MIT Research News' Journal

Dark matter experiment finds no evidence of axions

Physicists from MIT and elsewhere have performed the first run of a new experiment to detect axions — hypothetical particles that are predicted to be among the lightest particles in the universe. 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.

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 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, along with researchers from eight other institutions.

Magnetars and munchkins

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,” Winslow says. “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.”

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.

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

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.”

In the sweet spot

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. 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.”

Seeing the swarm

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.”

This research was funded, in part, by the National Science Foundation, the Department of Energy, and the Simons Foundation.

Commerce and coercion

Growing up on the Big Island of Hawaii, Kacie Miura says she felt removed from issues roiling the mainland U.S. and the rest of the world. "We were insulated in our own bubble and I wasn't that interested in domestic or international politics," says the fifth-year doctoral candidate. But while serving a two-year Peace Corps mission in China, Miura's view of the world changed dramatically.

In 2010, she was stationed in Chongqing, teaching English to rural teachers and to students of Yangtze Normal University, when tensions flared around the arrest by Japan of a Chinese fishing boat captain.

"Major anti-Japanese protests erupted throughout China," she recalls. "It was the first time I was confronted with the history between these nations, and it made me quite interested in the role of nationalism in politics."

Gripped by this drama, Miura decided to return to academics and study the role and impact of nationalist sentiment in Chinese foreign policy. Today, she is in the midst of writing a dissertation that offers fresh insights on the way economic factors and domestic politics, especially at the local government level, shape China's international relations.

"Those who study China see nationalism as a sort of narrative that the state actively creates, helping to create legitimacy for the [Communist] party," says Miura. She set out to learn whether all Chinese politics followed the central government's nationalist narrative.

In the past decade, several events involving foreign players have served to provoke an official reaction of nationalist outrage in China. For instance, in 2012, Japan procured islands in the East China Sea, a move that China strongly disputed. "There were massive protests throughout China, but not everywhere," she said. "Certain cities appeared surprisingly quiet, and one of them was Dalian — a place with a long history of Japanese investment, and home to many Japanese enterprises."

Reducing friction through trade

For her doctoral research, Miura decided to look closely at local responses to this incident, comparing Dalian with its provincial neighbor Shenyang, which shares geography, politics, and administration, but not the tight commercial connection to Japan. Might economic dependency in Dalian soften any local, antiforeign political protests, she wondered, and would Shenyang prove to be more overtly anti-Japanese, in line with the central government's stance?

To answer these questions, Miura conducted interviews with former officials, local residents and scholars, and scraped data from newspapers in each city to gauge sentiment about Japan during the months-long dispute in 2012. The results reinforced her initial hunch — for the most part.

"At Shenyang, leaders were permissive about anti-Japanese protests, including a huge one outside the Japanese consulate," says Miura. "Protest organizers, who claimed to have the support of the local government, were allegedly so eager for a successful event that they arranged transportation to bring in more people."

In Dalian, leaders found understated ways of supporting Japan. "As a Japanese businessperson put it: They were extending fists above the table, but reaching out under the table to shake hands," she says.

Miura is building an argument that China's central government is not a monolithic authority in determining political responses to international disputes. To bolster this case, she is also researching retaliation against South Korea in Chinese cities with different commercial ties to Seoul, after that nation installed an antiballistic missile defense system China found objectionable.

She is also teasing out the role of the central government's anticorruption crusades on local politics, as well as whether growing unemployment and associated social unrest, viewed with great alarm by Beijing, might factor into local government compliance with the central government's xenophobic policies.

While it's still early for any conclusions, Miura hopes her work will have implications for people eager to understand China's growing influence. "My research might encourage policymakers and businesses to seek allies at the local level, perhaps in cities that already have lots of American firms, where they can expect to be relatively protected even when political tensions are high."

Identity crisis

Miura confesses she is surprised to find herself in the midst of such research, or even pursuing a PhD. In college, she imagined she would remain in Hawaii and become a journalist. But with the 2008 recession, and newsrooms across the state downsizing, she thought her stint in the Peace Corps would buy her some time to figure out next steps.

Her transformative experience in China wasn't just about living far away from home and learning another language. Miura is a fourth-generation Japanese-American, which complicated her interactions with Chinese hosts and students. "I had a full-blown identity crisis," she recalls. "People didn't see me as really American because I didn't have blonde hair and blue eyes." She says she "blended in," which meant "locals often chose not to acknowledge the Japanese side of me."

This sharpened Miura's sensitivity to the rise of anti-Japanese anger, feeding her concern about nationalism. She pursued a master's degree at Yale in international relations, and interned at the International Crisis Group in Beijing, where she helped draft a report on regional responses to China's actions in the South China Sea. Through this work, she connected with Taylor Fravel, the Arthur and Ruth Sloan Professor of Political Science and an authority on China and international security. When she decided that her depth of interest required an advanced degree, MIT and work with Fravel seemed a natural fit.

She is deeply committed to contributing as a scholar to US-China relations.

"There is so little debate in policy circles, and I worry about the rhetoric — that people have concluded China is a threat," she says. "I would like to provide a voice of reason to persuade decision makers not to overreact to every single thing China does, and to realize that oftentimes what China does is in response to what we do."