New results from the world’s most sensitive dark matter detector have provided the best constraints to date on particles known as WIMPs, a leading candidate for what constitutes the invisible mass in our universe.
Dark Matter and LUX-ZEPLIN (LZ)
Understanding the nature of dark matter, the invisible substance that makes up most of the mass in our universe, is one of the biggest puzzles in physics. New results from the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), have narrowed down the search for one of the top dark matter candidates: Weakly Interacting Massive Particles, or WIMPs.
Berkeley Lab and the LZ Detector
The LZ detector, located at the Lawrence Berkeley National Laboratory under the U.S. Department of Energy, has been hunting for dark matter from a cavern nearly a mile underground at the Sanford Underground Research Facility in South Dakota. The new results from the experiment have explored weaker dark matter interactions than ever before and further narrowed what could be WIMPs.
Berkeley Lab with the LZ detector.
Achievements and Limitations of LZ
Chamkaur Ghag, the LZ operations lead and a professor at University College London (UCL), stated: “These are the latest constraints in the world with a significantly narrowed range in the search for dark matter and WIMPs.” He noted that the analysis techniques and the detector are performing even better than the collaboration expected. “If WIMPs are in the region we are searching, we would be able to confidently speak about them. We know that the tool is sensitive enough to see if they exist there during the search for lower energies.”
The collaboration found no evidence of WIMPs with energies above 9 gigaelectronvolts/c² (GeV/c²). (For comparison, the energy of a proton is slightly less than 1 GeV/c².) The experiment’s sensitivity to weak interactions allows researchers to rule out potential WIMP dark matter models that do not fit the data, focusing instead on more plausible hypotheses. The new results were presented at two physics conferences on August 26: the TeV Particle Astrophysics 2024 in Chicago, USA, and LIDINE 2024 in São Paulo, Brazil.
Diving into LZ’s Experimental Methods
The data analysis results come from 280 days of data collection: a new set of 220 days (collected from March 2023 to April 2024) combined with 60 days from the initial run of LZ. The experiment plans to collect data for 1,000 days before concluding in 2028.
Scott Kravitz, the LZ deputy operations lead and a professor at the University of Texas at Austin, compared: “If you think about searching for dark matter like digging for buried treasure, we have dug deeper about five times than anyone else in the past. To dig that deep, you cannot do it with a million shovels; you need to invent a new tool to do it.”
Improvements and Techniques in Dark Matter Detection
The sensitivity of LZ comes from countless ways the detector can mitigate background noise, false signals that could mislead or obscure dark matter interactions. By being placed deep underground, the detector is shielded from cosmic rays coming from space. To reduce natural radiation from everyday objects, LZ is constructed from thousands of ultra-clean, low-radiation components. The detector is built like an onion, with each layer blocking external radiation to eliminate particles that mimic dark matter interactions. Additionally, new sophisticated analysis techniques help exclude background interactions, especially from the most common culprit: radon.
This result also marks the first time LZ has employed “salt” – a technique adding fake WIMP signals during data collection. By disguising real data until “unsalted” at the end, researchers can avoid unconscious bias and prevent illusions or alterations in their analysis.
Scott Haselschwardt, a member of the LZ operations team and now an assistant professor at the University of Michigan, stated: “We are on the verge of entering the dark matter search field like never before. Humans tend to want to see patterns in the data, so it is really important when you step into this new field that there is no bias. If it has been discovered, everyone wants to explore in the right direction.”
The Importance of Dark Matter
Dark matter is named so because it does not emit, reflect, or absorb light. Dark matter is estimated to constitute 85% of the mass in 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 yet fundamental piece of the universe; the mass of dark matter contributes to the gravitational forces that help galaxies form and exist together.
LZ uses 10 tons of liquid xenon as a dense, transparent material for potential dark matter particles to collide with. They hope that a WIMP will collide with a xenon nucleus, causing it to move, similar to a billiard ball collision. By collecting the light and electrons emitted during the interaction, LZ can obtain signals from potential WIMPs along with other data.
Amy Cottle, the head of the WIMP search campaign and an assistant professor at UCL, stated: “We have demonstrated our strength as a WIMP search machine and we will continue to operate better and better. However, there is still much more we can do with LZ.
The next phase is to use this data to explore other interesting and rare physical processes, like the rare decay of xenon atoms, neutrinoless double beta decay, boron-8 neutrinos from the sun, and other physical processes beyond the Standard Model. Therefore, probing some dark matter models is very exciting and something we have not been able to approach in the last 20 years.”
Future Directions and Collaborative Efforts
LZ is a collaboration of about 250 scientists and engineers from 38 institutions in the U.S., UK, Portugal, Switzerland, South Korea, and Australia; most of the construction, operation, and analysis work for this record-setting experiment has been carried out by leading researchers. This collaboration is expected to analyze the next dataset and utilize new analysis tricks to search for dark matter with even lower mass. Scientists are also considering potential upgrades to further improve LZ and are planning for the next-generation dark matter detector named XLZD.
Kravitz remarked: “Our ability to search for dark matter is improving at a rate faster than Moore’s Law. If you look at the exponential curve, everything before was nothing. Stay tuned to see what comes next.”
