Physicists at MIT and elsewhere have conducted the first run of a new experiment to detect axions – hypothetical particles believed to be among the lightest particles in the universe. If they exist, axions would be virtually invisible, but inevitable. They could make up nearly 85 percent of the mass of the universe, in the form of dark matter.
Axions are particularly unusual because they are expected to change the rules of electricity and magnetism at the lowest level. In an article published today Physical Review Letters the MIT-led team reported that the experiment did not detect any signs of axions in the mass range of 0.31
For the first time, someone has been directly involved with this Axion space, "says Lindley Winslow, chief investigator of the experiment, and Jerrold R. Zacharias career development assistant at MIT." We are pleased that we can now say, "We have Here's a way, and we know how to do it 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 as well as researchers from eight other institutions.
Magnetars and Munchkins
. It is predicted that axions are virtually ghostly and have only tiny interactions with other elements in the universe.
"As dark matter, they should not affect your everyday life," says Winslow. "But they are 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."
Due to their interaction with electromagnetism, it is believed that axions have a surprise behavior around magnetars – a kind of neutron star that stirs up an enormously strong magnetic field. When axions are present, they can use the magnetic field of the magnetar to transform into radio waves that can be detected with special telescopes on Earth.
A trio of MIT theorists conducted a thought experiment in 2016 to detect axions, inspired by the magnetar. The experiment was named ABRACADABRA for the A broadband / resonance approach to cosmic axis detection using a B-field ring apparatus and was designed by Thaler, an associate professor of physics and research at the Nuclear Science Laboratory and the Center for Theoretical Physics , along with Benjamin Safdi, then MIT Pappalardo Fellow, and former student Yonatan Kahn.
The team proposed a design for a small, donut-shaped magnet that was 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 says, "where the munchkin should be." However, when axions are present, a detector should "see" a magnetic field in the middle of the donut
After the group had published their theoretical design, the experimenter Winslow looked for ways to actually build the experiment.
"We wanted to look for a signal from an axion where, if we see it, it really is the axion," says Winslow. "That was the elegant thing about this experiment. Technically, if you've seen this magnetic field, it could just be the axion because of the particular geometry they thought of."
In the Sweet Spot  It's a challenging experiment as the expected signal is less than 20 atto-tesla. For reference, the earth's magnetic field is 30 micro-Tesla and the human brain waves are 1 pico-tesla. In setting up the experiment, Winslow and her colleagues had to deal with two main design challenges. The first task was to keep the entire experiment at ultracold temperatures. The refrigerator contained a system of mechanical pumps whose activity could produce very slight vibrations that Winslow had sought to mask an axion signal.
The second challenge was noise in the area. For example, from nearby radio stations, electronics throughout the building switching on and off, and even LED lights on computers and electronics that could create competing magnetic fields.
The team solved the first problem by suspending the entire device and using a tooth filament as thin as dental floss. The second problem was solved by a combination of cold superconductive shielding and warm shielding on the outside of the experiment.
"We were finally able to record data, and there was a sweet region where we stood above the vibrations of the refrigerator and the environmental noise that probably came from our neighbors in which we were able to carry out the experiment."  The researchers first performed a series of tests to confirm that the experiment worked and showed magnetic fields accurately. The most important test was to inject a magnetic field to simulate a false axion and to see that the detector of the experiment produced the expected signal – indicating that a real axion would interact with the experiment. At this point, the experiment was ready.
"When you take the data and run it through an audio program, you can hear the sounds that the refrigerator makes," says Winslow. "We also see other sounds when someone next door does something, and then the sound disappears, when we look at this sweet spot, it stops, we understand how the detector works, and it gets quiet enough to hear the axes.
See the Swarm
In 2018, the team performed the first run of ABRACADABRA, with samples continuously sampled between July and August. After analyzing the data from this period, they found no evidence for axions in the mass range of 0.31 to 8.3 nanoelectronvolts, which change electricity and magnetism by more than a fraction of 10 billion.
The experiment is designed to detect even smaller axes masses of up to about 1 femtoelectron volt and axions of up to 1 microelectron volt.
The team will continue the current experiment, which is about the size of a basketball, to look for even smaller and weaker axions. Meanwhile, Winslow is figuring out how to scale up the experiment to the size of a small car – dimensions that can be used to detect even weaker axles.
"There is a possibility of a major discovery in the world next steps of the experiment," says Winslow. "What motivates us is the ability to see something that would change the field, it's a high-risk, high-reward physics."
The team simulates a magnetar to search for dark particles
Design and implementation of the ABRACADABRA 10 cm Axion dark matter search, journals.aps.org/prd/accepted/… a284b0eb5cd5d60d137