“Once an axion is detected,” astrophysicist Raymond Co at the University of Minnesota wrote in an email to The Daily Galaxy, “the implications to cosmology will be profound. For instance, signals from experiments with different search strategies will determine whether the axion is dark matter. If it is, with the measured axion properties, one can narrow down its possible cosmological origins.
“Such origins,” Co continued, “will in turn shed light on whether the axion dark matter is expected to be warm, which affects structure formation and can be verified in astrophysics experiments, and on how the early universe evolved. Certain axion properties—the mass and interaction strengths—will also reveal the roles of the axion in other open questions in particle physics and cosmology, including the strong CP problem and/or the observed cosmological excess of matter over antimatter.”
While they are thought to be everywhere, axions are predicted to be virtually ghost-like, having only tiny interactions with anything else in the universe. “As dark matter, they shouldn’t affect your everyday life,” said MIT physicist Lindley Winslow in 2019 about the first run of a new experiment to detect axions—hypothetical particles that are predicted to be among the lightest particles in the universe. “But they’re thought to affect things on a cosmological level,” Winslow noted, “like the expansion of the universe and the formation of galaxies we see in the night sky.”
Physicists from MIT and elsewhere have suggested that If they exist, axions would be virtually invisible, yet inescapable; they could make up nearly 85 percent of the mass of the universe, in the form of dark matter.
Will Modify the Rules of Electricity and Magnetism
Axions are particularly unusual in that they are expected to modify the rules of electricity and magnetism at a minute level. In a paper published today in Physical Review Letters, the MIT-led team reports that in the first month of observations the experiment detected no sign of axions within the mass range of 0.31 to 8.3 nanoelectronvolts. This means that axions within this mass range, which is equivalent to about one-quintillionth the mass of a proton, either don’t exist or they have an even smaller effect on electricity and magnetism than previously thought.
“This is the first time anyone has directly looked at this axion space,” says Winslow, principal investigator of the experiment and the Jerrold R. Zacharias Career Development Assistant Professor of Physics at MIT. “We’re excited that we can now say, ‘We have a way to look here, and we know how to do better!’”
Can Convert into Detectable Radio Waves
Because of their interaction with electromagnetism, axions are theorized to have a surprising behavior around magnetars—a type of neutron star that churns up a hugely powerful magnetic field. If axions are present, they can exploit the magnetar’s magnetic field to convert themselves into radio waves, which can be detected with dedicated telescopes on Earth.
A Broadband/Resonant Approach (ABRA)
In 2016, a trio of MIT theorists drew up a thought experiment for detecting axions, inspired by the magnetar. The experiment, which searches for ultra-light axion and axion-like dark matter, was dubbed ABRACADABRA, for the A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus, and was conceived by Jessie Thaler, a Professor in the Center for Theoretical Physics, along with Benjamin Safdi, then an MIT Pappalardo Fellow, and former graduate student Yonatan Kahn.
The MIT video below is a fascinating tour de force of the search for elusive dark matter and the “ABRA” experiment.
The team proposed a design for a small, donut-shaped magnet kept in a refrigerator at temperatures just above absolute zero. Without axions, there should be no magnetic field in the center of the donut, or, as Winslow puts it, “where the munchkin should be.” However, if axions exist, a detector should “see” a magnetic field in the middle of the donut.
After the group published their theoretical design, Winslow, an experimentalist, set about finding ways to actually build the experiment.
“We wanted to look for a signal of an axion where, if we see it, it’s really the axion,” Winslow says. “That’s what was elegant about this experiment. Technically, if you saw this magnetic field, it could only be the axion, because of the particular geometry they thought of.”
A Challenging Experiment
It is a challenging experiment because the expected signal is less than 20 atto-Tesla. For reference, the Earth’s magnetic field is 30 micro-Tesla and human brain waves are 1 pico-Tesla, reports MIT. In building the experiment, Winslow and her colleagues had to contend with two main design challenges, the first of which involved the refrigerator used to keep the entire experiment at ultracold temperatures. The refrigerator included a system of mechanical pumps whose activity could generate very slight vibrations that Winslow worried could mask an axion signal.
The second challenge had to do with noise in the environment, such as from nearby radio stations, electronics throughout the building turning on and off, and even LED lights on the computers and electronics, all of which could generate competing magnetic fields.
The team solved the first problem by hanging the entire contraption, using a thread as thin as dental floss. The second problem was solved by a combination of cold superconducting shielding and warm shielding around the outside of the experiment.
“We could then finally take data, and there was a sweet region in which we were above the vibrations of the fridge, and below the environmental noise probably coming from our neighbors, in which we could do the experiment.”
The researchers first ran a series of tests to confirm the experiment was working and exhibiting magnetic fields accurately. The most important test was the injection of a magnetic field to simulate a fake axion, and to see that the experiment’s detector produced the expected signal—indicating that if a real axion interacted with the experiment, it would be detected. At this point the experiment was ready to go.
“If you take the data and run it through an audio program, you can hear the sounds that the fridge makes,” Winslow says. “We also see other noise going on and off, from someone next door doing something, and then that noise goes away. And when we look at this sweet spot, it holds together, we understand how the detector works, and it becomes quiet enough to hear the axions.”
The First Run –High Risk, High Reward Physics
In 2018, the team carried out ABRACADABRA’s first run, continuously sampling between July and August. After analyzing the data from this period, they found no evidence of axions within the mass range of 0.31 to 8.3 nanoelectronvolts that change electricity and magnetism by more than one part in 10 billion.
The experiment is designed to detect axions of even smaller masses, down to about 1 femtoelectronvolts, as well as axions as large as 1 microelectronvolts.
The team will continue running the current experiment, which is about the size of a basketball, to look for even smaller and weaker axions. Meanwhile, Winslow is in the process of figuring out how to scale the experiment up, to the size of a compact car—dimensions that could enable detection of even weaker axions.
“There is a real possibility of a big discovery in the next stages of the experiment,” Winslow says. “What motivates us is the possibility of seeing something which would change the field. It’s high-risk, high-reward physics.”
The Last Word –Detection on an Axion will have Profound Implications
“Axions may or may not constitute the entire observed dark matter abundance,” Raymond Co wrote in his email to The Daily Galaxy. “Haloscopes such as ABRACADABRA and CASPER search for axions as dark matter, while helioscopes like the CERN Axion Solar Telescope or light-shining-through-walls look for axions produced from the Sun or on the Earth.”
“Detecting axions would be great,” Stacy McGaugh, Chair, Department of Astronomy and Director of the Warner & Swasey Observatory at Case Western Reserve University told The Daily Galaxy. “Being able to show that they have the right properties and exist in sufficient numbers to be the dark matter would be even better – just finding that a particle exists does not suffice. Thirty years ago we thought it would be a slam dunk to detect WIMPs within five years. That didn’t happen, so don’t hold your breath.”
MIT’s Chiara Pancaldo Salemi, an experimental astroparticle physicist working with the ABRACADABRA experiment and DM Radio, both of which search for low-mass axion dark matter, wrote in an email to The Daily Galaxy: “Since the 2019 results we actually upgraded the detector and published world-leading limits that go below the CAST bound on solar axions. We are now working on larger versions of the experiment, DMRadio-50L and DMRadio-m^3, in a larger collaboration. We expect to begin construction on DMRadio-50L very soon. The larger DMRadio-m^3 was recently selected under the DOE Dark Matter New Initiatives program, and we have been working on tuning its design.
“At the low masses we are looking at, the axion field would have been around during inflation, and so discovering it could give us insight into that early time period where we have very few handles on what was happening,” Salemi concluded.
The Dark Matter Radio, or DM Radio for short, reports the Irwin Lab at Stanford University, is an experiment that detects dark matter like an AM radio. But unlike a radio it uses exquisitely sensitive superconducting devices, including Superconducting Quantum Interference Devices (SQUIDs), and Quantum Sensors based on photon upconversion, to search for elusive axions and hidden photons.
Stanford’s Irwin group is focused on searching for “light” dark matter that weighs so little that it acts more like a field than a particle. Field-like dark matter must be a boson. Two theoretically well motivated light-field dark matter candidates are the axion (spin 0) and the hidden photon (spin 1). Both couple weakly to photons.
The CERN Axion Solar Telescope (CAST) is an experiment to search for axions proposed by some theoretical physicists to explain why there is a subtle difference between matter and antimatter in processes involving the weak force, but not the strong force. If axions exist, says CERN, they could be found in the center of the Sun and they could also make up invisible dark matter.
CAST is searching for these particles with a telescope designed to detect axions from the Sun. It uses an unexpected hybrid of equipment from particle physics and astronomy.
The image at the top of the page shows the supergiant elliptical galaxy M87 in the constellation Virgo, one of the most massive galaxies in the local universe notable for its large population of globular clusters. M87 is in the X-ray from Chandra (blue) and in radio emission from the Very Large Array (red-orange). Astronomers used the X-ray emission from M87 to hopefully detect the properties of axions. (X-ray NASA/CXC/KIPAC/N. Werner, E. Million et al.; Radio NRAO/AUI/NSF/F. Owen)