New findings from the world’s most precise dark matter detector have established the strongest limits to date on particles known as WIMPs, which are a key candidate for the mysterious invisible mass that constitutes much of our universe.
Understanding dark matter—the unseen substance that accounts for a significant portion of the universe’s mass—represents a major challenge in physics. Recent findings from the LUX-ZEPLIN (LZ) detector, recognized as the most sensitive dark matter detector globally, have tightened the possibilities regarding weakly interacting massive particles, or WIMPs, which are leading candidates for dark matter.
The LZ experiment, managed by the Department of Energy’s Lawrence Berkeley National Laboratory, operates from a cavern nearly a mile beneath the surface at the Sanford Underground Research Facility in South Dakota. The latest results from the experiment investigate interactions of dark matter that are weaker than any previously examined, providing deeper insights into the nature of WIMPs.
“These latest findings create new, leading constraints that are significantly better than before concerning dark matter and WIMPs,” noted Chamkaur Ghag, spokesperson for LZ and a professor at University College London (UCL). He highlighted that the performance of the detector and the analytical methods surpassed the expectations of the collaboration. “If WIMPs were within the searched range, we would effectively have been able to derive concrete conclusions about them. We know we possess the sensitivity and tools needed to detect them as we continue to explore lower energy levels and gather data throughout the lifetime of the experiment.”
The team found no signs of WIMPs with a mass greater than 9 gigaelectronvolts/c2 (GeV/c2), for context, a proton has a mass just under 1 GeV/c2. The experiment’s ability to detect faint signals enables researchers to dismiss many potential WIMP dark matter models that do not align with their data, significantly reducing the number of hiding spots for WIMPs. These new results were shared at two physics conferences on August 26: TeV Particle Astrophysics 2024 in Chicago, Illinois, and LIDINE 2024 in São Paulo, Brazil. An official scientific paper detailing these findings will be published shortly.
The analysis is based on 280 days of data, combining a new set of 220 days collected from March 2023 to April 2024 with 60 days from the initial run of LZ. The experiment aims to compile a total of 1,000 days of data before concluding in 2028.
“If we compare the search for dark matter to seeking buried treasure, we’ve dug nearly five times deeper than any previous efforts,” said Scott Kravitz, LZ’s deputy physics coordinator and a professor at the University of Texas at Austin. “That’s not something you achieve with a million shovels. It’s done by creating an innovative tool.”
The LZ detector’s high sensitivity stems from its multiple methods to reduce background noise, which might mimic or obscure dark matter signals. Positioned deep underground, the detector is protected from cosmic rays originating from space. Furthermore, LZ was constructed using thousands of ultra-clean, low-radiation components to minimize natural radiation interference. The design features concentric layers, each serving either to shield against outside radiation or to monitor particle interactions, helping to eliminate dark matter imitators. Advanced analysis techniques also assist in filtering out background interactions, particularly those caused by radon, the most common contaminant.
This study marks the first instance where LZ has adopted “salting”—a technique involving the addition of fake WIMP signals during data collection. By disguising the authentic data until the “unsalting” process at the end, researchers can mitigate unconscious bias and better interpret their findings.
“We are venturing into territories where dark matter hasn’t been explored before,” explained Scott Haselschwardt, LZ’s physics coordinator and a recent Chamberlain Fellow at Berkeley Lab, currently an assistant professor at the University of Michigan. “There’s a natural human inclination to identify patterns in data, so it’s crucial to eliminate any bias in this uncharted area. When a discovery occurs, precision is key.”
Dark matter is termed “dark” because it neither emits, reflects, nor absorbs light, and it’s estimated to constitute around 85% of the universe’s total mass. Although it has yet to be directly observed, dark matter’s effects have been inferred through various astronomical observations. Its mass is vital for the gravitational forces that enable galaxies to form and maintain their structure.
The LZ experiment utilizes 10 tons of liquid xenon, creating a dense, transparent medium for dark matter particles to potentially collide with. The goal is for a WIMP to strike a xenon nucleus, causing it to move, similar to how a cue ball in pool collides with other balls. By capturing the emitted light and electrons from these interactions, LZ can record potential WIMP signals in addition to other data.
“We have demonstrated our capabilities as a WIMP detection machine, and we are committed to continuing our operations and enhancing our methods. However, there are many other interesting physics processes we can explore with this detector,” said Amy Cottle, the lead for the WIMP search initiative and an assistant professor at UCL. “In the next phase, we will analyze this data to investigate rare phenomena, including unique decays of xenon atoms, the neutrinoless double beta decay, solar neutrinos from boron-8, and other physics beyond the Standard Model, along with probing some intriguing dark matter models from the past two decades.”
The LZ collaboration involves around 250 scientists from 38 institutions across the U.S., United Kingdom, Portugal, Switzerland, South Korea, and Australia. Much of the work related to building, operating, and analyzing this groundbreaking experiment is performed by early-career researchers. The collaboration is eager to evaluate the next data set and apply fresh analytical techniques to search for even lower-mass dark matter. Scientists are also considering enhancements to further improve LZ and planning for a next-generation dark matter detector known as XLZD.
“Our capacity to search for dark matter is advancing faster than Moore’s Law,” Kravitz remarked. “If you look at an exponential growth chart, everything up to this point seems insignificant. Just wait to see what lies ahead.”
LZ is backed 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. Support also comes from the Science & Technology Facilities Council in the UK, the Portuguese Foundation for Science and Technology, the Swiss National Science Foundation, and the Institute for Basic Science in Korea. Over 38 institutions of higher education and advanced research have contributed to the LZ project. The LZ collaboration also appreciates the support from the Sanford Underground Research Facility.
