"Axion radio" may let physicists listen for dark matter signals
Dark matter is said to be all around us – if it exists, that is – but the fact that it’s invisible makes it pretty hard to detect. Rather than trying to see it, researchers at Stockholm University have designed an experiment to listen for it instead, using what they call an “axion radio.”
Over 85 percent of all the matter in the universe is unaccounted for, known only through its gravitational interactions with regular matter. This mysterious stuff – referred to as dark matter – has yet to be directly detected, but it’s not from a lack of trying. Many different experiments have been run over the years to try to pick up signals from various proposed dark matter particles.
But one explanation that’s still possible is a hypothetical elementary particle called an axion. It’s believed that axions wouldn’t be discrete particles, but act like waves, flowing throughout space and only rarely interacting with normal matter. In particular, axions are thought to have weak – but detectable – interactions with electricity and magnetism, and this might be how they ultimately reveal themselves.
The Stockholm researchers designed a new experiment that could listen in for these kinds of interactions. The detector would be made up of a chamber filled with a cold plasma, with a forest of wires thinner than a hair running through it. This would be encased inside a large, powerful magnet.
The idea is that inside this magnetic field, any axions passing through would produce a small electric field. That, in turn, would drive oscillations in the plasma that could then be detected as evidence of the axions themselves. By moving those wires closer together or further apart, this device can be tuned like a radio to find just the right frequency of the axions.
“Without the cold plasma, axions cannot efficiently convert into light,” says Matthew Lawson, an author of the study. “The plasma plays a dual role, both creating an environment which allows for efficient conversion, and providing a resonant plasmon to collect the energy of the converted dark matter.”
This design differs from previous attempts at finding axions. A few years ago the nEDM experiment checked whether neutrons, kept in a very controlled environment free of interference, would change their spin over time. If so, this could be evidence of axions at work. Another experiment, called ABRACADABRA, used a ring-shaped magnet. Technically there should be no magnetic field in the center – but if axions are floating around everywhere, they could create one in that dead zone.
Both of these experiments came up empty-handed. But a null result doesn’t necessarily rule out the existence of axions – it just means they may have smaller masses, or weaker interactions. The beauty of the new experimental design is that the cold plasma would amplify any potential signal, so these weaker interactions could be detected. The researchers say that their system could also be scaled up relatively easily.
“This is totally a new way to look for dark matter, and will help us search for one of the strongest dark matter candidates in areas that are just completely unexplored,” says Alexander Miller, an author of the study. “Building a tuneable plasma would allow us to make much larger experiments than traditional techniques, giving much stronger signals at high frequencies.”
While the design remains theoretical for now, the concept is already in development for practical experiments based on the idea.
The research was published in the journal Physical Review Letters.
Source: Stockholm University