What it’s like to run the world’s best dark matter detector

Deep beneath the surface of South Dakota lies the LUX-ZEPLIN (LZ) experiment, a vital endeavor in the quest to unveil the mysteries of dark matter. Physicist Chamkaur Ghag from University College London is at the forefront of this ambitious scientific collaboration, which aims to uncover the elusive 85% of the universe’s matter that has yet to be identified.

As Ghag and his team reach a pivotal moment in their search, plans are in motion for a successor detector called XLZD, designed to significantly surpass LZ in size and sensitivity. However, should both detectors fail to provide evidence, physicists may need to reevaluate their understanding of dark matter. As Ghag discussed in anticipation of his appearance at New Scientist Live this October, he has already ventured into developing a smaller prototype of a dark matter detector.

Leah Crane: First things first, why is dark matter so important?

Chamkaur Ghag: Dark matter plays a crucial role in our understanding of the universe. While particle physics explains the behavior of particles and atoms, gravity operates on a much larger scale. The two frameworks conflict when we try to explain the stability of galaxies; they shouldn’t exist as they are. The gravitational forces holding them together stem from an invisible mass—dark matter—which constitutes around 85% of all matter in the universe.

Why have we been searching for it for so long without success?

Currently, the leading theory suggests dark matter is composed of WIMPs, or weakly interacting massive particles, which emerged shortly after the Big Bang. These particles rarely interact with ordinary matter, making their detection incredibly challenging. Therefore, we construct large detectors like LZ to increase the probability of dark matter interaction by providing a greater target area. Additionally, these experiments must be located deep underground to minimize interference from cosmic rays and other background noise.

The craft of searching for dark matter is painstaking; for instance, we must ensure minimal background noise. Most metals exhibit some level of radioactivity, so we carefully select construction materials to minimize that interference. LZ stands as the instrument with the lowest background noise and highest radio-purity on the planet.

So, how does LZ work to achieve this sensitivity?

At its core, LZ resembles a double-walled Thermos flask several meters tall and wide, filled with 7 tons of liquid xenon. Within this vessel, the xenon dwells in a reflective barrel and is monitored by light sensors located at both the top and bottom. An electric field spans this barrel, facilitating detection. When a WIMP collides with a xenon nucleus, it produces a faint flash of light. The electric field then releases electrons created by this interaction, resulting in an even brighter flash of light.

This mechanism provides us with two distinct light signals—one indicating the location of the event and the other offering insights into the type of interaction that occurred, whether it was a WIMP strike or another particle hitting the xenon nucleus. The detector’s placement a mile underground shields it from cosmic radiation, with additional water tanks providing further protection from geological interference.

What challenges emerged while developing such an intricate instrument?

A predecessor experiment, LUX, helped inform the advancements needed to enhance LZ’s sensitivity tenfold; however, implementing these improvements proved challenging yet rewarding. For me, ensuring the instrument remained clean and quiet posed the greatest difficulty. The entire LZ detector could occupy a football-sized area, yet we could only tolerate a single gram of dust across that expanse.

What does it feel like to work in such an ultra-clean environment so deep underground?

The facility is located in a former gold mine, leading to an industrially recognizable surface environment. Participants don hard hats and descend a mile, followed by a trek to the lab. Inside, the atmosphere shifts to that of a clean room devoid of natural light, filled solely with computers and equipment. But the journey into the lab feels almost otherworldly.

With the absence of detected WIMPs to date, when will we reconsider their viability as dark matter candidates?

We will need to reconsider the WIMP hypothesis if XLZD—a more advanced detector—fails to observe them. If we find ourselves exploring beyond the capabilities of XLZD, we may have to accept that traditional WIMP models could not exist. However, until we reach that point, WIMPs remain a valid area of investigation. The gap between current findings and where XLZD will be exploring is particularly intriguing.

What can you tell us about the smaller dark matter detector you’ve created?

We have developed a small, 150-nanometer-wide glass bead levitated by lasers, turning it into a sensitive force detector. This innovative approach allows us to gauge movement in three dimensions, enabling us to determine the exact source direction of interactions while effectively isolating environmental disturbances, such as terrestrial radioactive decay.

What motivated the shift towards smaller-scale detectors, and can we expect more of these in the future?

Large-scale detectors offer incredible sensitivity but often come with limitations. If a dark matter particle strikes a small detector, it generates signals that are easier to identify. In contrast, larger tanks may lead to photon losses due to scattering, limiting overall detection efficiency. Consequently, there is growing interest in seeking lower-mass dark matter candidates often beyond the range of devices like LZ, which supports the development of alternative detector designs.

Should we find dark matter, what implications would that hold for physics and our universe?

Discovering dark matter answers a fundamental question regarding the composition of the universe. This momentous find would also suggest new physics beyond the standard model, which is the foundational framework of particle physics. Such a revelation would not only illuminate the nature of dark matter but would also provide unprecedented insights into the fundamental constituents of reality.