Deep in South Dakota, scientists hunt for the unseen. Physicists like me don’t fully understand what makes up about 83% of the matter in the universe we call “dark matter.” But with a tank full of xenon buried about a mile below South Dakota, we might one day be able to measure what dark matter really is.

In the standard model, dark matter is responsible for most of the gravitational force in the universe and provides the glue that enables structures like galaxies to form, including our own Milky Way. As the Solar System orbits the center of the Milky Way, Earth moves within the halo of dark matter that makes up most of the matter in our galaxy.

A popular hypothesis is that dark matter is a new type of particle, weakly interacting large particles, or WIMPs. “WIMP” describes the essence of a particle pretty well – it has mass, meaning it interacts gravitationally, but otherwise it interacts very weakly – or rarely – with ordinary matter. Theoretically, WIMPs in the Milky Way fly past us all the time on Earth, but because they interact weakly, they don’t collide with anything.

WIMP search

Over the past 30 years, scientists have developed an experimental program to try to detect rare interactions between WIMPs and ordinary atoms. However, on Earth we are constantly surrounded by low, safe levels of radioactivity from the surrounding trace elements, particularly uranium and thorium, as well as cosmic rays from outer space. The goal of finding dark matter is to create a detector as sensitive as possible so that it can see the dark matter, and to place it in as quiet a place as possible so that the dark matter signal can be seen against the background of radioactivity.

With results published in July 2023, the LUX-ZEPLIN, or LZ, collaboration did just that, building the largest dark matter detector to date and placing it 4,850 feet (1,478 meters) underground at the Sanford Subsurface Research Center in Leeda, South Dakota. threw it. At the center of the LZ is 10 metric tons (10,000 kilograms) of liquid xenon. As the particles pass through the detector, they can collide with xenon atoms, causing a flash of light and the release of electrons.

In LZ, two large electric grids create an electric field in the volume of the liquid, pushing these released electrons to the surface of the liquid. As they cross the surface, they are drawn into the void above the liquid filled with xenon gas and are accelerated by another electric field, producing a second flash of light. Two large arrays of light sensors collect these two bursts of light and together allow researchers to reconstruct the position, energy and type of interaction that occurred.

reduction of radioactivity

All materials on Earth, including those used in the design of the WIMP detector, emit some amount of radiation that could potentially mask dark matter interactions. That’s why scientists are building dark matter detectors using the most “radio-clean” materials they can find—that is, free of radioactive contamination—both inside and outside the detector.

Working with foundries, for example, LZ was able to use the purest titanium in the world to create the central cylinder (or cryostat) that holds liquid xenon. Using this particular titanium reduces the radioactivity in the LZ, creating an open space to view dark matter interactions. Also, liquid xenon is so dense that it actually acts as a radiation shield and it’s easy to clean up any radioactive contaminants that might get in.

In the LZ, the central xenon detector is inside two other detectors called the xenon shell and the outer detector. These support layers capture the radioactivity on its way to or from the central xenon chamber. Because dark matter interactions are so rare, a dark matter particle interacts only once in the entire apparatus. Therefore, if we observe a multiple interactive event in xenon or external detector, we can assume that it is not caused by a WIMP.

the hunt continues

In the newly published results, using 60 days of data, LZ recorded about five events per day on the detector. That’s roughly a trillion fewer events than a typical surface-based particle detector can record in a day. Looking at the characteristics of these events, researchers can safely say that dark matter has not yet caused any interactions. The result is unfortunately not a new physics discovery, but because it remains invisible to LZ, we can place a limit on how weakly dark matter must interact.

These constraints help physicists understand what dark matter isn’t, and LZ does it better than any experiment in the world. Meanwhile, there is hope for what will happen next in the search for dark matter. LZ is currently collecting more data and we expect to have more than 15 times the data in the next few years. A WIMP interaction may already be in this dataset and await discovery in the next round of analysis. Source