Understanding dark matter — that invisible component that represents most of the mass of the cosmos — continues to be one of the great challenges of modern physics. However, scientists may be one step closer to a major breakthrough: at stake is the LUX-ZEPLIN (LZ) experiment, considered the most sensitive detector in the world, which has just published new results that refine the search for one of the main theoretical candidates: weakly interacting massive particles, known as WIMPs.
“We always hope to discover a new particle, but it is also essential to be able to establish limits for what dark matter can be,” explained Hugh Lippincott, an experimental physicist at the University of California, Santa Barbara (UCSB). Although scientists have been convinced of its existence for decades, dark matter remains elusive, even as it shapes galaxies and holds the cosmic fabric together.
Almost 1.6 km below the surface is LUX-ZEPLIN (LZ), at the Sanford Underground Research Center (SURF), in South Dakota (USA), indicated the website ‘Tempo.pt’. There, protected from background radiation, it looks for tiny signs that could reveal the presence of a WIMP. In its most recent analysis, the team examined data collected over 280 days of observation, adding 220 new days — between March 2023 and April 2024 — to the 60 days of its first operational cycle.
By 2028, it is expected to complete 1,000 days of measurements. The heart of the experiment are two titanium chambers filled with 10 tons of ultrapure liquid xenon, a silent and dense environment that records the faintest flashes of light generated by a potential collision with a WIMP. Around it, an External Detector (OD) with sparkling liquid loaded with gadolinium helps distinguish authentic signals from background noise.
The secret to LZ’s sensitivity lies in its ability to reduce false signals. Because it is buried underground, the detector is protected from cosmic rays, and its structure — made up of thousands of low-radiation components — minimizes natural interference from the environment. Each layer of the system has a function: blocking external radiation or tracking interactions that could mimic dark matter.
Additionally, the team uses advanced analytics techniques to filter out spurious events and maintain data integrity.
Among the main enemies of the experiment are neutrons, subatomic particles present in almost all atoms and capable of producing signals indistinguishable from WIMPs. To address this challenge, UCSB scientists led the External Detector project, essential for ruling out neutron interactions and validating possible real detections.
“The problem with neutrons is that they generate the same type of signal that we expect from a WIMP,” explained researcher Makayla Trask. “OD allows us to detect them and rule out false positives.” Another frequent imitator is radon, a radioactive gas that can emit a sequence of decays easily confused with dark matter. “At this stage, we are able to identify these complete sequences in the detector and avoid confusion”, explained physicist Jack Bargemann.
To avoid misinterpretation by humans, the LZ collaboration uses a method called “salting,” which introduces false WIMP signals into the data during collection. Only at the end of the analysis — when the data is “salting” — do scientists discover which events were real. This eliminates any unconscious bias in interpretation.
“We are exploring a region where no one has ever looked,” said Scott Haselschwardt, study coordinator. “When working at the frontier of knowledge, maintaining objectivity is essential.”
With more than 250 scientists from 38 institutions in six countries, the LUX-ZEPLIN collaboration is preparing to continue collecting data and developing an even more advanced version: the XLZD, the future next-generation detector that promises to take humanity one step closer to understanding the invisible matter of the cosmos.