Reaching New Horizons in the Dark Matter Exploration

New findings from the world's most sensitive dark matter detector, LUX-ZEPLIN (LZ), have further narrowed down the possibilities for one of the most promising dark matter candidates: weakly interacting massive particles (WIMPs).

One of the most challenging mysteries in physics is understanding the nature of dark matter, the unseen material that makes up the majority of the universe's mass.

The LZ experiment, led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), is dedicated to searching for dark matter deep underground at the Sanford Underground Research Facility in South Dakota. The latest findings from this experiment delve deeper into potential dark matter interactions, further narrowing down the possibilities for WIMPs, a leading dark matter candidate.

These are new world-leading constraints by a sizable margin on dark matter and WIMPs.

Chamkaur Ghag, Professor, University College London

The team found no evidence of WIMPs with masses greater than 9 gigaelectronvolts/c² (GeV/c²), noting that a proton's mass is slightly less than 1 GeV/c². The experiment’s high sensitivity to weak interactions allows researchers to rule out many plausible WIMP dark matter models that do not match the data, significantly narrowing the range of possibilities for where WIMPs could exist.

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

The conclusions are based on 280 days of data, including a recent batch of 220 days collected between March 2023 and April 2024, along with 60 days from LZ's initial run. The experiment aims to gather 1,000 days of data before its expected conclusion in 2028.

If you think of the search for dark matter like looking for buried treasure, we’ve dug almost five times deeper than anyone else has in the past. That’s something you don’t do with a million shovels – you do it by inventing a new tool.

Scott Kravitz, Professor, University of Texas at Austin

LZ's remarkable sensitivity is due to several advanced methods for minimizing backgrounds, or misleading signals that could mimic or obscure a dark matter interaction. The detector is located deep underground to shield it from cosmic rays, and it is built with hundreds of ultraclean, low-radiation components to eliminate natural radiation from ordinary materials.

The detector's design resembles an onion, with multiple layers that either block external radiation or detect particle interactions to differentiate them from potential dark matter signals. Additionally, sophisticated research techniques are employed to rule out background interactions, especially those caused by radon, a common source of interference.

For the first time, LZ has also utilized a technique known as "salting," in which fake WIMP signals are inserted into the data during collection. This method helps prevent unconscious bias and overinterpretation by masking the real data until the "unsalting" process at the end.

We are pushing the boundary into a regime where people have not looked for dark matter before. 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.

Scott Haselschwardt, Assistant Professor, University of Michigan

Dark matter, so termed because it does neither emit, reflect, or absorb light, is thought to account for 85 % of the universe's mass but has never been directly discovered, despite leaving its fingerprints on several astronomical measurements. Humans would not exist without this enigmatic yet essential component of the cosmos; dark matter's bulk contributes to the gravitational force that allows galaxies to form and remain together.

LZ employs 10 tons of liquid xenon to create 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, much like a cue ball in a game of pool. LZ detects possible WIMP signals by collecting light and electrons released during interactions, in addition to other data.

Amy Cottle, lead on the WIMP search effort and an assistant professor at UCL, added, “We have demonstrated how strong we are as a WIMP search machine, and we are going to keep running and getting even better – but there are lots of other things we can do with this detector.”

Cottle added, “The next stage is using these data to look at other interesting and rare physics processes, like rare decays of xenon atoms, neutrinoless double beta decay, boron-8 neutrinos from the sun, and other beyond-the-Standard-Model physics. And this is in addition to probing some of the most interesting and previously inaccessible dark matter models from the last 20 years.”

About 250 scientists and engineers from 38 institutions in the US, UK, Portugal, Switzerland, South Korea, and Australia collaborated to create LZ; early career researchers handled a large portion of the building, operation, and analysis of the world-record experiment.

The team is eager to examine the next data collection and apply new analytic techniques to search for even more low-mass dark matter. In addition, scientists are planning for XLZD, a next-generation dark matter detector, and considering possible improvements to LZ.

“Our ability to search for dark matter is improving at a rate faster than Moore’s Law. If you look at an exponential curve, everything before now is nothing. Just wait until you see what comes next”, Kravitz noted.

The National Energy Research Scientific Computing Center, a DOE Office of Science user facility, and the U.S. Department of Energy’s Office of Science, Office of High Energy Physics, sponsor LZ. In addition, LZ receives funding from the Portuguese Foundation for Science and Technology, the Swiss National Science Foundation, the Institute for Basic Science in Korea, and the Science & Technology Facilities Council of the United Kingdom. LZ received funding from more than 38 high-end research and education institutes. The LZ collaboration acknowledges the Sanford Underground Research Facility for its support.