A minimalist approach to dark matter hunting

Specifically, Antypas’ team is using their experiment to search for a class of dark matter known as ultralight dark matter. In its heaviest form, an ultralight dark matter particle is still about a trillion times lighter than an electron. According to quantum mechanics, all matter has particle and wave-like properties, with larger objects typically having more particle-like properties and smaller ones more wave-like properties. “When you talk about ultralight dark matter, you mean that dark matter is more like a wave,” says physicist Kathryn Zurek of the California Institute of Technology, who was not involved in the experiment.

Like all other dark matter experiments so far, Antypas’ search has found nothing. However, the lack of a discovery is helping to narrow down the properties of dark matter, as the experiment reveals what dark matter is not. In addition, the team’s approach differs from more well-known dark matter experiments that look for particles known as WIMPs (which are weakly interacting massive particles). These experiments typically involve collaborations of 100 or more scientists, and the detectors have dramatic technical requirements. For example, the LZ detector in South Dakota contains 7 tons of liquid xenon, a rare element found in the atmosphere at less than 1 part in 10 million. To shield the detectors from unwanted radiation, physicists station them in laboratories deep inside the mountains or underground in former mines.

In contrast, Antypas’ entire experiment fits on a tabletop, and his collaboration consisted of 11 scientists. The search for dark matter was actually a side project in his lab. They typically use the equipment to study the weak nuclear force in atoms responsible for radioactive decay. “It was a quick and interesting thing for us,” says Antypas. “We use these methods for other applications.” Compared to WIMP detectors, the tabletop experiments are simple and inexpensive, says Gehrlein.

Over the past decade, these tabletop approaches to dark matter searches have become increasingly popular, Zurek says. Physicists who pioneered the development of super-precise tools and lasers to study and control individual atoms and molecules sought other ways to use their new machines. “More and more people turned to the field, not as their main discipline, but to find new creative uses for their measurements,” says Zurek. “They can repurpose their experiments to look for dark matter.”

In one notable example, physicists have redesigned atomic clocks to look for dark matter instead of telling time. These precise machines, not losing or gaining a second for millions of years, rely on the energy levels of atoms, which are determined by interactions between their nuclei and electrons, which depend on fundamental constants. Similar to Antypas’ experiment, these researchers searched for dark matter by precisely measuring the energy levels of atoms to look for changes in fundamental constant values. (You didn’t find any.)

But these relatively minimalist experiments won’t replace more conventional dark matter experiments because the two species are sensitive to different hypothetical types — and masses — of dark matter. Theorists have hypothesized a multitude of dark matter particles with masses exceeding 75 orders of magnitude, Gehrlein says. At their lightest, the particles could be more than a trillion times lighter than even the ultralight dark matter Antypas is looking for. The heaviest dark matter candidates are actually astrophysical objects the size of black holes.

Unfortunately for physicists, their experiments have not provided any evidence that makes one mass range more likely than others. “That tells us that we have to look everywhere,” says Gehrlein. With so few leads, the dark matter hunters need all the reinforcements they can get.

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