Deep underground in South Dakota, the most advanced dark matter detector on Earth awaits its moment of discovery. This is the Lux-Zeplin (LZ) experiment, highlighting a vast tank of liquid xenon. Physicist Shankaur Ghag from University College London is among the key leaders in this large scientific collaboration, which aims to unravel about 85% of the universe’s mysteries that still elude us.

Currently, Ghag and his team find themselves at a crucial juncture in the quest for this elusive substance. They are considering plans for a more significant detector called xlzd, which promises to be many times the size of the LZ and even more precise. However, if neither detector can uncover the dark matter, they may need to reassess their understanding of what dark matter is. As Ghag suggests, future dark matter detectors may not be massive underground structures but rather smaller, unassuming devices. He has already devised a prototype of such a detector ahead of his upcoming talks at New Scientist Live this October.

Leah Crane: To start, why is dark matter so essential?

Chamkaur Ghag: On one side, we have all the knowledge that particles and atoms, alongside particle physics, provide about the components of matter. On the contrary, we understand gravity as well. While this may seem comprehensive, a significant issue arises when attempting to merge gravity and particle physics. Our galaxy shouldn’t exist as it does. It remains intact through gravity, which seems to derive from unseen matter. This isn’t just a tiny glue; around 85% of the universe comprises this so-called dark matter.

Why have our efforts to find it been so prolonged, with little success?

At present, we hypothesize that dark matter likely consists of what we term “wimps”—massive, weakly interacting particles that originated in the early universe. Consequently, these rarely interact with other particles, providing only a faint signature, which necessitates a large detector for detection. The larger these detectors are, the greater the chance that dark matter particles will pass through them. Additionally, they must be extremely quiet since even slight vibrations can obscure the signal.

We discuss the theoretical landscape of dark matter, which encompasses the range of masses and characteristics such particles could possess. We’ve already excluded certain regions of this landscape, making it essential to delve even deeper underground with larger detectors to explore where dark matter may still exist.

This painstaking endeavor requires minimizing background noise. For instance, many metals emit small radioactive levels, necessitating rigorous efforts to reduce construction material noise. The LZ detector boasts the lowest background noise and the highest level of radio-purity on the planet.

The LZ is currently the most sensitive detector we have. How does it function?

In essence, it operates as a double-walled thermos, containing several meters of liquid xenon. This xenon resides within a reflective tank, equipped with light sensors positioned above and below. Additionally, an electric field exists within this tank. When a wimp collides with a xenon nucleus, it generates a brief flash of light. However, due to the electric field, it causes the electrons to split apart, producing a second flash from the nucleus.

This two-signal output enables us to ascertain the exact location of an event. The intensity of both the primary and secondary flashes informs us about the microphysics of whether the interaction was caused by a wimp or an unrelated phenomenon, such as gamma rays. To ensure optimal detection, we are positioned miles underground to shield against cosmic rays and also encapsulated in an aquarium to safeguard against the surrounding rock.

This endeavor is undoubtedly complex. What has been the most challenging aspect of making it operational?

In an earlier experiment with a smaller prototype called Lux, I understood what was required to create an instrument tenfold more sensitive. Bringing that theoretical knowledge into practice proved challenging. For me, the toughest challenge lay in ensuring the instrument remained clean and quiet enough to achieve required sensitivity. When deployed with the LZ, it occupies a vast area equivalent to a football pitch, where it must tolerate only a gram of dust spread across its surface.

What is it like working with such an ultra-clean detector underground?

The environment, once a gold mine, retains its industrial atmosphere. You don a hard hat, descend a mile down, and then trek to the lab. Upon entering the lab, you lose sense of the surroundings; it transforms into a clean room filled with computers and equipment—essentially a lab devoid of windows. But the journey underground feels otherworldly.

Historically, wimps have been the primary suspect for dark matter. At what point do we consider the wimp hypothesis invalid if we find no evidence?

Should we construct the XLZDs, the larger detectors intended for this purpose, and reach a point where they fail to detect wimps, it would be hard to sustain the idea of a standard wimp existing if we must venture beyond the capabilities of those instruments. However, until that happens, wimps are still in the game. The void between our current findings and those of the XLZD remains intriguing.

We’ve also developed a much smaller, entirely different detector for dark matter. Can you tell me more about it?

We’ve engineered 150 nanometer wide glass beads coated with lasers. This highly sensitive force detector can determine interactions in three dimensions, allowing us to ascertain which direction an event originated from. This capability is significant as it enables us to filter out terrestrial background influence, such as radioactive decay from geological materials.

This concept seems far removed from large detectors like the LZ. What’s the logic behind its creation? Will we see further advancements in smaller detectors?

Large-scale underground experiments, while large and sensitive, can paradoxically limit sensitivity due to their size. For instance, when a dark matter particle collides with my xenon detector, it may produce 10 photons. A smaller tank can capture all of them, but in a larger tank, these photons could bounce around and only a few are detected.

Furthermore, when a dark matter particle interacts with my detector, it only generates two photons initially. In this scenario, the maximal signal from a detector akin to the LZ diminishes. This has spurred the motivation to search for low-mass dark matter particles beyond the LZ’s detection range, leading us toward alternative detection methods.

If dark matter were to be discovered, what implications would that hold for physics and our understanding of the universe?

The implications would be two-fold: it would conclusively provide answers to what constitutes 85% of the universe, and it would challenge the standard model of particle physics, which currently outlines the known components of reality. Thus, if we discovered dark matter, it may offer the first glimpse beyond this conventional framework. Up until now, we’ve had no solid evidence to deviate from the standard model—this would serve as the first ray of hope.