In 2012, two previous dark matter detection experiments—the Large Underground Xenon (LUX) and ZonEd Proportional scintillation in Liquid Noble gases (ZEPLIN)—came together to form the LUX-ZEPLIN (LZ) experiment. Since it commenced operations, this collaboration has conducted the most sensitive search ever mounted for Weakly Interacting Massive Particles (WIMPs) – one of the leading Dark Matter candidates. This collaboration includes around 250 scientists from 39 institutions in the U.S., U.K., Portugal, Switzerland, South Korea, and Australia.

On Monday, August 26th, the latest results from the LUX-ZEPLIN project were shared at two scientific conferences. These results were celebrated by scientists at the University of Albany‘s Department of Physics, including Associate Professors Cecilia Levy and Matthew Szydagis (two members of the experiment). This latest result is nearly five times more sensitive than the previous result and found no evidence of WIMPs above a mass of 9 GeV/c². These are the best-ever limits on WIMPS and a crucial step toward finding the mysterious invisible mass that makes up 85% of the Universe.

Led by the Department of Energy’s (DoE) Lawrence Berkeley National Laboratory, the LZ experiment is located at the Sanford Underground Research Facility in South Dakota, about 1,500 meters (nearly a mile) beneath the surface. The experiment relies on an ultra-sensitive detector made of 10 tonnes (11 U.S. tons) of liquid xenon to hunt for the elusive signals caused by WIMP-nucleus interactions. While direct detections are yet to be made, these latest results have helped scientists narrow the search.

As Levy explained in a recent UofA press release:

“Dark matter interacts very, very rarely with normal matter, but we don’t know exactly how rarely. The way we measure it is through this cross-section or how probable an interaction is within our detector. Depending on the mass of a dark matter particle, which we don’t know yet, an interaction within the detector is more or less probable. What the new LZ results tell us is that dark matter interacts with normal matter even more rarely than we thought, and the only instrument in the world that is sensitive enough to measure that is LZ.”

The existence and nature of Dark Matter are among the greatest mysteries in modern astrophysics. Originally proposed to explain the rotational curves of galaxies, the existence of Dark Matter is vital to the most widely accepted cosmological model – the Lambda Cold Dark Matter (LCDM) model. Unfortunately, according to the prevailing theories, DM only interacts with normal (aka. “luminous”) matter via gravity, the weakest of the four fundamental forces. Detecting these interactions requires incredibly sensitive instruments and an environment free of electromagnetic energy (including heat and light).

While no direct detections have been made, the latest results from LZ have narrowed the range of possibilities for one of the leading DM candidates. As Szydagis said:

“It’s often misunderstood what is meant by the phrase ‘world’s best dark matter experiment’ since no one has made a conclusive, unambiguous discovery yet. However, new, stricter null results like LZ’s are still extremely valuable for science. UAlbany, as one part of the multinational collaboration that is LZ, has been making important contributions ensuring the robustness of LZ’s results, going back to the very beginning of the experiment.”

Although DM remains “invisible” to us, the presence of its gravitational pull is fundamental to our understanding of the Universe. For example, the formation and movement of galaxies are attributed to DM, and its existence is vital for explaining the large-scale structure and evolution of the Universe. If DM does not exist, then our understanding of gravity – as described by Einstein’s Theory of General Relativity – is essentially wrong and needs revision. However, General Relativity has been experimentally validated again and again over the past century.

Therefore, narrowing the search for its constituent particle is vital to proving that our foundational theories about the Universe are correct. As Levy noted, UAlbany scientists have been making integral contributions to LZ for over a decade, and their work is far from done! “Working on LZ is always so exciting, even if we still have not made a discovery yet,” she said. “We all know that if it were easy, someone else would have done it already! I think right now what we need to take out of this result is that LZ is a great team of scientists, our detector is working superbly, our analysis is extremely robust, and we are nowhere near done taking data.”

Further Reading: University at Albany