Missing in axion: Dark matter experiment rules out more candidate particles
Roughly 85 percent of the mass in the universe is missing. Known as dark matter, this stuff is invisible but makes itself known through very weak interactions with regular matter, and physicists have been hunting for it for decades. Hypothetical particles called axions are a long-standing candidate, but a new experiment has now found that they don't exist – at least within a certain mass range.
Many theories abound on what dark matter might be made of, and different experiments to find the stuff usually focus on one specific type of particle. None have managed to find anything concrete yet, but even a null result helps by narrowing the search parameters. So far, the HADES particle detector has ruled out "dark photons," while LUX and XENON1T have both eliminated some types of weakly-interacting massive particles (WIMPs).
The new study zeroed in on a different suspect: axions. If they exist, these particles are thought to be neutral, very light and effectively everywhere, drifting around and through regular matter without interacting with it. But they are predicted to interact with electricity and magnetism, which could potentially be detected by specially-designed experiments.
"As dark matter, (axions) shouldn't affect your everyday life," says Lindley Winslow, principal investigator of the new study. "But they're thought to affect things on a cosmological level, like the expansion of the universe and the formation of galaxies we see in the night sky."
A few years ago, researchers at MIT came up with a thought experiment that could theoretically be used to detect axions. Essentially, because of the nature of electromagnetic fields, there should be no magnetic field in the center of a ring-shaped magnet. But if axions really are drifting around everywhere, they could create a signal in that dead zone – albeit a tiny one.
"We wanted to look for a signal of an axion where, if we see it, it's really the axion," says Winslow. "That's what was elegant about this experiment. Technically, if you saw this magnetic field, it could only be the axion, because of the particular geometry they thought of."
The team named the experiment ABRACADABRA, which apparently stands for A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus. And now they've built the system, conducted the experiment and announced the results.
The setup is essentially a small, donut-shaped magnet kept in a chamber chilled to almost absolute zero. To counteract vibrations from the refrigeration system, the basketball-sized chamber was suspended from a thin thread, while layers of cold superconducting shielding and warm shielding around the experiment's exterior prevented outside noise and electromagnetic interference.
In the first month of running the ABRACADABRA experiment, the team detected no sign of axions with a mass between 0.31 and 8.3 nanoelectronvolts. From that, the team draws two possible conclusions – either axions are more or less massive than that range, or their effect on electromagnetism is even smaller than previously thought. And the effect that this experiment tested for was already incredibly tiny – just one part in 10 billion.
Again, a null result on this study doesn't disprove the dark matter theory. There's a long list of candidate particles, so each experiment that crosses off another one of them brings us closer to the answer. Other experiments like the nEDM have looked for other types of axions, and there are still mass ranges that might bear fruit in future experiments.
"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 team plans to keep running the ABRACADABRA experiment to look for smaller and weaker axions, and is looking for ways to scale up the apparatus, which could eventually allow even weaker axions to be detected.
The research was published in the journal Physical Review Letters.