SLAC fellow searches for dark matter axions using radio waves

Experimental physicist Chelsea Bartram has been fascinated by radio since childhood. She earned a ham radio license, built a few antennas and realized she could hear people speaking in foreign countries.

Years later, she continued to pursue radio technology but used it to tune into dark matter airwaves, she said in a lecture on Aug. 2 hosted by the Stanford Linear Accelerator Center (SLAC), a federally funded laboratory where Bartram is a Panofsky Fellow.

“Designing radios became a game to me,” she said. “With a few supplies and a lot of creativity, you could talk to someone on the other side of the globe. You could even reflect signals off the moon and you could observe the effects of the sun on the Earth’s atmosphere.”

Bartram said she was astonished to realize she could also use radios to observe more hidden phenomena, like dark matter airwaves.

Dark matter is an invisible material that makes up 85% of the mass of the universe, she said. This means that everything that we can see and touch consists of only 15% of all matter. Dark matter is believed to have played an important role in the formation of galaxies and the evolution of the universe.

If the dark matter axion — a hypothetical particle with no charge, zero spin and a small mass — is found, then we want to study it very intensely and “learn as much as we can about the dark matter halo around us,” she said. The halo is a hypothetical structure of the universe.

Bartram joined University of Washington’s Axion Dark Matter eXperiment (ADMX) in 2019 to collect data and help with analysis. ADMX created a cavity “haloscope” — about the size of a coffee can and made of copper with a magnetic field as strong as that of eight Teslas — designed to detect axions by converting them into photons, particles of electromagnetic energy, by cooling the cavity to be “colder than space” at 0.1 degrees Kelvin. 

For reference, the temperature of outer space is about 2.7 degrees Kelvin. Dark matter axions would interact with the magnetic field and turn into a photon. The photon signal would then go through several amplifiers to create a power spectrum measuring the signal’s frequency.

This frequency “can then be converted to the mass of the axion,” Bartram said. To identify the exact frequency, researchers would tune a radio inside the cavity haloscope. 

But why use a radio to detect the axion’s existence?

“The fact that these axions have a long wavelength does mean that its behavior is different than a normal particle,” Bartram said. “It means it behaves more like waves, which is why radios specifically are used for this experiment. They are good at detecting waves that are coherent, which in turn detects for the presence of axions.”

While no exact frequencies of axions have been found yet, ADMX has made significant progress and will continue tuning in to higher frequencies in search of axions.

Bartram is also involved with Dark Matter Radio at SLAC, a research group that uses Lumper Element Haloscope with a distinct magnetic field to detect axions.

One day, Bartram hopes to create “axion kits” for high school laboratories, allowing students to research axions using radio technology themselves.

“I just think that’s the most fantastic thing, so I sincerely hope we discover [the axion],” she said.

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