In one of the most rigorous searches for dark matter ever conducted, a highly sensitive experiment has failed to find any evidence of the elusive particles. The LUX-ZEPLIN (LZ) experiment, operating from a former gold mine a mile underground, completed its latest run without detecting a single confirmed interaction from a dark matter candidate, further constraining theories about the nature of the universe’s missing mass.
The result, while not the discovery scientists had hoped for, represents a significant milestone in physics. By finding nothing, the LZ collaboration has essentially drawn the tightest net yet around the primary suspect for dark matter: a class of particles known as WIMPs. This null finding effectively rules out a significant range of properties that these particles could have, forcing theorists to reconsider their models and refining the search for the next generation of experiments. The findings, based on 280 days of data, are nearly five times more stringent than the previous best results in the field.
An Unprecedentedly Sensitive Instrument
The search for dark matter requires instruments of exquisite sensitivity, capable of detecting the faintest of interactions while shielded from the constant barrage of cosmic radiation. The LZ experiment is the apex of this technological quest, located 4,850 feet deep at the Sanford Underground Research Facility in South Dakota. This subterranean laboratory provides a natural shield from cosmic rays that could otherwise mimic a dark matter signal. But depth alone is not enough. The detector itself was constructed from thousands of low-radiation components to minimize background noise from the materials themselves.
At the heart of the detector is a time projection chamber filled with 10 tonnes of liquid xenon, with seven tonnes considered the active target region. Xenon is used because its heavy nucleus increases the probability of a collision with a dark matter particle. If a particle like a WIMP passes through the dense, transparent liquid, it is expected to bump into a xenon nucleus, much like a cue ball striking another in a game of pool. This collision would cause the nucleus to recoil, producing a tiny flash of light and liberating a small number of electrons. These two distinct signals—light and charge—are crucial for identifying a potential dark matter event.
The Hunt for WIMPs
The primary quarry of the LZ experiment is the Weakly Interacting Massive Particle, or WIMP. For decades, WIMPs have been the leading theoretical candidate for dark matter. The existence of this type of particle is predicted by theories such as supersymmetry, and if they exist, their collective mass could account for the 27% of the universe that is believed to be dark matter. This invisible substance is a fundamental piece of the cosmos; its gravitational pull is what helps form and hold galaxies together. Without it, galaxies as we know them would not exist.
Because WIMPs, as their name suggests, interact very weakly with ordinary matter, they are incredibly difficult to detect. Countless billions of them may be passing through the Earth and everything on it every second, but the probability of one hitting the nucleus of an atom is vanishingly small. The LZ experiment was designed to maximize the chances of seeing such a rare event. The massive volume of xenon provides a vast number of atomic targets, increasing the odds that at least one WIMP will interact during the experiment’s run.
Methodology of Detection
The experiment’s cleverness lies in its ability to distinguish a potential WIMP signal from other background events, which are primarily caused by trace radioactivity. When a particle interacts with the xenon, the initial flash of light is detected almost instantly by an array of photomultiplier tubes. The freed electrons, meanwhile, are drifted upwards by an electric field toward a layer of gaseous xenon at the top of the chamber, where they generate a second, brighter flash of light. By analyzing the timing and relative brightness of these two signals, scientists can determine the location and energy of the interaction with extreme precision. This dual-signal technique allows them to reject most background events, which have a different signature from the expected WIMP interaction. The latest results are based on a combined 280 days of exposure, collected between 2023 and 2024.
A Null Result with Major Implications
After analyzing the data, the LZ collaboration found no evidence of WIMPs with a mass greater than 9 GeV/c², a measure of mass in particle physics. For comparison, a single proton has a mass slightly less than 1 GeV/c². This null result places the most stringent limits ever on the possible interaction strength of these particles with ordinary matter. Specifically, it establishes a new upper bound for the cross-section—a measure of the probability of interaction—at a minuscule 2.2 × 10⁻⁴⁸ cm² for a WIMP with a mass of 40 GeV/c². This means that if WIMPs of that mass do exist, they are even more elusive and interact more weakly than previously ruled out.
In the search for dark matter, a null result is not a failure. Instead, it is a crucial piece of the puzzle. Each new limit that fails to find a signal systematically closes the windows where these particles could be hiding. By ruling out a significant portion of the theoretically favored parameter space for WIMPs, the LZ results provide invaluable guidance for future experiments and force theorists to refine or reconsider their models. As one physicist described the process, searching for dark matter is like looking for buried treasure; the LZ experiment has now dug almost five times deeper than any previous search.
The Future of the Search
The search is far from over. The recent findings represent only about a quarter of the data the LZ experiment plans to collect. The experiment is scheduled to continue running until 2028, with the goal of acquiring a total of 1,000 days of data. With each new batch of data, the experiment’s sensitivity will improve, allowing scientists to probe even weaker interactions and previously unexplored regions. The collaboration remains optimistic that the detector is performing exceptionally well, even better than expected, and is fully capable of making a discovery if dark matter particles are within its range of sensitivity.
Scientists involved with the project emphasize that they have only just begun to scratch the surface of what the technology can achieve. As the experiment continues to accrue data, it will either find the first direct evidence of WIMPs or place such tight constraints on their properties that the physics community may need to pivot toward alternative candidates for dark matter, such as axions or other exotic particles. The ongoing work will continue to sweep through previously inaccessible and uncharted territory, holding the potential to deliver a definitive observation in the coming years.
A Global Collaborative Effort
The LUX-ZEPLIN experiment is a testament to the global nature of modern scientific research. The collaboration includes approximately 250 scientists from 39 institutions across the United States, United Kingdom, Portugal, Switzerland, South Korea, and Australia. The name “LZ” itself reflects this collaborative spirit, stemming from the merger of two previous-generation dark matter experiments: the Large Underground Xenon (LUX) experiment and the ZonEd Proportional Scintillation in Liquid Noble gases (ZEPLIN) experiment. This international effort combines expertise and resources to tackle one of the most profound and persistent mysteries in all of science: the fundamental nature of the dark universe.