New Experiment to Detect Axions Yields Negative Results

A new experiment to detect axions has been run for the first time by physicists from MIT and other institutions. Axions are speculative particles predicted to be one of the lightest particles to exist in the universe.

If axions occur, they would be almost invisible, but inescapable. They could constitute almost 85% of the mass of the universe, in the form of dark matter. Axions are specifically peculiar because it is expected that they will modify the rules of electricity and magnetism at the minuscule levels.

In a study reported in Physical Review Letters on March 28^th, 2019, the MIT-led group has described that in the first month of observations, no sign of axions was detected by the experiment between the mass range of 0.31–8.3 neV. This implies that axions lying in this mass range, equivalent to nearly one-quintillionth the mass of a proton, either do not exist or they have much more negligible effect on electricity and magnetism than thought earlier.

“This is the first time anyone has directly looked at this axion space,” says Lindley 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!’”

Winslow’s MIT co-authors include lead author Jonathan Ouellet, Chiara Salemi, Zachary Bogorad, Janet Conrad, Joseph Formaggio, Joseph Minervini, Alexey Radovinsky, Jesse Thaler, and Daniel Winklehner, together with scientists from eight other institutions.

Magnetars and Munchkins

Although they are considered to be omnipresent, it is predicted that axions are virtually ghost-like, and have only very little interaction with other matters in the universe.

As dark matter, they shouldn’t affect your everyday life. But they’re thought to affect things on a cosmological level, like the expansion of the universe and the formation of galaxies we see in the night sky.

Lindley Winslow, Jerrold R. Zacharias Career Development Assistant Professor of Physics, MIT

Due to their interaction with electromagnetism, axions are speculated to have a mind-blowing behavior around magnetars—a kind of neutron star that swirls up an immensely powerful magnetic field. In case axions exist, they can make use of the magnetic field of the magnetar to transform themselves into radio waves, which can be detected using dedicated telescopes on Earth.

In 2016, inspired by the magnetar, three MIT theorists designed a proposed experiment for detecting axions. The experiment was named ABRACADABRA, short for A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus, and was conceived by Thaler, an associate professor of physics and a researcher in the Laboratory for Nuclear Science and the Center for Theoretical Physics, together with Benjamin Safdi, then an MIT Pappalardo Fellow, and former graduate student Yonatan Kahn.

The researchers put forward a design for a tiny, donut-shaped magnet placed in a refrigerator at temperatures just above absolute zero. With axions missing, magnetic field in the middle of the donut should be zero, or as Winslow states, “where the munchkin should be.” In contrast, if axions occur, a detector should “see” a magnetic field at the center of the donut.

Following the publishing of the theoretical design by the team, Winslow, an experimentalist, started looking for ways to actually develop the experiment.

We wanted to look for a signal of an axion where, if we see it, it’s really the axion. 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.

Lindley Winslow, Jerrold R. Zacharias Career Development Assistant Professor of Physics, MIT

In the Sweet Spot

It is a difficult experiment since the expected signal is well below 20 atto-Tesla. As a reference, the magnetic field of Earth is 30 micro-Tesla and that of human brain waves is 1 pico-Tesla. While developing the experiment, Winslow and her team faced two key design challenges.

The first one was due to the refrigerator used to maintain the whole experiment at ultracold temperatures. The refrigerator was outfitted with a system of mechanical pumps, the activity of which could produce very slight vibrations that, according to Winslow, could mask an axion signal.

The second one was due to the noise in the environment, for example, from adjacent radio stations, turning on and off electronics through the entire building, and even LED lights on the electronics and computers, all of which could produce competing magnetic fields.

The researchers overcame the first challenge by hanging the entire contraption using a thread as thin as dental floss. The second challenge was overcome by a combination of warm shielding and cold superconducting shielding surrounding the outer side 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.”

At first, the team performed a series of tests to validate that the experiment was working and exhibiting magnetic fields exactly. The most significant one of all the tests was the injection of a magnetic field to simulate a fake axion, and to observe that the experiment’s detector generated the anticipated signal—pointing toward the fact that if a real axion interacted with the experiment, then it would be detected. At this point, the experiment was ready to progress.

If you take the data and run it through an audio program, you can hear the sounds that the fridge makes. 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.

Lindley Winslow, Jerrold R. Zacharias Career Development Assistant Professor of Physics, MIT

Seeing the Swarm

In 2018, the researchers conducted the first run of ABRACADABRA, continuously sampling between July and August. Following the analysis of the data from this period, there was no evidence of the existence of axions within the mass range of 0.31–8.3 neV that change magnetism and electricity by more than one part in 10 billion.

The experiment has been developed to detect axions of further smaller masses, down to about 1 feV, and also axions as large as 1 μeV.

The researchers will continue running the existing experiment, with nearly the size of a basketball, to search for even weaker and smaller axions. At the same time, Winslow has been trying to find out how to scale the experiment up to the size of a compact car—dimensions that could allow detection of axions that are even weaker.

There is a real possibility of a big discovery in the next stages of the experiment. What motivates us is the possibility of seeing something which would change the field. It’s high-risk, high-reward physics.

Lindley Winslow, Jerrold R. Zacharias Career Development Assistant Professor of Physics, MIT

This study was partially funded by the National Science Foundation, the Department of Energy, and the Simons Foundation.