Experiment sets new record in search for dark matter

The nature of dark matter, the invisible substance that makes up most of the mass in our universe, is one of the greatest mysteries in physics. New results from the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), have narrowed the possibility of identifying one of the leading candidates for dark matter: weakly interacting massive particles, or WIMPs.

The LZ experiment, led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), is searching for dark matter in a cavern nearly a kilometer underground at the Sanford Underground Research Facility in South Dakota. The experiment’s new results explore weaker dark matter interactions than ever before studied and further constrain what WIMPs might be.

“These are new world-leading constraints on dark matter and WIMPs,” said Chamkaur Ghag, a spokesperson for LZ and a professor at University College London (UCL). He noted that the detection and analysis techniques are performing even better than the collaboration expected.

“If WIMPs had been present in the region we studied, we could have said something about it reliably. We know we have the sensitivity and the tools to see if they are present as we search lower energies and accumulate most of the lifetime of this experiment.”

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

The new results were presented at two physics conferences on August 26: TeV Particle Astrophysics 2024 in Chicago, Illinois, and LIDINE 2024 in São Paulo, Brazil. A paper will be published in the coming weeks.

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

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

The LZ’s sensitivity comes from the many ways the detector can reduce background noise, false signals that can mimic or mask interactions with dark matter. Located deep within the Earth, the detector is shielded from cosmic rays from space.

To reduce the background radiation from everyday objects, LZ was built from thousands of ultra-clean, low-radiation parts. The detector is built like an onion, with each layer blocking outside radiation or tracking particle interactions to eliminate dark matter mimics. Sophisticated new analysis techniques help eliminate background interactions, especially those of the most common culprit: radon.

This result is also the first time LZ has applied “salting,” a technique that involves adding false WIMP signals during data collection. By hiding the real data until “unsalting” at the very end, researchers can avoid unconscious bias and avoid overinterpreting or changing their analysis.

“We’re pushing the boundaries of a regime in which researchers have never looked for dark matter before,” said Scott Haselschwardt, LZ physics coordinator and recent Chamberlain Fellow at Berkeley Lab, now an assistant professor at the University of Michigan. “Humans tend to want to see patterns in data, so it’s very important when you’re entering this new regime that you don’t let bias creep in. If you’re going to make a discovery, you want to make it right.”

Dark matter, so named because it does not emit, reflect or absorb light, is thought to make up 85% of the mass of the universe, but has never been directly detected, although it has left its mark on many astronomical observations. We would not exist without this mysterious but fundamental piece of the universe; the mass of dark matter contributes to the gravitational pull that helps galaxies form and stay together.

LZ uses 10 tons of liquid xenon to provide a dense, transparent material for dark matter particles to potentially collide with. The goal is for a WIMP to collide with a xenon nucleus, causing it to move, much like a cue ball being hit in a game of billiards. By collecting the light and electrons emitted during the interactions, LZ captures potential WIMP signals and other data.

“We’ve demonstrated how powerful we are as a WIMP search machine, and we’re going to keep running and improving it, but there are lots of other things we can do with this detector,” said Amy Cottle, leader of the WIMP search effort and assistant professor at UCL.

“The next step is to use these data to study other interesting and rare physical processes, such as rare decays of xenon atoms, neutrinoless double beta decay, neutrinos from the Sun’s boron-8, and other physical phenomena beyond the Standard Model. And this is in addition to studying some of the most interesting and previously inaccessible models of dark matter from the last 20 years.”

LZ is a collaboration of about 250 scientists from 38 institutions in the United States, the United Kingdom, Portugal, Switzerland, South Korea, and Australia; much of the work of building, operating, and analyzing this record-breaking experiment is being done by early-career researchers.

The collaboration is already looking forward to analyzing the next data set and using new analysis tricks to search for even lower-mass dark matter. Scientists are also thinking about potential upgrades to further improve LZ and are planning 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 that is nothing. Wait until you see what comes next.”