Dark matter appears to dominate the mass of the Universe, yet nobody knows what it is. Deep beneath South Dakota, the LUX-ZEPLIN experiment has completed its most sensitive search yet — and by finding no evidence for one of the leading candidates, it has significantly reduced the places where dark matter can still be hiding.

Look at a galaxy and something does not add up.

The stars, gas and dust we can see cannot provide enough gravity to explain how galaxies rotate and how large cosmic structures formed.

Physicists therefore conclude that the Universe contains enormous quantities of invisible material known as dark matter.

It does not emit, absorb or reflect light. According to current estimates, dark matter accounts for roughly 85% of the Universe’s matter.

Yet despite decades of searching, scientists still do not know what dark matter actually is.

One of the world’s most ambitious attempts to answer that question is taking place almost a mile beneath the surface of South Dakota.

A detector buried beneath the Earth

The LUX-ZEPLIN experiment, usually called LZ, operates at the Sanford Underground Research Facility in the United States.

Its location deep underground is deliberate.

Cosmic rays constantly bombard Earth’s surface and can produce signals that resemble the extremely rare particle interactions scientists are trying to detect.

Nearly a mile of rock provides a gigantic natural shield.

At the centre of LZ is approximately 10 tonnes of liquid xenon.

Scientists hope that a particle of dark matter will occasionally collide with the nucleus of a xenon atom.

Such an interaction would be extraordinarily subtle.

But it should produce tiny quantities of light and free electrons that LZ’s sensitive instruments can detect.

Hunting the WIMP

The experiment is particularly designed to search for weakly interacting massive particles, better known as WIMPs.

WIMPs have long been considered one of the leading hypothetical candidates for dark matter.

They would interact extremely weakly with ordinary matter — which would explain why billions of them could potentially pass through us without being noticed.

But “weakly interacting” does not necessarily mean “never interacting.”

If enough xenon atoms are observed for long enough, scientists hope that eventually a WIMP will collide with one.

The difficulty is separating such an event from everything else occurring inside the detector.

280 days of searching

The results announced on 26 August 2024 analysed 280 days of data.

That consisted of 220 days of observations collected between March 2023 and April 2024, combined with 60 days from LZ’s first science run.

The result?

No evidence for WIMPs was found in the region searched.

At first sight, that might sound disappointing.

Scientifically, it is anything but meaningless.

The new result established the world’s strongest constraints at the time on WIMP interactions over the investigated mass range. Berkeley Lab reported that the result was nearly five times better than the previous world’s best published result.

Learning from finding nothing

Experiments do not only make progress when they discover something.

Imagine searching an enormous building for a hidden object.

Every room you examine carefully and eliminate reduces the number of places where the object can still be.

Dark-matter experiments operate in a similar way.

LZ can determine that if WIMPs possessed certain combinations of mass and interaction strength, the detector should have seen them.

Because it did not, those possibilities become increasingly difficult to maintain.

The 2024 analysis found no evidence of WIMPs above a mass of 9 GeV/c² in the search reported by Berkeley Lab, further narrowing the parameter space available to WIMP models.

The problem of false signals

Searching for dark matter is like listening for a whisper inside a noisy room.

Radioactive materials can produce particle interactions.

Cosmic radiation can interfere.

Even trace amounts of radon can generate events that might initially resemble the signal researchers are seeking.

LZ was therefore constructed from thousands of specially selected components designed to have extremely low levels of radioactivity.

The detector also contains several layers capable of identifying unwanted particle interactions.

Researchers at Imperial College London made an important contribution to this process by developing a technique known as radon tagging.

It allows scientists to follow radioactive atoms through their decay chain as they move through the liquid xenon.

Those events can then be identified as background rather than mistaken for dark matter.

Scientists hid fake dark matter in their own data

The researchers also used an ingenious technique known as salting.

Artificial WIMP signals were secretly inserted into the experimental data.

The scientists analysing the results did not know which potential events were real and which had deliberately been added.

Only after the analysis procedure had been finalised was the dataset “unsalted.”

The purpose is to prevent unconscious human bias.

Researchers cannot subtly modify their selection criteria because a particular event looks exciting if they do not know whether that event is genuine or deliberately planted.

An international machine

LZ is not the work of a single university.

The experiment is led by the US Department of Energy’s Lawrence Berkeley National Laboratory and involves roughly 250 scientists from 38 institutions across the United States, United Kingdom, Portugal, Switzerland, South Korea and Australia.

A British team of around 60 researchers from ten institutions was led by Imperial College London.

Professor Henrique Araújo of Imperial’s Department of Physics co-led development of the xenon detector at the heart of LZ and coordinated many of the UK contributions.

The experiment is far from finished

The 280 days analysed for the August 2024 result represented only part of LZ’s planned scientific programme.

The experiment aims to accumulate approximately 1,000 days of data before the end of its planned run in 2028.

More observation time means greater sensitivity.

The longer LZ operates, the greater its chances of observing an exceptionally rare interaction — or of ruling out still more of the territory in which WIMPs might exist.

Researchers are also developing techniques to investigate dark-matter candidates at lower masses.

The invisible Universe

There is something extraordinary about the LZ experiment.

Scientists have built a gigantic instrument, placed it almost a mile underground, filled its heart with tonnes of liquid xenon and spent years eliminating almost every possible source of contamination.

All to detect something nobody has ever seen.

The 2024 result did not reveal dark matter.

Instead, it told physicists more precisely what dark matter is probably not.

That may sound like a subtle distinction, but it is one of the fundamental ways science advances.

Every excluded possibility makes the remaining possibilities more interesting.

Somewhere between the motion of galaxies on the largest scales and the faintest particle interaction inside a tank of xenon may lie the answer to one of modern physics’ greatest mysteries:

What is most of the matter in our Universe actually made of?

Source: Based on “New record set in ongoing search for dark matter,” published by Imperial College London on 26 August 2024, together with the announcement from Lawrence Berkeley National Laboratory, which leads the LUX-ZEPLIN collaboration.