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On the first floor of the MIT Nuclear Science Laboratory hangs an instrument called “A Broadband / Resonant Approach to Cosmic Axis Sensing with an Amplifying B-Field Ring Apparatus”, or ABRACADABRA for short. As the name states, ABRACADABRA’s goal is to detect axions, a hypothetical particle that could be the main constituent of dark matter, the invisible and yet unexplained material that makes up most of the universe.
For Chiara Salemi, a fourth year graduate student in physics in the group of Lindley Winslow, Associate Professor of Physics in Professional Development Jerrold R. Zacharias, ABRACADABRA is the perfect tool to work on during her doctorate. “I wanted a little experiment so that I could do all the different pieces of the experiment,” Salemi says. ABRACADABRA, which consists of an extremely well shielded magnet, is the size of a basketball.
Salemi’s willingness to work on all aspects is unique. “Experimental physics has roughly three components: hardware, computation and phenomenology,” explains Winslow, with students approaching one of the three. “Chiara’s affinity and strengths are evenly distributed across the three areas,” says Winslow. “Makes her a particularly strong student.”
Since the beginning of his PhD, Salemi has worked on everything from updating ABRACADABRA’s circuitry for its second run to analyzing the instrument’s data to look for the first sign of a dark matter particle.
A happy accident
When Salemi started college, he had no plans to devote himself to physics. “I was inclined to science, but I wasn’t entirely sure about this or what field of science I would like.” During her first semester at the University of North Carolina at Chapel Hill, she took physics with the goal of determining if this could be a field she might be interested in. “Besides, I totally fell in love with it, because I started doing research and research is fun.”
During his university career, Salemi has collected research experiences. He used radio telescopes in West Virginia. He spent a semester in Geneva, Switzerland researching Higgs boson decays at the European Organization for Nuclear Research, better known as CERN. At Lawrence Berkeley National Laboratory, he tinkered with the design of semiconductors for detecting neutrinos. It was during one such research experience, a summer program at Fermilab in Illinois, that he began working with axions. “Like many things in life, it was an accident.”
Salemi had applied for the summer program because he wanted to continue working on neutrinos and “Fermilab is the hub of everything related to neutrinos”. But when she arrived, Salemi found that she had been assigned to work on axions. “I was extremely disappointed, but I ended up falling in love with axions, because they are really interesting and different from other particle physics experiments.”
The elementary particles in the universe and the forces that regulate their interactions are explained by the standard model of particle physics. The name belies the importance of this theory; the Standard Model, developed in the early 1970s, describes almost everything in the subatomic world. “But there are some huge open holes,” Salemi says. “And one of these huge, gaping holes is dark matter.”
Dark matter is matter we cannot see. Unlike normal matter, which interacts with light – absorbing it, reflecting it, emitting it – dark matter hardly interacts or interacts with light, making it invisible to both the naked eye and current instruments. Its existence is deduced from its impact on visible matter. Despite its invisibility, dark matter is much more abundant, Salemi says. “There is five times more dark matter in the universe than normal matter.”
Like its visible counterpart, which is made up of particles like neutrons, protons, and electrons, dark matter is also made up of particles, but physicists still don’t know exactly what type. One candidate is Axion and ABRACADABRA was designed to find him.
Small but powerful
Compared to CERN’s Large Hadron Collider, which is an instrument tasked with detecting the proposed particles and has a circumference of 16.6 miles, ABRACADABRA is tiny. For Salemi, the instrument is representative of a new era of tabletop physics. Creating ever larger tools to search for ever more elusive particles had been the strategy to follow, but these have gotten more and more expensive. “Because of this, people are coming up with all sorts of really cool ideas on how to make more discoveries, but on a smaller budget,” Salemi says.
ABRACADABRA’s design was developed in 2016 by three theorists: Jesse Thaler, associate professor of physics; Benjamin Safdi, then an MIT Pappalardo Fellow; and Yonatan Kahn PhD ’15, then Thaler’s graduate student. Winslow, an experimental particle physicist, took that project and figured out how to make it a reality.
ABRACADABRA is composed of a series of toroid-shaped magnetic coils – imagine an elongated donut – wrapped in a superconducting metal and kept refrigerated around absolute zero. The magnet, which Salemi says is the size of a large grapefruit, generates a magnetic field around the toroid but not in the donut’s hole. Explain that if axions existed and interacted with the magnetic field, a second magnetic field will appear inside the donut hole. “The idea is that that would be a zero-field region, unless there is an axion.”
It can take 10 or more years to take a theoretical design for an experiment and make it operational. ABRACADABRA’s journey was much shorter. “We went from a theoretical paper published in September 2016 to a result in October 2018,” says Winslow. The geometry of the toroidal magnet, Winslow says, provides a naturally low background region, the donut hole, in which to look for axions. “Unfortunately, we got over the easy part and now we have to cut down on those already low backgrounds,” says Winslow. “Chiara led the effort to increase the sensitivity of the experiment by a factor of 10,” says Winslow.
To detect a second magnetic field generated by an axion, you need an incredibly sensitive instrument, but also shielded from external noise. For ABRACADABRA, that shielding comes from the superconducting material and its freezing temperature. Even with these shields, ABRACADABRA can detect people walking around the lab and even pick up radio stations in Boston, Massachusetts. “We can actually hear the station from our data,” Salemi says. “It’s like the most expensive radio.”
If an axion signal is detected, Salemi and colleagues strive to disprove it first, looking for all potential sources of noise and eliminating them one by one. According to Salemi, detecting dark matter means prizes, even a Nobel Prize. “So you don’t post that kind of result without spending a lot of time making sure it’s correct.”
The results of the first run of ABRACADABRA were published in March 2019 in Physical Review Letters by Salemi, Winslow and others in the MIT Physics Department. No axions were detected, but the run did highlight changes the team could make to increase the sensitivity of the instrument ahead of its second run which began in January 2020. “We have been working on the setup, execution and analysis of the second run for about a year and half, “Salemi says. All data has now been collected and the team is finishing the analysis. The results will be published by the end of the year.
As they prepare the results for publication, Salemi and his colleagues are already thinking about the next generation of axion detectors, called DM Radios, for Dark Matter Radios. Salemi says it will be a much broader multi-institute collaboration and that the design of the new tool is still being worked on, including the decision on the shape of the magnet. “We have two possible designs: one is in the shape of a donut and the other is in the shape of a cylinder.”
The search for axions began in 1977, when they were first theorized, and since the 1980s experimental physicists have designed and improved the tools to detect this elusive particle. For Salemi, it would be great to continue working on the axions until they are discovered, although no one can predict when that might happen. “But, see the low-mass experimental axion dark matter from start to finish? What could I do,” he says. “Fingers crossed.”
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