Physicists tighten the net on elusive dark matter
Results from the LUX-ZEPLIN experiment, led in part by UC Santa Barbara, mark a major step in defining what dark matter can — and cannot — be
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The outer detector of the LZ dark matter experiment
view moreCredit: Matt Kapust/Sanford Underground Research Laboratory
Determining the nature of dark matter, the invisible substance that makes up most of the mass in our universe, is one of the greatest puzzles in physics. New results from the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), have narrowed down the possibilities for one of the leading dark matter candidates: weakly interacting massive particles (WIMPs).
“While we always hope to discover a new particle, it is important for particle physics that we are able to set bounds on what the dark matter might actually be,” said UC Santa Barbara experimental physicist Hugh Lippincott. Scientists have suspected the existence of dark matter for decades, but it remains a mysterious substance — one that nevertheless plays a fundamental role in the structure of the universe.
LZ hunts for dark matter from a cavern nearly one mile underground at the Sanford Underground Research Facility (SURF) in South Dakota. The experiment’s new results explore weaker dark matter interactions than ever searched before and further limit what WIMPs could be. 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.
The inner portion of the LZ detector consists of two nested titanium tanks filled with ten tonnes of transparent pure liquid xenon, which is so dense it creates a highly isolated environment, free from the “noise” of the outside world and perfect for capturing the faintest of faint signals that could be indicative of a WIMP. 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. This liquid xenon core is surrounded by a much larger Outer Detector (OD) — acrylic tanks filled with gadolinium-loaded liquid scintillator.
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.
UCSB was one of the founding groups in LZ, led by UCSB physicist Harry Nelson, who hosted the first LZ meeting at UCSB in 2012. The team currently consists of faculty members Lippincott and Nelson, postdoctoral researchers Chami Amarasinghe and TJ Whitis, and graduate students Jeonghwa Kim, Makayla Trask, Lindsey Weeldreyer, and Jordan Thomas. Other contributors to the result include recent Ph.D. recipient Jack Bargemann, now a postdoctoral researcher at Pacific Northwest National Laboratory, and former undergraduate researcher; Tarun Advaith Kumar, now a graduate student at the Perimeter Institute. The physics coordinator for the result was Scott Haselschwardt, who received his Ph.D. from UCSB in 2018 and is now an assistant professor at the University of Michigan.
Neutrons, subatomic particles that exist in every atom save hydrogen, are among the most common confounders of WIMP signals. Nelson and UCSB led the design, fabrication, and commissioning of the OD, the critical component that allows the collaboration to rule out these particles and enable a real discovery.
“The tricky thing about neutrons is that they also interact with the xenon nuclei, giving off a signal identical to what we expect from WIMPs,” Trask said. “The OD is excellent at detecting neutrons, and confirms a WIMP detection by not having any response.” Presence of a pulse in the OD can veto an otherwise perfect candidate for a WIMP detection.
Radon is also a WIMP mimic, for which the scientists must be vigilant. “Radon undergoes a particular sequence of decays, some of which could be mistaken for WIMPs,” Bargemann said. “One of the things we were able to do in this run was look out for the whole set of decays in the detector to identify the radon and avoid confusing them for WIMPs.”
To enable a strong result and eliminate unconscious bias, the LZ collaboration applied a technique called “salting,” which 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. “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.”
With these results, the field of possibilities for what WIMPs may be has narrowed dramatically, allowing all scientists searching for dark matter to better focus their searches and reject incorrect models of how the universe operates. It’s a long game, with more data collection in the future and one that will do more than accelerate the search for dark matter.
“Our experiment is also sensitive to rare events with roots in diverse areas of physics,” Amarasinghe said. “Some examples are solar neutrinos, the fascinating decays of certain xenon isotopes, and even other types of dark matter. With the intensity of this result behind us, I’m very excited to spend more time on these searches.”
“The UCSB Physics Department has a long history of devising searches for dark matter, starting with one of the first published results of a search in 1988,” Nelson said. Previous faculty members include David Caldwell (now deceased), and Michael Witherell, now director of the Lawrence Berkeley Laboratory. David Hale (now retired) pioneered many of the techniques for suppressing fake dark matter signals which are now employed throughout the field of dark matter searches. “UCSB, through the Physics Department, the College of Letters and Science, the administration, and through private donations, has strongly supported the dark matter effort for decades, and made substantial contributions to LZ.”
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 extending our data analysis techniques to seek signals from 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.
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. More than 38 institutions of higher education and advanced research provided support to LZ. The assistance of the Sanford Underground Research Facility has at all times been critical for UCSB efforts to LZ.
Journal
Physical Review Letters
Article Title
Dark Matter Search Results from 4.2 Tonne−Years of Exposure of the LUX-ZEPLIN (LZ) Experiment
Astronomy breakthrough: The mystery of dark matter can be unraveled using radio telescopes
New findings about the early Universe, 100 million years after the Big Bang
Tel-Aviv University
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The black-and-white image shows the distribution of dark matter, concentrated in dense clumps in the darker regions.
Credit: Tel Aviv University
A new study from Tel Aviv University has predicted, for the first time, the groundbreaking results that can be obtained from detecting radio waves coming to us from the early Universe. The findings show that during the cosmic dark ages, dark matter formed dense clumps throughout the Universe, which pulled in hydrogen gas and caused it to emit intense radio waves. This leads to a novel method to use the measured radio signals to help resolve the mystery of dark matter.
The study was led by Prof. Rennan Barkana from Tel Aviv University’s Sackler School of Physics and Astronomy and included his Ph.D. student Sudipta Sikder as well as collaborators from Japan, India, and the UK. Their novel conclusions have been published in the prestigious journal Nature Astronomy.
The researchers note that the cosmic dark ages (the period just before the formation of the first stars) can be studied by detecting radio waves that were emitted from the hydrogen gas that filled the Universe at that time. While a simple TV antenna can detect radio waves, the specific waves from the early Universe are blocked by the Earth’s atmosphere. They can only be studied from space, particularly the moon, which offers a stable environment, free of any interference from the Earth’s atmosphere or from radio communications. Of course, putting a telescope on the moon is no simple matter, but we are seeing an international space race in which many countries are trying to return to the moon with space probes and, eventually, astronauts. Space agencies in the U.S., Europe, China and India are searching for worthy scientific goals for lunar development, and the new research highlights the potential of detecting radio waves from the cosmic dark ages.
Prof. Barkana explains: “NASA’s new James Webb space telescope discovered recently distant galaxies whose light we receive from early galaxies, around 300 million years after the Big Bang. Our new research studies an even earlier and more mysterious era: the cosmic dark ages, only 100 million years after the Big Bang. Computer simulations predict that dark matter throughout the Universe was forming dense clumps, which would later help form the first stars and galaxies. The predicted size of these nuggets depends on, and thus can help illuminate, the unknown properties of dark matter, but they cannot be seen directly. However, these dark matter clumps pulled in hydrogen gas and caused it to emit stronger radio waves. We predict that the cumulative effect of all this can be detected with radio antennas that measure the average radio intensity on the sky.”
This radio signal from the cosmic dark ages should be relatively weak, but if the observational challenges can be overcome, it will open new avenues for testing the nature of dark matter. When the first stars formed a short time later, in the period known as cosmic dawn, their starlight is predicted to have strongly amplified the radio wave signal. The signal from this later era should be easier to observe, and this can be done using telescopes on Earth, but the radio measurements will be more challenging to interpret, given the influence of star formation with all of its complexity. In this case, though, a great deal of complementary information is potentially available from large radio telescope arrays that will attempt to produce a complete map of the radio waves on the sky, looking for patterns of strong and weak emission that should also reveal the presence of the same dark matter clumps. Prof. Barkana is part of the largest such international collaboration, the Square Kilometre Array (SKA), which includes a massive array of 80,000 radio antennas currently being rolled out in Australia.
The researchers assess that the findings may be very significant for the scientific understanding of dark matter. In the present Universe, dark matter has had billions of years to interact with stars and galaxies, making it more difficult to decode its properties. In contrast, the pristine conditions in the early Universe offer a potentially perfect laboratory for astrophysicists.
Prof. Barkana concludes: “Just as old radio stations are being replaced with newer technology that brings forth websites and podcasts, astronomers are expanding the reach of radio astronomy. When scientists open a new observational window, surprising discoveries usually result. The holy grail of physics is to discover the properties of dark matter, the mysterious substance that we know constitutes most of the matter in the Universe, yet we do not know much about its nature and properties. Understandably, astronomers are eager to start tuning into the cosmic radio channels of the early Universe.”
Link to the article:
https://www.nature.com/articles/s41550-025-02637-0
DOI
The colored image presents the corresponding temperature map of the gas.
Credit
Tel Aviv University
