Getting Under the (Neutron) Skin

October 20, 2022

Every atomic nucleus, miniscule or massive, is governed by a basic law of nature: the strong force holding it together. The more we know about the complex workings of nuclei the better we understand different kinds of matter, including elements produced in cosmic collisions (like the gold in our jewelry boxes). In Nature Physics, UT’s physicists and their colleagues at Chalmers University of Technology present a groundbreaking model to calculate the properties of a lead nucleus with a method that can be used across the nuclear landscape—all the way to neutron stars.

The Story of Stars in a Single Nucleus

Scientists use computational models to get the most complete picture of a nucleus. Heavier nuclei typically don’t fit well in those models. They may be unstable and fall apart quickly. They might have so many protons and neutrons that it’s difficult to keep track of all the interactions between them. Nuclei with lots of neutrons present a particular challenge. The new approach turns that obstacle into an asset.

When neutrons outnumber protons, equal numbers of each gather in the nucleus’s center while the surplus neutrons collect on its surface, forming a skin. Lead (Pb-208) is a good example. With 126 neutrons and 82 protons, it’s a heavy (yet stable) nucleus covered in a neutron skin. This outer layer is sensitive to the strong force, which holds its protons and neutrons together.

Professor Thomas Papenbrock described how he and his colleagues connected this single, simple nucleus to massive neutron stars, a teaspoon of which would weigh 4 billion tons on Earth.

Combining advanced methods, statistical tools, and computational modeling, they were able to calculate the neutron skin in lead. This informs scientists about the “nuclear equation of state,” which describes matter under various conditions like pressure, volume, or temperature. That equation in turn links neutron skin thickness to the structure and size of neutron stars. Further, the model can be adapted across the nuclear landscape to predict properties of other heavy nuclei that have eluded models in the past.

The work also shows that a heavy nucleus can be computed with tools theorists routinely use: Hamiltonians (functions that describe a dynamic system) rooted in the theory describing the strong force (quantum chromodynamics, or QCD). These tools have well-understood parameters that help quantify uncertainties, making predictions more precise.

The Physics in Your Jewelry Box

Papenbrock outlined multiple reasons why understanding nuclear structure and properties is important. First, there’s understanding the complexity arising from the strong force, one of nature’s four basic interactions (along with the weak, electromagnetic, and gravitational forces).

“Second,” he said, “is understanding the production of elements in the early universe and in stellar processes such as neutron-star mergers and supernovae explosions.”

(Helium and hydrogen were born in the early seconds of our universe, while gold and platinum trace their beginnings to merging neutron stars.)

“This require us to understand very neutron-rich nuclei that cannot all be produced and studied in laboratories,” Papenbrock said. “So, we have to compute them.”

Understanding the nucleus not only helps us understand what happened eons ago or what goes on in far-away objects. It also helps scientists shape future discoveries.

“Searches for physics beyond the Standard Model of elementary particles almost all involve hypothetical reactions of such particles with or inside atomic nuclei,” he explained.

James Bond and Nuclear Physics

Papenbrock pursues nuclear studies through a joint faculty appointment with Oak Ridge National Laboratory. He worked with ORNL’s Zhonghao Sun (former UT postdoc) and Gaute Hagen (an adjunct professor with UT Physics) to calculate lead’s neutron skin, a feat made possible by ORNL’s Summit supercomputer.

“Computing has been a game changer over the last two-to-three decades,” Papenbrock said. “Moore’s law states that computing power doubles roughly every 16 months. Our phones can do things today that seemed science fiction (or James Bond) just a few decades ago.

“This has forced nuclear physicists to collaborate with computer scientists to really use all these technologies,” he continued. “We had to make so many changes to our codes over the years to adapt to this evolution of computing.”

Using collaborative strengths and computing power for nuclear physics is familiar territory for Papenbrock and Hagen. They’re part of the NUCLEI (the NUclear Computational Low Energy Initiative) collaboration that recently won $13 million in funding through SciDAC, the U.S. Department of Energy’s Scientific Discovery Through Advanced Computing program (part of the Office of Science). Papenbrock became the group’s principal investigator this year.

Comprising five national laboratories and seven universities, NUCLEI aims to advance the computations of atomic nuclei and processes. The Nature Physics paper was a result of the collaboration’s research, earning a News and Views article in the journal for its significance. This work, the review explains, “has allowed practitioners to reach for the stars.”

Andrew W. Steiner Elected APS Fellow

October 19, 2022

When a giant star dies, Andrew W. Steiner gets a little more lab space.

An associate professor with a joint appointment at Oak Ridge National Laboratory, Steiner studies the neutron stars left after massive stars collapse. His outstanding research has earned him election as a Fellow of the American Physical Society (APS), making him the 10th APS Fellow on the current physics faculty.

APS Fellowship recognizes members who have made advances in physics through original research and publication or significant innovative contributions in applying physics to science and technology. They may also have made significant contributions to physics teaching or service. Steiner was cited “for pioneering a data-driven approach to constraining neutron star properties and the dense matter equation of state that combines advanced statistical methods, state-of-the-art nuclear theory, experimental constraints on bulk nuclear properties, and astrophysical data.”

A Stellar Laboratory

Dense, with a mass much larger than that of our sun, an average neutron star could fit in the space between the Tennessee Theatre and McGhee Tyson Airport. Yet these stellar bodies are actually good models for atoms. Steiner uses neutron star observations as a kind of laboratory to learn more about how neutrons and protons interact, especially in terms of the strong force (one of nature’s four basic forces) that holds them together. He was a bit surprised the citation mentioned “advanced statistical methods,” because he’s never actually had any courses in statistics.

“It is, in part, a testament to how much our field has changed over the past decade or so,” he said. “Statistical methods, computational tools, and machine learning continue to transform our ability to use data to elucidate our knowledge of the strong nuclear force.”

Steiner’s work has ties to important progress in nuclear astrophysics. The team that first detected gravitational waves produced by colliding neutron stars cited his group’s predictions about tidal deformability (what he calls “squishiness”) in these massive objects. He’s also director of the Nuclear Physics from Multi-Messenger Mergers (NP3M) research hub. The National Science Foundation (NSF) awarded UT $3.25 million to launch this national effort, bringing together scientists with expertise in nuclear physics and astrophysical simulations to study the properties of matter not available on Earth. Among the hub’s goals is training a diverse new generation of scientists.

“Professor Steiner works at the forefront of theoretical nuclear astrophysics where he uses cutting edge computational tools to probe the properties of hot and dense strongly interacting matter,” said Physics Professor and Department Head Adrian Del Maestro. “His dedication to building open source tools ensures that his research has a broad impact on the field as recognized by his peers through election as a Fellow of the APS. We are proud to add him to the list of 10 APS Fellows forging new knowledge and teaching and mentoring students in the Department of Physics and Astronomy at the University of Tennessee.”

Steiner earned a bachelor’s degree in physics at Carnegie Mellon University, followed by master’s and doctoral degrees in physics at the State University of New York at Stony Brook. He joined the UT Physics faculty in 2014. In 2016 he won an NSF Faculty Early Career Development Program (CAREER) Award, following that up in 2019 with a Senior Career Award for Research and Creative Achievement from the College of Arts and Sciences.

Steiner credits a long list of supporters for his career success: his wife, his daughter, his parents, and colleagues from UT Physics and around the country (including Madappa Prakash, his graduate advisor).

Each year, no more than one half of one percent of the society’s membership (excluding student members) is recognized by their peers for election to the status of Fellow. Steiner was recommended for Fellowship by his colleagues in the APS Division of Nuclear Physics. He will be acknowledged at the Division’s annual meeting later this month.