From atomic nuclei to astrophysics, collaborative program builds basis for scientific discoveries

Oak Ridge National Laboratory is leading two nuclear physics research projects within the Scientific Discovery through Advanced Computing, or SciDAC, program from the Department of Energy Office of Science. One of the projects is called Nuclear Computational Low-Energy Initiative, or NUCLEI. The other is Exascale Nuclear Astrophysics for FRIB, or ENAF. FRIB refers to the Facility for Rare Isotope Beams, a DOE-supported research, teaching and training center located at Michigan State University.

The NUCLEI project involves computing properties and reactions of atomic nuclei that are important for particle accelerator-based experiments on Earth and for thermonuclear reactions occurring in stars. NUCLEI is composed of 35 principal and co-principal investigators, or PIs, from 11 institutions.

"It's a broad perspective," said NUCLEI PI and ORNL nuclear physicist Thomas Papenbrock. "To advance science and provide our experimental facilities with the necessary theory, we develop and improve models of the forces between two and three nucleons."

The researchers perform increasingly precise calculations of experimentally observable quantities that probe the fundamental symmetries of nuclei, and neutrino and electron interactions with nuclei and dense nuclear matter found in neutron stars. Additionally, they conduct quantitative studies of nuclear fission and fusion.

The observables the NUCLEI project team computes are relevant to DOE facilities. Accordingly, they interpret and guide experiments at FRIB; the Argonne Tandem Linac Accelerator System, or ATLAS, at Argonne National Laboratory; and the Thomas Jefferson National Accelerator Facility, or Jefferson Lab. FRIB and ATLAS probe the structures and reactions of rare nuclei. Researchers at Jefferson Lab are investigating nucleon substructure and atomic nuclei. The NUCLEI project also involves computing the nuclear physics input to neutrinoless double beta decay.

Neutrinoless double beta decay is a radioactive transformation in which the type of nucleus of an atom is changed and energy is released, without emission of neutrinos. This not-yet-observed radioactive decay is speculated to exist, and the search for this process is key to the efforts of ORNL's Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay, or LEGEND, collaboration. LEGEND aims to detect this rare decay, which would provide evidence of phenomena beyond the Standard Model of particle physics, the theoretical framework that describes the fundamental particles and their interactions that make up the universe.

The NUCLEI collaborating institutions are ORNL; Argonne, Lawrence Berkeley, Lawrence Livermore and Los Alamos national laboratories; Iowa State University; Massachusetts Institute of Technology; Michigan State University; Ohio State University; the University of North Carolina at Chapel Hill; the University of Notre Dame; the University of Oregon; and Washington University in St. Louis.

Finding where we came from

The ENAF project studies merging neutron stars and their role in the origins of the heaviest elements, such as silver, gold and uranium. Toward this goal, the collaboration, composed of seven PIs from six institutions, is developing some of the most advanced simulation codes to study the lives and deaths of stars and how neutrinos change flavors, or types.

Ghostlike, tiny and almost weightless, neutrinos zoom around the universe at incredibly high speeds. Although they are one of the building blocks of matter, they are exceptionally elusive because they scarcely interact with anything. Understanding how changing neutrino flavors, or neutrino flavor oscillations, affect astrophysical events is one of ENAF's key goals because it constrains the masses of neutrinos and the way they interact with matter. This information will help unravel some of the mysteries of the universe's composition, the behavior of these fundamental particles, and how stars produce energy.

The importance of ENAF's work to humanity is in what it tells us about the chain of events from the origin of the universe all the way to our beginning and how the universe will evolve.

"This nucleosynthesis, the formation of the elements, is one of the links in that chain, so comprehending all of those things is required if we really want to know where we came from," said ORNL nuclear astrophysicist Raph Hix, who heads the ENAF project. "Like studying planetary science and how planets form, how stars form, how galaxies form—all of these things contribute to our understanding of how we came to be."

The ENAF research team uses computer simulations that contain the most complex physics, which requires tapping into the world's fastest supercomputers. In a collaborative workflow of experimentation and simulation, ENAF and FRIB have a reciprocal relationship. FRIB applies its next-generation technology to conducting experiments that reveal properties of nuclei that it conveys as input to ENAF's simulations. In turn, the simulations illuminate the thermodynamic conditions within the experiments, helping FRIB scientists choose their next measurements.

The ENAF collaborating institutions are ORNL, Argonne National Laboratory, North Carolina State University, Penn State University, the University of Notre Dame, and the University of California, Berkeley.

Bridges for conquering the complex

Supercomputing has been integral to nuclear physics research for several decades and has become essential to addressing scientific topics of national interest, including clean energy, new materials, climate change, proteins and the COVID-19 virus, the origins of the universe, and the nature of matter. Both Hix and Papenbrock said that the continually growing complexity of computers and the associated scientific software has made the creation of multidisciplinary, collaborative teams with computational expertise crucial to advancing nuclear physics and other kinds of scientific research.

SciDAC connects domain scientists, computer scientists and applied mathematicians to develop the scientific computing software and hardware infrastructure needed to advance scientific discovery using supercomputers.

"The program is a real blessing," Hix said. "All computational science is, by its very nature, cross-disciplinary, because if you want to use these computers well, you need computing experts. We have relied heavily on the computational science and applied mathematics experts that SciDAC has made available through this project."

Papenbrock referred to SciDAC as a game-changer for nuclear physics research.

"Fifteen years ago, the types of calculations that we do now were at the fringe; very few people did them," he said. "They were, and still are, very costly and could be accomplished only for very few nuclei. Now, we can perform such calculations for literally hundreds of nuclei. Because of SciDAC, many groups are performing the calculations, and we can run so many simulations that we can advance nuclear models and quantify uncertainties. It's just a dream come true."

Nuclear physicists can make predictions with quantified uncertainties related to the properties of nuclear reactions, decay processes, and nuclear structure to anticipate the behavior of atomic nuclei and their interactions. The scientists assess and express the potential errors or variations in their predictions and endeavor to refine the precision and accuracy of their calculations.

The breadth of SciDAC

DOE initiated the SciDAC program in 2001. It partners all six Office of Science programs—Advanced Scientific Computing Research, Basic Energy Sciences, Biological and Environmental Research, Fusion Energy Sciences, High Energy Physics and Nuclear Physics—as well as the Office of Nuclear Energy to dramatically accelerate progress in scientific computing that delivers breakthrough scientific results. The awards are recompeted every five years, and SciDAC is in its fifth cycle.

The low-energy nuclear physics community, the people who compute atomic nuclei, have been involved with SciDAC since 2007, when the program was in its second cycle. Papenbrock has been a collaborator since then and has witnessed the evolution of staff composition and computational complexity.

"Although we've used the name "NUCLEI' the last three cycles of the collaboration, one-third of the people are new," Papenbrock said. "As computers have become incredibly more powerful over the years, new challenges and opportunities have arisen, and the program has allowed people to confront the challenges."

The nuclear astrophysics community has been involved with SciDAC even longer, since the first cycle of SciDAC. "I've grown up, professionally, within SciDAC, from a postdoctoral researcher under the first round of SciDAC collaborations to the PI for this fifth cycle," Hix said. "The advances the nuclear astrophysics community have made in that time have been truly exciting but would have simply been impossible without SciDAC and our computational partners."

The ENAF project received its SciDAC award in June 2023. Funding began in July, joining NUCLEI and two other projects supported by the program's Partnership in Nuclear Physics for which funding started in September 2022.

The other two projects are Femtoscale Imaging of Nuclei Using Exascale Platforms, led by Ian Cloet of Argonne National Laboratory, and Fundamental Nuclear Physics at the Exascale and Beyond, led by Robert Edwards of Jefferson Lab.

ORNL theoretical physicist Balint Joo, who specializes in simulations of lattice quantum chromodynamics, is a participant in the Fundamental Nuclear Physics at the Exascale and Beyond project.

UT-Battelle manages ORNL for DOE's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit http://energy.gov/science/.

Provided by Oak Ridge National Laboratory