Imagine standing on a seashore, watching the waves roll in. Now imagine trying to describe what’s happening inside a wave, an eddy, and a single drop of water—all at one time.

That’s the kind of challenge Professor Thomas Papenbrock and Adjunct Assistant Professor Gaute Hagen are up against as they calculate physics at different energy scales inside an atomic nucleus. With colleagues from Oak Ridge National Laboratory, Louisiana State University, and Chalmers University of Technology in Sweden, they’ve developed a model that not only computes multiscale physics, but does so drawing from the roots of fundamental nuclear theory.

This kind of research is important because the more researchers discover about the nucleus, the better chance that knowledge can find its way to improving our lives (medical imaging and food safety) and answering bigger scientific questions (what fuels a massive star?). Those applications require understanding the essential properties of these complex systems. 

Calculating Nuclear Waves and Eddies

A nucleus isn’t one simple entity. Inside are protons and neutrons, formed by quarks bound together by gluons. That’s a lot of moving parts for structures so small that a typical grain of sand includes more than 10 million trillion of them.

Papenbrock said that like any system consisting of many particles, nuclei show different phenomena at different length (or energy) scales. To visualize this, consider the ocean. Papenbrock explained that seawater can appear as waves (on the scale of yards), eddies (which can range from tens of yards to inches in scale), and even motions of individual water molecules (microscopic scale).

In a nucleus, different phenomena are realized at different energy scales. At low energy the entire nucleus might rotate. At the next energy level, many (but not all) of the particles vibrate. At even higher energies a nucleus (or a single proton or neutron) could break up.

“The more microscopic the approach one uses, starting at smaller length scales, the more cumbersome it is to compute these collective phenomena, such as eddies and waves in the case of water,” Papenbrock said.

A Computational Two-Step

To tackle this cumbersome task, Papenbrock, Hagen, and their colleagues studied neon nuclei using the power of Oak Ridge National Laboratory’s Frontier supercomputer. In a sort of computational two-step, they captured large energies by including short-range correlations and then computed the details by including long-range correlations. This approach gave them a high-resolution look into the intricate workings inside a nucleus.

A significant achievement in this work is that the team overcame a long-standing challenge in the field by describing multiscale physics with roots in quantum chromodynamics, or QCD. This is the fundamental theory of the strong nuclear force: one of the four forces of physics. It’s part of the Standard Model that describes all known particles and forces: the building blocks of matter and how they interact.

Papenbrock said that “one of the goals in nuclear physics is to describe phenomena starting from the most basic theories: in our case QCD. While we are not yet quite there, we used effective theories of QCD.”

He explained that earlier research computed nuclear binding and rotation energies, but relied on existing theories to model different phenomena at different energy scales. Here, he and his colleagues have developed a unified model that ties directly to bedrock theory.

The findings were published in Physical Review X and selected for a Physics Viewpoint highlight. Learn more about this research from Oak Ridge National Laboratory.

Rebecca Godri isn’t afraid of the hard work needed to test the Standard Model of Physics. As the department’s newest student selected for the prestigious Department of Energy (DOE) Office of Science Graduate Student Research (SCGSR) program, she’s won financial support to pursue this effort at Oak Ridge National Laboratory (ORNL).

Looking for new physics

As part of UT’s fundamental neutron physics group, Godri investigates the weak force, one of the four fundamental interactions in nature. She’s working on the Nab experiment at ORNL; a project designed to discover the nuances of neutron beta decay, a weak process.

A free (unbound) neutron is unstable: it will decay into a proton, electron, and antineutrino. Professor Nadia Fomin, one of Godri’s advisors, explained that “As experimentalists, we can ‘observe’ the neutron spin, momenta of the charged particles, and the angles between them. While the antineutrinos are detectable, the efficiency is very low, and we can get all the necessary information from basic principles of energy and momentum conservation.”

The Nab experiment will make precise determinations of “little a” and “little b,” decay correlation parameters of neutron beta decay. These determinations provide a stringent test for physics beyond the Standard Model.

Flipping the Spin

As small as protons, electrons, and antineutrinos are, the Nab experiment requires mighty tools to study them. Godri is working at the Fundamental Neutron Physics Beamline (FnPB) at the ORNL Spallation Neutron Source, a state-of-the-art facility that directs a powerful neutron stream down beamlines for instruments purpose-built for specific kinds of measurements. As their name implies, neutrons have no positive or negative charge. They do, however, have spin, a magnetic property that points them in certain direction. When neutrons decay, their speed, energy, and direction can give scientists critical information. The FnPB is unique in that can support different experiments with different full-scale apparatus, such as Nab.

Godri’s work, which she recently presented at an American Physical Society meeting, is getting neutrons to reveal more clues about the weak interaction in physics. To do that she’s measuring the residual polarization of the neutron beam at the FNPB and characterizing the Nab experiment’s spin flipper—a device that lets scientists control the neutron’s spin. She works with both Fomin at UT and Chenyang Jiang, a research scientist in ORNL’s Neutron Technologies Division.

While some may find all the preparation tedious, Godri said she enjoys both the groundwork and the payoff.

“The hands-on work is exciting,” she said. “My favorite part is being able to get data from the measurements we’ve spent months preparing for!”

An Easy Decision

Godri earned a bachelor’s degree in physics from the University of Tennessee at Chattanooga in 2021 and started her graduate studies in Knoxville that fall. She finished a master’s in physics last year and is working toward a PhD.

UT’s nuclear physics research opportunities made it an obvious choice for her graduate studies.

“Nadia’s research at ORNL aligned with my research interests post-undergrad, so UT was an easy decision,” she said. Godri’s SCGSR award is the 14^th for UT Physics since 2016 and the fifth this year. She’s part of the latest cohort, comprising 62 PhD students from 24 states

Imagination, quite literally, made Lindsey Hessler a Vol before she even started high school. Now a UT senior, she has won an assistantship from Jefferson Lab to support her research in nuclear physics.

It’s the Small Things in Life

Hessler said her interest in physics came about because she is “incredibly fascinated by the intricacies of the universe and understanding how the small things in life work.”

During COVID she spent hours on YouTube watching videos about stars, galaxies, energy—anything explaining the building blocks of our world and universe.

For nearly a year she’s been working with Professor Nadia Fomin and Assistant Professor Dien Nguyen as part of the nuclear physics research group. She was one of the department’s 2024 Summer Research Fellows and learned in August she had won a Jefferson Science Associates Minority/Female Undergraduate Research Assistantship.

The program supports minority or female undergraduates working on projects that are part of the Jefferson Lab research program or are directly related its scientific or engineering aspects. Situated in Virginia, this United States Department of Energy facility is a leader in accelerator science, dedicated to probing the particles and forces that comprise and govern the matter that makes up our world. With this award, Hessler will contribute to the lab’s scientific mission.

“This assistantship will cover a variety of projects,” she explained. “I will be working to collect data with a Helium-3 Polarization apparatus as well as developing a projection of runtime for upcoming experiments at CEBAF (the Continuous Electron Beam Accelerator Facility).”

Her physics studies are driven by a creative and curious worldview that began when she was still in middle school and ultimately brought her to Knoxville.

Solving Problems in the Lab and Industry

Hessler is from Germantown, Wisconsin, but early on her college die was cast in Tennessee orange.

“I chose UT because as a kid I was involved in a competition/club called Destination Imagination (DI),” she said. “The Global Finals were held at UT after the college semester ended, so I spent a week in the dorms at age 12 and fell in love with the campus, ambience, and culture in Knoxville. Ever since then I was determined to be a Volunteer.”

She explained that she began her UT studies as a management major but quickly discovered she had a knack for science.

“At this point I had a decent amount of business classes completed (so) I decided to add physics as a second major,” Hessler said. “I have loved learning two very different studies and have found that both educational paths teach me how to work through problems (and life) in very different but beneficial ways.”

Despite the demands of a double major in business administration (management) and physics, she’s on track academically and said that “ideally (she) will be graduating next December holding a diploma from the Haslam College of Business and UT’s College of Arts and Sciences.”

Her plans include going on to graduate school with a focus on nuclear physics, using the full advantage of her combined majors to design her career.

“I would love to work in the energy and/or defense industry and apply my physics degree as well as my BS in management to project management and operations,” she said.

From her first days as an imaginative kid visiting campus, Hessler has followed her passion and is headed for a promising next destination as a nuclear scientist.

Ian Cox is proof that you don’t always have to travel far to go a long way. He grew up in Knoxville, graduated from Hardin Valley Academy, and came to UT on a physics scholarship. Now he’s finishing a PhD in nuclear physics with Professor Robert Grzywacz and is first author on a Physical Review Letters publication detailing a new approach to understanding exotic nuclei.  

A Touch of Magic

Researchers from 13 universities and five national laboratories collaborated on this investigation at the Facility for Rare Isotope Beams (FRIB), a premiere research hub at Michigan State University. The nucleus is the heart of every atom, and since 2022 FRIB has produced hundreds of rare isotopes so that scientists can unearth how the most exotic nuclei hold together or decay. FRIB explores this unknown territory by creating extremely imbalanced and short-lived assemblies of protons and neutrons, helping physicists gain a deeper understanding of these quantum mechanical systems. The more complete that picture, the greater the likelihood scientists can predict how nuclei form, what their properties are, and how those properties can be of use. In this case, the starring isotope was chlorine-45. With 17 protons and 28 neutrons, it has a touch of what physicists categorize as magic.

Protons and neutrons in a nucleus are known collectively as nucleons and they’re arranged in shells. When they appear in certain numbers (2, 8, 20, 28, 50, 82, and 126), scientists call them “magic” because they fill complete shells and make the nucleus more stable (although that may be a lifetime of only a few milliseconds). Magic numbers are like sentinels in what’s known as the valley of stability. A proton or neutron count with a magic number (as in chlorine-45) resides at the border, where on one side you have nucleons bound strongly enough to hold the nucleus together and on the other their imbalance causes it to fall apart. This isn’t always straightforward, however. Magic numbers change for nuclei rich with neutrons, and scientists want to know how that alters the shell structure.

In this experiment the scientific team found that the beta decay of chlorine-45 converts one of its 28 neutrons to one of the 18 protons in argon-45. This lies outside the magic threshold of a 20-proton shell, creating a particularly unstable, unbound system. Grzywacz said “the experiment provided a unique method to study how the protons behave in a very neutron-rich nucleus. Understanding the persistence of nuclear shell gaps is crucial to describe the properties and formation of atomic nuclei.”

A Nuclear Symphony

A combined arrangement of innovative tools, talent, and effort made this work successful.

“This was an important experiment because we tried a lot of new things,” Grzywacz said.

His team was particularly pleased to use the full capabilities of the FRIB Decay Station Initiator (FDSi) for the first time. Grzywacz is the spokesperson for the FDSi, a years-long collaboration that designed, built, and implemented a modular combination of beta, neutron, and gamma-ray detectors to measure the decay of the most exotic nuclei produced at FRIB.

The FDSi played a crucial role in determining the complete decay pattern of chlorine-45. Cox identified the isotopes in this experiment as interesting candidates to show what the decay station could do. A key element was the two-focal plane detection system, which allows for simultaneous measurements and ultimately a combined and consistent data analysis that wouldn’t be possible with a single multi-detector system.

This is what Grzywacz called a scientific “symphony,” where “the instruments combined produce a different result than if played separately and individually.”

That metaphor extends to the scientists involved in the experiment.

Cox and Zhengyu Xu (a UT postdoctoral research associate) helped install FDSi at FRIB from the ground up. They’ve supported every FDSi experiment since, and led the analysis on the work published in PRL. Wei Jia Ong of Lawrence Livermore National Laboratory (LLNL) directed the measurement. Her interest was to measure decay of another isotope, calcium-54; this experiment was a prelude to the 2024 measurement.

Navigating multiple instruments and working with a large team were part of the learning experience for Cox.

“Working at FRIB on FDSi, I have learned a great deal about the complex nature of radioactive ion beam facilities and specifically the challenges which come with combining multiple different detector systems for a single experiment,” he said. “The varying types of detectors require a large collaboration of researchers, each with their own expertise to handle the individual detector systems, while also having to work together to ensure a successful experiment.”

Cox explained that frustrations could arise while everyone was trying to optimize their system in a limited amount of time. However, he found that having a sizable collaboration was helpful because in the end, working together, smaller groups could focus on individual detectors. Sharing the responsibility meant the science moved forward more seamlessly. This is the fourth publication based on FDSi findings and the third published in PRL.

To See the World, Stay Close to Home

Cox stayed in Tennessee for his education and ended up travelling far and wide. In addition to FRIB, he’s worked at the Radioactive Isotope Beam Factory at RIKEN in Japan and presented his research at conferences all over the world. He arrived on campus with a William Bugg Physics Scholarship and a spot in the Chancellor’s Honors Program. Over his undergraduate and graduate studies, he won the department’s Robert Talley Award for Outstanding Undergraduate Research, secured a Graduate Advancement and Training Education Fellowship from the UT-Oak Ridge Innovation Institute (UT-ORII), and won the department’s Paul Stelson Fellowship for Professional Promise in recognition of his outstanding research contributions as well as his departmental citizenship.

He plans to finish his PhD this summer and work in either the private sector or at a national laboratory. Whatever comes next, his time at UT has prepared him well, especially with the nuclear research group collaborating at laboratories in different states and countries.

“I have really enjoyed both travelling all over the world, for participating in experiments and presenting results, and the ability to meet many different researchers from all corners of the globe,” Cox said. “I believe this will greatly help me in my career, as I have been able to form many connections and establish myself as a young scientist.”

Mother Nature keeps moving the goalposts for Anthony Mezzacappa and he wouldn’t have it any other way. His years of dedication to computational and theoretical astrophysics research, even as the landscape shifts, have earned him election as a Fellow of the American Association for the Advancement of Science (AAAS).

The AAAS Council bestows this honor on members whose “efforts on behalf of the advancement of science, or its applications, are scientifically or socially distinguished.”

Mezzacappa is the Newton W. and Wilma C. Thomas Chair in Theoretical and Computational Astrophysics and a College of Arts and Sciences Excellence Professor. He develops sophisticated models of supernovae through roles in UT’s Physics Department (since 1994) and at Oak Ridge National Laboratory (since 1996). It’s the foundation for his AAAS Fellowship, where he was cited “for distinguished contributions to the field of computational and theoretical astrophysics, particularly for developing theoretical frameworks and computational methods to model core collapse supernovae.”

“The AAAS Fellowship is about the advancement of science writ large,” Mezzacappa said. “This Fellowship really is special in that sense. I’ve spent a lot of time doing things for the field and for computational science. It’s really nice to receive a recognition of that.”

From Upstart to Leadership

Early in his career Mezzacappa was the principal investigator for the first large-scale, multi-investigator, multi-institutional computational astrophysics effort in the country to focus on core collapse supernovae. That was the beginning of a long list of professional accomplishments in astrophysics.

“When you first start out, there are more senior people who are leaders in the field and you’re a young upstart,” he said. “I was glad to get the vote of confidence by senior people to lead that effort.”

He said he knew after directing a program of that size there was no going back to smaller initiatives where he’d be the lone PI on a single project. He also noticed that over time, he and his contemporaries evolved from upstarts just beginning in the field to leaders who were guiding it.

“That changes the whole responsibility,” Mezzacappa said. “It changes how you think about yourself and your field. One of your jobs is to usher the science forward, which implicitly means with integrity and scientific accuracy.”

For him, it also means cultivating a network of fellow scientists to dive into the mysteries of astrophysics.

“Over time, you wind up contributing where you can, where your strengths are,” he said. “One of the things I know how to do is build and manage projects and programs. I created the largest core collapse supernova theory group in the world here between (ORNL) and the university.”

That gift for organizing expertise and assets has certainly benefited the research and teaching environment at UT.

“Professor Mezzacappa is the definition of a fearless scientist who chases after the solutions to some of the most challenging and interesting problems underlying how the universe functions,” said Adrian Del Maestro, professor of physics and head of the department­. “The recognition by the AAAS for his dedication to discovery in computational astrophysics is well deserved, and the department (and especially his students and postdocs) are extremely lucky to have Professor Mezzacappa at the University of Tennessee.” 

Moving the Goalposts

When a gigantic star can no longer support its own weight, gravity will eventually cause it to collapse on itself in some of the Universe’s most spectacular fireworks. When Mezzacappa and his colleagues were first developing models for core-collapse supernovae, they began with spherical symmetry, though they knew the physics would eventually lead them to more complex models.

“We always knew that we had a long way to go,” he said.

As they accumulate a deeper knowledge base and their tools (supercomputers) get bigger and faster, Mezzacappa said astrophysicists can now create beautiful three-dimensional supernovae models. As amazing as they are, however, those models still don’t answer all their questions.

“The thing that’s really changed is that Mother Nature is moving the goalposts,” he said.

“When you start out you think it’s a fixed target, because you have limited knowledge,” he went on to explain. “As you learn more and move forward, you realize the problem is even harder than you imagined to begin with. As the models have gotten more sophisticated, we’ve discovered the physics of supernovae is richer, and some of that richness is very challenging to model.”

To Mezzacappa, that’s part of the what makes fundamental science so meaningful.

“As humans, we always want to find the answers,” he explained. “As I’ve grown, one of the things that’s changed is that I kind of enjoy the mystery more now. I think knowing everything would be quite boring.”

Back to the Beginning, and the Future

Mezzacappa was in high school when he first learned about Einstein’s theories and decided what he wanted to study for the rest of his life.

“Relativity is what pulled me into physics from the get-go,” he said.

Decades later, his work keeps him close to the spark that ignited his imagination. He explained that core-collapse supernovae are one of the primary sources of gravitational waves and the only known source from which they’ve yet to be detected.

“We are in a position now, because we are a leading group and we have some of the leading models, to make predictions for what the gravitational waves for the supernovae look like,” he said. “I feel like I’m really contributing—not just to astrophysics but to relativity, which is special for me.”

The Most Important Impact

The AAAS Fellowship is one of many honors Mezzacappa has earned over a distinguished career. In the past year he’s been named a College of Arts and Science Excellence Professor, won the College’s Senior Award for Excellence in Research & Creative Achievement, and been recognized with the university’s Alexander Prize. He was elected a Fellow of the American Physical Society in 2004 and a UT-Battelle Corporate Fellow in 2005 in recognition of his supernova research and his broader role in the development of computational science in the United States.

“They all mean a lot,” Mezzacappa said of his honors, but “your most important impact is the impact you have for others, not for yourself.”

Professor Mezzacappa becomes the fourth member of the current physics faculty elected to AAAS Fellowship, joining Professors Elbio Dagotto, Adriana Moreo, and Hanno Weitering.