Particle Physicist Hunting for Dark Matter Candidates that Contain the Secrets of the Universe

Many scientists say that visible matter, including that on Earth, accounts for only about 4.4% of the entire universe. They explain that 27% consists of dark matter, and the remaining 68.6% is dark energy. The name "dark matter" hints at its nature—it does not interact with light, making it detectable only through its gravitational effects. However, dark matter is merely one hypothesis that aims to explain the universe, as its existence has yet to be conclusively proven.

One of the prominent candidates for dark matter is the axion, a hypothetical particle proposed to resolve mysteries in particle physics. For decades, researchers worldwide have been searching for axions. In Korea, the Institute for Basic Science (IBS) Center for Axion and Precision Physics Research (CAPP) is conducting research to find axions. In August, CAPP published two significant studies that narrowed down the parameters to determine the existence of axions, drawing considerable attention.

Research Fellow YOON Seong Woo of CAPP, who participated in the study, explained, “Searching for axion dark matter is like finding a needle in a haystack. If you have a rough idea of its location, you can find the needle faster, much like our research demonstrates the importance of designing experiments based on theoretical predictions.” This work helps outline the direction for the next generation of axion search experiments.

Dr. Yoon joined CAPP in 2015 and embarked on his journey as an "axion hunter." Now, in his 9th year, he commented, “Searching for particles often leads to philosophical reflections, like questioning the origins of the universe and life itself,” emphasizing the allure of dark matter research. We spoke with Dr. Yoon to discuss the significance of axion research, the recent achievements, and his path into particle physics.

When did you first become interested in AI?

Around 20 years ago, I started to believe that AI would become important in chemistry. I was considering ways to make new discoveries in chemistry without relying on serendipity. I thought it would be possible to predict chemical reactions using algorithms. From 1995 to 2003, I was at Harvard University, where I interacted with people who were developing chess AI. I figured that if AI could learn to play chess, it could also learn about chemical molecules. This led to my first paper in 2005.

Please introduce yourself.

Hello, I’m Dr. YOON Seong Woo, a Research Fellow at IBS’s CAPP. During my school days, I became fascinated with particle physics after learning about Dr. Benjamin LEE Whisoh, a Korean-American physicist. I graduated from the Department of Physics at Korea University in 2000, earned my master’s in 2002, completed my Ph.D. at Northwestern University in 2009, and then worked as a postdoctoral researcher at Fermilab until 2014. After a year of research at the University of Maryland, I joined CAPP at IBS as a Young Scientist Fellow (YSF) in 2015. YSF is a program by IBS supporting scientists under 40. Now, I continue to study dark matter, particularly focusing on axions, as a Research Fellow at CAPP.

Please introduce the Center for Axion and Precision Physics Research (CAPP).

CAPP is dedicated to determining whether axions make up a portion of cosmic dark matter. The center is led by Director Yannis SEMERTZIDIS, who conducted the world’s first axion-related experiments in the 1980s. CAPP aims to discover particles that challenge the Standard Model of particle physics.

Researchers exclude regions where dark matter may not exist, based on theoretical predictions. Generally, scientists use strong magnets, as theory suggests axions convert to photons with frequencies matching their mass when exposed to a magnetic field. By amplifying these frequencies with a resonator, we can detect whether axions are present within that range.

The challenge lies in that the exact mass of the axion, or its frequency when converted, is unknown, requiring us to search across a broad frequency range like tuning a radio.

Please explain CAPP's recent achievements and their impact on academia.

There are two main theoretical models for axions: the DFSZ axion and the KSVZ axion. The DFSZ axion has weaker interactions, making it harder to detect experimentally than the KSVZ axion. CAPP focused on developing equipment to search for DFSZ axions. Only two research teams globally, including ours, are currently equipped for this. Recently, we managed to confirm the non-existence of axions within a specific range.

The probability of detecting axions increases with a stronger magnetic field. Our team successfully conducted a high-sensitivity search for axions with masses between 4.24 and 4.91 µeV within the frequency range of 1.025–1.185 GHz using a powerful superconducting magnet. To date, CAPP is the only research group worldwide to achieve high-sensitivity searches above 1 GHz. After only ten years of research, we achieved world-class results.

Additionally, we maximized the search for the KSVZ axion model at high frequencies, as suggested by Professor KIM Jihn-eui of Seoul National University. Recent simulations suggest that axions may exist within the mass range of 20–30 µeV, corresponding to 4.8–7.25 GHz. Our research revealed that axions do not exist within the 21.86–22.00 µeV range, providing the most sensitive results to date.

What role did you play in this achievement?

I contributed to the study focusing on KSVZ axions in high-frequency ranges. I worked on designing an optimized resonator capable of rapid axion searches without reducing its volume. To search for high-frequency signals, reducing the resonator's volume is typically necessary, but this decreases the likelihood of detecting axions, requiring more time to gather data. Thus, we developed a “multiple-cavity resonator,” which resembles slices of a pie in a cylindrical design. This resonator was instrumental in achieving our research results.

Please tell us any challenges you faced during this experiment and what led you to begin this research.

Optimizing the resonator design to suit axion search experiments required a massive number of simulations and a great deal of time. Additionally, because we used sensitive quantum sensors to detect axions, it was essential to minimize noise interference. This involved cooling the equipment, including the resonator, to reduce thermal noise, which required reaching cryogenic temperatures. It took several days to achieve the low temperatures, and due to the complex structure of the detection system, we had to repeat the process until every component functioned correctly.

I started this research because searching for axions is CAPP's ultimate goal. Studying particles inevitably raises philosophical questions about the origins of the universe and life. What does the universe look like and how did it form? Who are we? Such questions often come up during research. Perhaps CAPP can contribute to finding answers to these questions?

Please share your future research plans.

We’ll continue to hunt for axions, gradually expanding the search range. There is intense competition worldwide in the search for axions, and IBS aims to deliver world-leading results.

What support would you need for future research?

I’d be grateful if our center’s research could be trusted and awaited with patience. It took several years just to get the equipment up and running after the center's establishment in 2013, and only now we’re seeing results. We aim to produce exceptional research results that will ultimately benefit society.

Any final remarks?

I’d like to emphasize the importance of basic science. Today, we see substantial investments in AI and quantum technology, both founded on concepts developed over 100 years ago. Thanks to the persistent efforts of basic science researchers, these fields have become investment-worthy today. I hope that ongoing investment in basic science will continue, allowing remarkable technologies to emerge in the future.