New result ends a 30-year research and development phase and begins the definitive search for axion particles
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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 the search for that particle has begun in earnest.
This week, the Axion Dark Matter Experiment (ADMX) unveiled a new result (published in Physical Review Letters) that places it in a category of one: it is the only experiment on Earth capable of probing for these low-mass particles, called axions, with the sensitivity necessary to find them. This technological breakthrough is the result of more than 30 years of research and development, beginning with the efforts of a team at University of Florida. The final piece of the puzzle now has come in the form of a quantum-enabled device that allows ADMX to listen for axions more closely than any experiment ever built.
ADMX is managed by the U.S. Department of Energy’s Fermi National Accelerator Laboratory and located at the University of Washington. This new result, the first from the second-generation run of ADMX, sets limits on a small range of frequencies where axions could have been hiding, and sets the stage for a wider search in the coming years.
“This result signals the start of the true hunt for axions,” said Fermilab’s Andrew Sonnenschein, the project manager for ADMX. “We know we have the sensitivity now to detect them, and if the theory is right, it’s only a matter of time before we find them.”
That theory suggests that the dark matter that holds galaxies together might be made up of a vast number of low-mass particles which act like waves streaming through the cosmos. The first efforts to find this particle, named the axion by theorist Frank Wilczek, took place in the 1980s. Pierre Sikivie, UF professor of physics, believed that axions could be the dark matter, and with UF professors David Tanner and Neil Sullivan, began working on a detector in 1983. “In principle, you could detect dark matter here, because it’s throughout the galaxy,” says Sikivie. The present iteration of the detector at Washington finally has the sensitivity necessary to detect axions.
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.
Sikivie wrote seminal papers on axion cosmology, including the notion that the galactic halo – the swath of matter enclosing a galaxy – could be made of axions, and he theorized a detection method to make the “invisible axion” visible. Pioneering experiments and analyses by UF scientists, in collaboration of the University of Rochester, Brookhaven National Laboratory and Fermilab, demonstrated the practicality of the experiment. They led to the construction in the late 1990s at Lawrence Livermore National Laboratory of a large-scale detector that is the basis of the current ADMX.
“If you think of an AM radio, it’s exactly like that,” said Gray Rybka, co-spokesperson for ADMX and assistant professor 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.”
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.
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, but after connecting with John Clarke, a professor at the University of California Berkeley, Clarke 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.
“It’s very satisfying to have this achievement,” says Tanner, who worked on the super-cooling systems for the detector. “Potentially, from the signal, you could trace back the history of the dark matter flow in the galaxy.”
“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,” said Gianpaolo Carosi, co-spokesperson for ADMX and scientist at the U.S. Department of Energy’s Lawrence Livermore National Laboratory.
“This result plants a flag,” said Leslie Rosenberg, professor 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 spend the next five years testing 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, it may force theorists to devise new solutions to those riddles.
“A discovery could come at any time over the next few years,” said 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.”
Read the paper in Physical Review Letters here.
This research is supported by the U.S. Department of Energy Office of Science, the Heising-Simons Foundation and research and development programs at the U.S. DOE’s Lawrence Livermore National Laboratory and the U.S. DOE’s Pacific Northwest National Laboratory.
The ADMX collaboration includes scientists at 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 at Berkeley, the University of Chicago, the University of Florida and the University of Sheffield.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC, a joint partnership between the University of Chicago and the Universities Research Association Inc. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
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