A Minimalist Approach to the Hunt for Dark Matter

Specifically, the Antipas team used their experiments to search for a class of dark matter known as ultralight dark matter. At their heaviest, ultra-bright dark matter particles are still about a trillion times lighter than electrons. 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 ones having more wave-like qualities. “When people talk about ultra-bright dark matter, they mean that dark matter is more like a wave,” said physicist Kathryn Zurek of the California Institute of Technology, who was not involved in the experiment.

Like all other dark matter experiments so far, the Antipas search has found nothing. However, the absence of their discovery helps limit the properties of dark matter, as experiments show what dark matter is not. Also, the team’s approach differs from that of the more famous dark matter experiment, which looks for particles known as WIMPs (i.e. weakly interacting massive particles). Such experiments typically involve the collaboration of 100 scientists or more, and detectors have dramatic engineering requirements. For example, the LZ detector in South Dakota contained 7 tons of liquid xenon, a rare element found in the atmosphere at levels less than 1 part per 10 million. To protect detectors from unwanted radiation, physicists place them in laboratories deep in mountains or underground in former mines.

Instead, the entire Antipas experiment was laid out on the table, and the collaboration consisted of 11 scientists. Searching for dark matter is actually a side project for his lab. They usually use equipment to study the weak nuclear force in atoms, which is responsible for radioactive decay. “This was a fast and exciting thing for us to do,” said Antypas. “We use this method for other applications.” Compared to the WIMP detector, the tabletop experiment is simple and cost-effective, says Gehrlein.

Over the past decade or so, this desk approach has become increasingly popular for dark matter searches, Zurek said. Physicists, who first developed super-precision tools and lasers to study and control single atoms and molecules, are looking for more ways to use their new machine. “More people are moving into the field, not as their primary discipline, but as a way to find new creative applications for their measurement,” says Zurek. “They can reuse their experiments to search for dark matter.”

In one notable example, physicists rearranged atomic clocks to look for dark matter, not for timekeeping. This precision machine, which has neither lost nor gained a second for millions of years, depends on the energy level of the atom, which is determined by the interaction between the nucleus and its electrons depending on a fundamental constant. Similar to the Antipas experiment, the researchers searched for dark matter by measuring the energy levels of atoms precisely, looking for changes in the value of the fundamental constant. (They didn’t find it.)

But these relatively minimalistic experiments will not replace more conventional dark matter experiments, because both types are sensitive to different hypothetical types—and masses—of dark matter. Theorists have hypothesized that dark matter particles range in mass more than 75 times, Gehrlein said. At its lightest, its particles can be more than a quadrillion times lighter than the ultra-bright dark matter that Antypas is looking for. The heaviest dark matter candidates are actually astrophysical objects the size of a black hole.

Unfortunately for physicists, their experiments don’t offer any clues as to what makes one mass range more likely than another. “This tells us that we have to look everywhere,” said Gehrlein. With a few clues, dark matter hunters need all the reinforcements they can get.

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