenon detector. Image credit: Matthew Kapust/Sanford Underground Research Facility

'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.

This result is also the first time that LZ has applied "salting"-a technique that adds fake WIMP signals during data collection. By camouflaging the real data until "unsalting" at the very end, researchers can avoid unconscious bias and keep from overly interpreting or changing their analysis.

"We're pushing the boundary into a regime where people have not looked for dark matter before," said Haselschwardt of U-M. "There's a human tendency to want to see patterns in data, so it's really important when you enter this new regime that no bias wanders in. If you make a discovery, you want to get it right."

[Link]Light bounces off the LZ detector's inner photomultiplier tubes and woven mesh wire grids. Image credit: Matthew Kapust/Sanford Underground Research Facility

Beyond WIMPs

Dark matter, so named because it does not emit, reflect or absorb light, is estimated to make up 85% of the mass in the universe but has never been directly detected. Still, it has left its fingerprints on multiple astronomical observations.

Life as we know it wouldn't exist without this mysterious yet fundamental piece of the universe. Dark matter's mass contributes to the gravitational attraction that helps galaxies form and stay together.

LZ uses 10 tonnes of liquid xenon to provide a dense, transparent material for dark matter particles to potentially bump into. The hope is for a WIMP to knock into a xenon nucleus, causing it to move, much like a hit from a cue ball in a game of pool. By collecting the light and electrons emitted during interactions, LZ captures potential WIMP signals alongside other data.

"We've demonstrated how strong we are as a WIMP search machine, and we're going to keep running and getting even better-but there's lots of other things we can do with this detector," said Amy Cottle, lead on the WIMP search effort and an assistant professor at UCL.

"The next stage is using these data to look at other interesting and rare physics processes, like rare decays of xenon atoms, neutrinoless double beta decay, boron-8 neutrinos from the sun, and other beyond-the-Standard-Model physics. And this is in addition to probing some of the most interesting and previously inaccessible dark matter models from the last 20 years."

[Link]Members of the LZ collaboration gather at the Sanford Underground Research Facility in June 2023, shortly after the experiment began the recent science run. Image credit: Stephen Kenny/Sanford Underground Research Facility

LZ is a collaboration of roughly 250 scientists from 38 institutions in the United States, United Kingdom, Portugal, Switzerland, South Korea and Australia; much of the work building, operating and analyzing the record-setting experiment is done by early career researchers. The collaboration is already looking forward to analyzing the next data set and using new analysis tricks to look for even lower-mass dark matter. Scientists are also thinking through potential upgrades to further improve LZ, and planning for a next-generation dark matter detector called XLZD.

"Our ability to search for dark matter is improving at a rate faster than Moore's Law," Kravitz said. "If you look at an exponential curve, everything before now is nothing. Just wait until you see what comes next."

LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. LZ is also supported by the Science & Technology Facilities Council of the United Kingdom, the Portuguese Foundation for Science and Technology, the Swiss National Science Foundation and the Institute for Basic Science, Korea. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.

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