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Radio receiver ‘listens’ for dark matter particles

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Researchers have developed a way to “listen” for the signs of dark matter axions, the particles that may make up dark matter.

“We’ve built a radio that looks for a radio station, but we don’t know its frequency.”

Forty years ago, scientists theorized a new kind of low-mass particle that could solve one of the enduring mysteries of nature: what dark matter is made of. Now a new chapter in the search for that particle has begun.

The Axion Dark Matter Experiment (ADMX) is the world’s first and only experiment to have achieved the necessary sensitivity to “hear” the telltale signs of dark matter axions.

This technological breakthrough is the result of more than 30 years of research and development, with the latest piece of the puzzle coming in the form of a quantum-enabled device that allows ADMX to listen for axions more closely than any experiment ever built.

ADMX is based at the University of Washington and managed by the US Department of Energy’s Fermi National Accelerator Laboratory. This new result, the first from the second-generation run of ADMX, sets limits on a small range of frequencies where axions may be hiding, and sets the stage for a wider search in the coming years.

Axion Dark Matter Experiment lab
Inside the ADMX experiment hall at the University of Washington. The ADMX detector is underground, surrounded by a magnetic field. (Credit: Mark Stone/U. of Washington)

“This result signals the start of the true hunt for axions,” says Fermilab’s Andrew Sonnenschein, the operations manager for ADMX. “If dark matter axions exist within the frequency band we will be probing for the next few years, then it’s only a matter of time before we find them.”

Tuning in to find dark matter

One theory suggests that the dark matter that holds galaxies together might be made up of a vast number of low-mass particles, which are almost invisible to detection as they stream through the cosmos. Efforts in the 1980s to find this particle, named the axion by theorist Frank Wilczek, currently of the Massachusetts Institute of Technology, were unsuccessful, showing that their detection would be extremely challenging.

Axion Dark Matter Experiment detector
View of the top portion of the ADMX detector. (Credit: Mark Stone/U. of Washington)

ADMX is an axion haloscope—essentially a large, low-noise, radio receiver, which scientists tune to different frequencies and listen to find the axion signal frequency. Axions almost never interact with matter, but with the aid of a strong magnetic field and a cold, dark, properly tuned, reflective box, ADMX can “hear” photons created when axions convert into electromagnetic waves inside the detector.

“If you think of an AM radio, it’s exactly like that,” says Gray Rybka, co-spokesperson for ADMX and assistant professor of physics at the University of Washington. “We’ve built a radio that looks for a radio station, but we don’t know its frequency. We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.”

A long journey

Invented by Pierre Sikivie of the University of Florida in 1983, the detection method might make the “invisible axion” visible. Sikivie also came up with the  notion that galactic halos could be made of axions.

Pioneering experiments and analyses by a collaboration of Fermilab, the University of Rochester, and the US Department of Energy’s Brookhaven National Laboratory, as well as scientists at the University of Florida, demonstrated the practicality of the experiment. This led to the construction in the late 1990s of a large-scale detector at the US Department of Energy’s Lawrence Livermore National Laboratory that is the basis of the current ADMX.

It was only recently, however, that the ADMX team has been able to deploy superconducting quantum amplifiers to their full potential, enabling the experiment to reach unprecedented sensitivity. Previous runs of ADMX were stymied by background noise generated by thermal radiation and the machine’s own electronics.

“No new technology is needed. We don’t need a miracle anymore, we just need the time.”

Fixing thermal radiation noise is easy: a refrigeration system cools the detector down to 0.1 Kelvin (roughly -460 degrees Fahrenheit). But eliminating the noise from electronics proved more difficult.

The first runs of ADMX used standard transistor amplifiers. Then, the researchers connected with John Clarke, a professor at the University of California, Berkeley, who developed a quantum-limited amplifier for the experiment. This much quieter technology, combined with the refrigeration unit, reduces the noise by a significant enough level that the signal, should ADMX discover one, will come through loud and clear.

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“The initial versions of this experiment, with transistor-based amplifiers would have taken hundreds of years to scan the most likely range of axion masses. With the new superconducting detectors we can search the same range on timescales of only a few years,” says Gianpaolo Carosi, co-spokesperson for ADMX and scientist at Lawrence Livermore National Laboratory.

No miracle necessary

“This result plants a flag,” says Leslie Rosenberg, professor of physics at the University of Washington and chief scientist for ADMX. “It tells the world that we have the sensitivity, and have a very good shot at finding the axion. No new technology is needed. We don’t need a miracle anymore, we just need the time.”

ADMX will now test millions of frequencies at this level of sensitivity. If axions are found, it would be a major discovery that could explain not only dark matter, but other lingering mysteries of the universe. If ADMX does not find axions, that may force theorists to devise new solutions to those riddles.

“A discovery could come at any time over the next few years,” says scientist Aaron Chou of Fermilab. “It’s been a long road getting to this point, but we’re about to begin the most exciting time in this ongoing search for axions.”

The researchers report their results in the journal Physical Review Letters.

Dark matter may be ‘cold’ not ‘fuzzy’

The ADMX collaboration includes scientists from Fermilab; the University of Washington; Lawrence Livermore National Laboratory; Pacific Northwest National Laboratory; Los Alamos National Laboratory; the National Radio Astronomy Observatory; the University of California, Berkeley; the University of Chicago; the University of Florida; and the University of Sheffield.

The US Department of Energy Office of Science, the Heising-Simons Foundation, and research and development programs at the US DOE’s Lawrence Livermore National Laboratory and the US DOE’s Pacific Northwest National Laboratory supported the research.

Source: University of Washington