Physicists build donut-shaped magnet to find 'ghost-like' dark matter particle

The Abracadabra experiment sounds like magic but it may help uncover an elusive, invisible particle.

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Jackson Ryan was CNET's science editor, and a multiple award-winning one at that. Earlier, he'd been a scientist, but he realized he wasn't very happy sitting at a lab bench all day. Science writing, he realized, was the best job in the world -- it let him tell stories about space, the planet, climate change and the people working at the frontiers of human knowledge. He also owns a lot of ugly Christmas sweaters.
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One of the central puzzles in particle physics is discovering what particle (or particles) makes up dark matter -- the form of matter that is responsible for 85 percent of the mass in the known universe. 

Some physicists believe searching for a hypothetical particle known as an "axion" could lead to a better understanding of dark matter and to hunt for it, a team of US physicists have recently designed and tested a basketball-sized, donut-shaped apparatus that can seek it out.

It has been believed that axions may be detectable by looking at an unusual type of neutron star known as a "magnetar." These small, erupting stars create some of the most powerful magnetic fields in the universe. Because of their giant magnetic power, axions would be converted to radio waves in the presence of the magnetar -- and thus, detectable by telescopes on Earth.

That strange cosmic phenomenon inspired theoretical physicists to create the impressively-named Abracadabra experiment. (The full name is "A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus" so the theorists deserve a round of applause for that backcronym.) The experiment consists of a donut (or "toroid") shaped device, dangled in a freezer just above absolute zero and fine-tuned to create its own magnetic field.

If axions exist, the magnetic field in the middle of the donut could reveal them.

"That's what was elegant about this experiment," said Lindley Winslow, principal investigator on the project, in a press release. "Technically, if you saw this magnetic field, it could only be the axion, because of the particular geometry they thought of."

The first run of Abracadabra took place in July and August 2018, looking for evidence that axions had interacted with the device.

However, the team detected no signs of the "ghost-like" particle. While that seems like bad news for axion enthusiasts, the experiment doesn't end there. The magnetic field -- the energy -- generated by the axion is expected to be so tiny that this particular run was unable to detect it because it only looked in a very specific, tight range. Like searching the house for a lost remote, researchers have only looked under the couch -- they can still look under the cushions, in the bedroom and behind the TV.

"This is the first time anyone has directly looked at this axion space," says Winslow. "We're excited that we can now say, 'We have a way to look here, and we know how to do better!'"

The study was published Thursday in the journal Physical Review Letters.

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