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.
This week, the (ADMX) unveiled a new result, in the journal , that places it in a category of one: it is the world鈥檚 first and only experiment to have achieved the necessary sensitivity to 鈥渉ear鈥� 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 天美影院 and managed by the U.S. Department of Energy鈥檚 . 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.
鈥淭his result signals the start of the true hunt for axions,鈥� said Fermilab鈥檚 Andrew Sonnenschein, the operations manager for ADMX. 鈥淚f dark matter axions exist within the frequency band we will be probing for the next few years, then it鈥檚 only a matter of time before we find them.鈥�
One theory suggests that the dark matter that holds galaxies together聽might be made up of a vast number of low-mass particles, which are聽almost invisible to detection as they stream through the cosmos. Efforts聽in the 1980s to find this particle, named the axion by theorist Frank聽Wilczek, currently of the Massachusetts Institute of Technology, were unsuccessful, showing that their detection would be聽extremely challenging.
ADMX is an axion haloscope 鈥� essentially a large, low-noise, radio聽receiver, which scientists tune to different frequencies and listen to聽find the axion signal frequency. Axions almost never interact with聽matter, but with the aid of a strong magnetic field and a cold, dark,聽properly tuned, reflective box, ADMX can 鈥渉ear鈥� photons created when聽axions convert into electromagnetic waves inside the detector.
鈥淚f you think of an AM radio, it鈥檚 exactly like that,鈥� said , co-spokesperson for ADMX and assistant professor of physics at the 天美影院. 鈥淲e鈥檝e built a radio that looks for a radio station, but we don’t know its frequency.聽We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.鈥�
This detection method, which might make the “invisible axion” visible, was聽invented by Pierre Sikivie of the University of Florida in 1983, as was聽the notion that galactic halos could be made of axions. Pioneering聽experiments and analyses by a collaboration of Fermilab, the University of Rochester and the U.S. Department of Energy鈥檚 Brookhaven National Laboratory, as well as scientists at the University of Florida, demonstrated the聽practicality of the experiment. This led to the construction in the late聽1990s of a large-scale聽detector at the U.S. Department of Energy鈥檚 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鈥檚 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. 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.
鈥淭he 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,鈥� said Gianpaolo Carosi, co-spokesperson for ADMX and scientist at Lawrence Livermore National Laboratory.
鈥淭his result plants a flag,鈥� said , professor of physics at the 天美影院 and chief scientist for ADMX. 鈥淚t 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鈥檛 need a miracle anymore, we just need the time.鈥�
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.
鈥淎 discovery could come at any time over the next few years,鈥� said scientist Aaron Chou of Fermilab. 鈥淚t鈥檚 been a long road getting to this point, but we鈥檙e about to begin the most exciting time in this ongoing search for axions.鈥�
The ADMX collaboration includes scientists at Fermilab, the 天美影院, 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. 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鈥檚 Lawrence Livermore National Laboratory and the U.S. DOE鈥檚 Pacific Northwest National Laboratory.
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For more information, contact Sonnenschein at 630-840-2883 or sonnenschein@fnal.gov and Rybka at 206-543-2797 or grybka@uw.edu.
This is a joint聽 by Fermilab and the 天美影院.