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Hanging on the first floor of MIT’s Nuclear Science Lab hangs an instrument called “A Wideband / Resonant Approach to Cosmic Axion Detection with a B-Field Ring Amplifier Apparatus,” or ABRACADABRA for short. As the name suggests, ABRACADABRA’s goal is to detect axions, a hypothetical particle that may be the main constituent of dark matter, the invisible and as yet unexplained material that makes up most of the universe.
For Chiara Salemi, fourth-year graduate physics student in Lindley Winslow’s group, Jerrold R. Zacharias career development associate professor in physics, ABRACADABRA is the perfect instrument to work on during her PhD. “I wanted a little experiment to be able to do all the different parts of the experiment,” says Salemi. ABRACADABRA, which consists of an extremely well shielded magnet, is the size of a basketball.
Salemi’s willingness to work in all aspects is unique. “Experimental physics has roughly three components: hardware, computation, and phenomenology,” says Winslow, with the students leaning toward one of the three. “Chiara’s affinity and strengths are spread evenly across all three areas,” says Winslow. “It makes her a particularly strong student.”
Since starting his PhD, Salemi has worked on everything from updating the circuits of ABRACADABRA for his second run, to analyzing data from the instrument to look for the first sign of a dark matter particle.
A happy accident
When Salemi started college, she had no intention of doing physics. “I was leaning towards science, but I was not completely 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 studied physics with the goal of determining if this might be an area that might interest her. “And then I just fell in love with it, because I started doing research, and research is fun.
Throughout his undergraduate career, Salemi has collected research experiences. She has operated radio telescopes in West Virginia. She spent a semester in Geneva, Switzerland, researching the Higgs boson decay at the European Organization for Nuclear Research, better known as CERN. At the Lawrence Berkeley National Laboratory, she tinkered with the design of semiconductors for neutrino detection. It was during one of these research experiences, a summer program at Fermilab in Illinois, that she began working with axions. “Like a lot of things in life, it was an accident.”
Salemi had applied for the summer program because she wanted to continue working on neutrinos and “Fermilab is the center of everything about neutrinos. But when she got there, Salemi found out that she was assigned to work on the axions. “I was extremely disappointed, but ended up falling in love with axions because they are really interesting and different from other particle physics experiments.
The elementary particles of 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, which was developed in the early 1970s, describes almost everything in the subatomic world. “But there are huge gaping holes,” says Salemi. “And one of those huge gaping holes is dark matter.”
Dark matter is matter that we cannot see. Unlike normal matter, which interacts with light – absorbing it, reflecting it, emitting it – dark matter does not or barely interact with light, making it invisible to 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, says Salemi. “There is five times more dark matter in the universe than normal matter.”
Like its visible counterpart, which is made up of particles such as neutrons, protons, and electrons, dark matter is also made up of particles, but physicists are still not sure exactly what types. One candidate is the axion, and ABRACADABRA was designed to find it.
Small but mighty
Compared to CERN’s Large Hadron Collider, which is an instrument tasked with detecting proposed particles and has a circumference of 16.6 miles, ABRACADABRA is tiny. For Salemi, the instrument is representative of a new era in table physics. Creating larger and larger instruments to search for increasingly elusive particles was the strategy of choice, but these became more and more expensive. “Because of this, people come up with all kinds of really interesting ideas on how to keep making discoveries, but on a smaller budget,” says Salemi.
The ABRACADABRA design was developed in 2016 by three theorists: Jesse Thaler, associate professor of physics; Benjamin Safdi, then MIT Pappalardo scholarship holder; and Yonatan Kahn PhD ’15, then graduate student of Thaler. Winslow, an experimental particle physicist, took this design and figured out how to make it a reality.
ABRACADABRA is made up of a series of torus-shaped magnetic coils – imagine an elongated donut – wrapped in superconductive metal and kept in the refrigerator at about absolute zero. The magnet, which Salemi says is about the size of a large grapefruit, generates a magnetic field around the torus but not in the donut hole. She explains that, if axions exist and interact with the magnetic field, a second magnetic field will appear in the ring hole. “The idea is that it would be a zero field region, unless there is an axion.”
It can take 10 years or more to take a theoretical design of an experiment and make it operational. ABRACADABRA’s trip was much shorter. “We went from a theoretical article published in September 2016 to a result in October 2018,” says Winslow. The geometry of the toroidal magnet, Winslow says, provides a naturally low bottom region, the ring hole, in which to look for axions. “Sadly, we’ve taken the easier part and now need to cut back 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 instrument that is incredibly sensitive, but also protected against external noise. For ABRACADABRA, this shielding comes from the superconducting material and its freezing temperature. Even with these shields, ABRACADABRA can detect people walking through the lab and even pick up radio stations around Boston, Massachusetts. “We can actually listen to the station from our data,” says Salemi. “It’s like the most expensive radio.”
If an axion signal is detected, Salemi and his colleagues will first attempt to refute it, looking for all potential sources of noise and eliminating them one by one. According to Salemi, detecting dark matter means rewards, even a Nobel Prize. “So you don’t publish that kind of result without spending a lot of time making sure it’s correct.”
The results of the first execution of ABRACADABRA were published in March 2019 in Physical examination letters by Salemi, Winslow and others from the Department of Physics at MIT. No axions were detected, but the analysis highlighted adjustments the team could make to increase the sensitivity of the instrument before its second run which began in January 2020. “We are working on the setup, the execution and analysis of the second round for about a year. and a half, ”says Salemi. Currently, all data has been collected and the group is completing the analysis. The results will be published later this year.
As they prepare these results for publication, Salemi and his colleagues are already thinking about the next generation of axis detectors, called DM Radios, for Dark Matter Radios. Salemi says it will be a much larger, multi-institute collaboration, and the design of the new instrument is still being designed, including the choice of the shape of the magnet. “We have two possible models: one is the shape of the ring and the other is a cylindrical shape.”
The search for axions began in 1977, when they were first theorized, and since the 1980s experimental physicists have been designing and improving instruments to detect this elusive particle. For Salemi, it would be amazing to continue working on axions until they are discovered, even though no one can predict when it might happen. “But, see experimental dark matter axion of low mass from start to finish?” That’s what I could do, ”she said. “Crossed fingers.”
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