The behavior of galaxies, particularly their rotation speeds, and the residual radiation from the Big Bang, suggest the presence of a vast, unseen substance—dark matter.
A staggering 85% of the matter in the universe is unaccounted for. Scientists believe that this missing matter is composed of dark matter, an invisible substance that we can only detect indirectly through its gravitational influence on surrounding matter.
Theresa Fruth, Lecturer in Physics at the University of Sydney, along with a global team of approximately 250 scientists working on the LUX-ZEPLIN (LZ) dark matter experiment, is dedicated to uncovering the true nature of this mysterious substance. While they have yet to identify the specific particles that make up dark matter, their latest findings have set the most stringent limits on their properties to date. Moreover, the results confirm that the detector is performing as expected, promising even more precise results in the near future.
These findings were presented at the TeV Particle Astrophysics 2024 conference in Chicago and the LIDINE 2024 conference in São Paulo, Brazil. The team is also preparing a journal paper for peer review to share their results with the wider scientific community.
What is Dark Matter?
Astronomical observations reveal that the visible matter in stars, gas, and galaxies is only a fraction of the total matter in the universe. The behavior of galaxies, particularly their rotation speeds, and the residual radiation from the Big Bang, suggest the presence of a vast, unseen substance—dark matter.
But what exactly is dark matter? Currently, no known particle can fully explain these observations. Numerous theories exist, ranging from the existence of exotic particles to tiny black holes, or even fundamental revisions to our understanding of gravity. However, none of these theories has been definitively proven.
One of the most widely accepted theories proposes that dark matter consists of “weakly interacting massive particles” (WIMPs). These particles, though relatively heavy, would rarely interact with ordinary matter, yet their gravitational effects could account for the anomalies observed in the cosmos.
To test this theory, scientists believe that WIMPs might be passing through Earth continuously. While most would pass through without interacting, occasionally, a WIMP might collide with the nucleus of an atom—an event the LZ experiment is designed to detect.
A Quest Deep Underground
The LUX-ZEPLIN (LZ) experiment is housed deep underground, in an old goldmine in South Dakota, USA, approximately 1,500 meters below the surface. This location helps shield the experiment from background radiation that could interfere with their observations.
At the heart of the experiment is a large, double-walled tank filled with seven tons of liquid xenon, a noble gas cooled to 175 kelvin (-98°C). If a dark matter particle collides with a xenon nucleus, it should produce a tiny flash of light. The detector is equipped with 494 light sensors to capture these flashes.
However, dark matter particles aren’t the only sources of these light flashes. Background radiation, even from the tank and detector materials, can also create similar signals. Distinguishing between potential dark matter signals and background noise is a significant part of the work. The team uses detailed simulations to predict what they would expect to see with and without dark matter, helping them to isolate any genuine dark matter interactions.
Since beginning her Ph.D. in 2015, Theresa Fruth has focused on developing these simulations, as well as monitoring the detectors. She also played a key role in integrating and commissioning the central detector, which began collecting data in 2021.
Tightening the Net
The most recent data, gathered over 280 days, has not yet revealed signs of dark matter. However, these results allow the team to rule out certain possibilities. Specifically, they found no evidence of particles with masses greater than 1.6 × 10⁻²⁶ kilograms, roughly 10 times the mass of a proton.
Looking ahead, the team aims to gather 1,000 days of observational data, which will enhance their ability to search for even more elusive dark matter particles. Should dark matter remain undetected, they are already planning the next generation of dark matter experiments. The XLZD (XENON-LUX-ZEPLIN-DARWIN) consortium is working towards building a detector nearly 10 times larger, increasing the chances of finally detecting these enigmatic particles.
The search for dark matter continues to challenge our understanding of the universe. While the specific nature of dark matter remains elusive, the progress made by the LUX-ZEPLIN experiment represents a significant step forward. As the team gathers more data and refines their techniques, they move closer to unraveling one of the greatest mysteries in modern physics.
