A Minimalist Approach to the Hunt for Dark Matter

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In particular, the Antipas team is using their experiment to search for a class of dark matter known as ultralight dark matter. At its heaviest, an ultralight dark matter particle is still about a trillion times lighter than an electron. According to quantum mechanics, all matter has both particle-like and wave-like qualities, with larger objects typically having more particle-like qualities and smaller objects more wave-like qualities. “When people talk about ultralight dark matter, what they mean is 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 to date, Antipas’ search has turned up nothing. However, its absence of discovery helps to constrain the properties of dark matter, as the experiment shows what dark matter is not. In addition, the team’s approach is distinctive compared to the best-known dark matter experiments, which search for particles known as WIMPs (that is, weakly interacting massive particles). These experiments often involve collaborations of 100 or more scientists, and the detectors have spectacular engineering 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 protect the detectors from unwanted radiation, physicists place them in laboratories deep inside mountains or underground in old mines.

Instead, the entire Antypas experiment fits into one table and his collaboration consisted of 11 scientists. The search for dark matter was actually a side project for his lab. They typically use the equipment to study the weak nuclear force in atoms, which is responsible for radioactive decay. “This was a quick and interesting thing for us,” says Antypas. “We use these methods for other applications.” Compared to WIMP detectors, benchtop experiments are simple and cost-effective, Gehrlein says.

Over the past decade or so, these benchtop approaches have become increasingly popular for dark matter searches, Zurek says. Physicists, who first developed super-precise tools and lasers to study and control individual atoms and molecules, looked for more ways to use their new machines. “More people moved into the field, not as their primary discipline, but as a way to find creative new applications for their measurements,” Zurek says. “They can reuse their experiments to search for dark matter.”

In one notable example, physicists reformed atomic clocks to search for dark matter instead of keeping time. These precise machines, which do not lose or gain a second for millions of years, are based on the energy levels of atoms, which are determined from the interactions between their nuclei and electrons that depend on fundamental constants . Similar to the Antipas experiment, these researchers searched for dark matter by precisely measuring the energy levels of atoms, to look for changes in the values ​​of the fundamental constants. (None found.)

But these relatively minimalist experiments will not replace more conventional dark matter experiments, as the two types are sensitive to different hypothetical types (and masses) of dark matter. Theorists have hypothesized a variety of dark matter particles whose masses range by more than 75 orders of magnitude, Gehrlein says. At the very least, the particles could be more than a quadrillion times lighter than even the ultralight dark matter that Antipas is looking for. The heaviest dark matter candidates are actually astrophysical objects as large as black holes.

Unfortunately for physicists, their experiments have offered no clues that make one range of masses more likely than others. “That tells us to look everywhere,” says Gehrlein. With so few clues, dark matter hunters need all the reinforcements they can get.

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