UNIVERSITY PARK, Pa. — The nature of dark matter, the invisible substance thought to make up most of the mass in our universe, is one of the greatest mysteries in physics. Using new results from the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), an international collaboration that includes Penn State researchers has narrowed down the possible properties of one of the leading candidates for the particles that compose dark matter: weakly interacting massive particles, or 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. Although it has never been directly detected, it has left its fingerprints on multiple astronomical observations. 

“Dark matter is a fundamental part of the universe; and we wouldn’t exist without it; dark matter’s mass contributes to the gravitational attraction that helps galaxies form and stay together,” said Carmen Carmona-Benitez, associate professor of physics and the LZ principal investigator at Penn State. “LZ is designed to detect cosmic particles passing through earth, including theorized dark matter particles called WIMPs, with great sensitivity. Based on what we detect — and more often, what we don’t detect — we can put additional limits or constraints on the potential characteristics and properties of WIMPs and get a better sense of what exactly these particles are and aren’t.”

LZ, led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), is located in a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota. Researchers at Penn State play a key role in the experiment, contributing to the operation of the detector and the analysis that led to the latest results. The experiment’s new results explore weaker dark matter interactions than ever searched before and further limits 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. 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. 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 nine gigaelectronvolts per the speed of light in a vacuum squared (GeV/c2). For comparison, the mass of a proton is slightly less than one 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. The new results were presented at two physics conferences on Aug. 26: TeV Particle Astrophysics 2024 in Chicago, Illinois, and LIDINE 2024 in São Paulo, Brazil. A scientific paper will be published in the coming weeks.

The results include analysis of 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 collaboration plans to collect 1,000 days’ worth of data before the experiment ends in 2028.

“LZ is at least 50 times more sensitive than previous dark matter detectors,” said Luiz de Viveiros, assistant professor of physics at Penn State, whose team is responsible for modeling and monitoring background signals in the detector. “Its sensitivity comes from the many ways the detector can reduce background noise, which are signals that can hide or impersonate a dark matter interaction.”

LZ’s deep underground location shields the detector from cosmic rays coming from space, helping reduce background noise. LZ was also built from thousands of ultraclean, low-radiation parts to reduce natural radiation from everyday objects. The detector is built like an onion, with each layer either blocking outside radiation or tracking particle interactions to rule out dark matter mimics. Additionally, sophisticated new analysis techniques help rule out background interactions.

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,” said David Woodward, former assistant research professor at Penn State, now program manager with LZ at Berkeley Lab.

LZ uses 10 tonnes, or 10,000 kilograms, of liquid xenon to provide a dense, transparent material for dark matter particles to potentially bump into. The hope, the researchers said, 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. 

“Researchers have only scratched the surface of what LZ can do,” Carmona-Benitez said. “With the detector’s exceptional sensitivity and advanced analysis techniques, our collaboration is primed to discover dark matter if it exists within the experiment’s reach and to explore other rare physics phenomena.” 

The next stage is using these data to look at other interesting and rare physics processes, like rare decays of xenon atoms, neutrino-less double beta decay, boron-8 neutrinos from the sun and other beyond-the-standard-model physics. Future data sets and new analysis techniques will allow the collaboration to look for even lower-mass dark matter.

LZ is a collaboration of roughly 250 scientists from 38 institutions in the United States, United Kingdom, Portugal, Switzerland, South Korea and Australia. 

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. Over 38 institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.

Editor’s Note: A version of this release originally appeared at the Berkeley Lab website.