When leading research teams from different disciplines come together, groundbreaking discoveries often follow. This was precisely the case when quantum physicists from the Physikalisch-Technische Bundesanstalt (PTB) and the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg collaborated to combine atomic and nuclear physics with unprecedented accuracy. Their work, which blends two distinct methods of measurement, has led to important findings about the fundamental nature of atomic nuclei, while simultaneously setting new limits on the possible existence of a “dark force” that could link neutrons and electrons.
Published in Physical Review Letters, their findings have the potential to significantly reshape how scientists think about both atomic structure and the possibility of dark matter interactions. The collaborative effort also includes the contributions of theoretical physicists from the Technical University of Darmstadt and Leibniz University Hannover, who, through new nuclear theory calculations, were able to connect the dots between measurements of electron shells and the deformation of atomic nuclei, offering new insights into previously unexplored areas of physics.
The Search for Dark Forces
For nearly a century, physicists have known that a significant portion of the universe’s mass is made up of dark matter—matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects on visible matter. Despite its strong presence, dark matter remains elusive, and the true nature of its interactions with ordinary matter is still a mystery. However, scientists have long speculated that dark matter may not only interact through gravity but also through other forces, often referred to as dark forces.
The concept of dark forces proposes that there could be forces capable of interacting with both dark and visible matter, but their effects are too subtle to be detected through conventional means. If these forces exist, they might leave subtle signatures in atomic systems—signatures that, with the right tools, could provide key insights into their nature. Researchers have been exploring whether such forces might manifest as shifts in the electron shells of atoms, and the team’s recent work provides important progress in this area.
Measuring the Electron Shells and the Structure of Atomic Nuclei
One of the most powerful methods for detecting potential dark forces is by measuring the shift in electron resonances in isotopes—atoms of the same element that differ only in the number of neutrons in the nucleus. These isotope shifts offer an invaluable window into the interaction between the nuclear structure and the electron shell of an atom. By analyzing how the electron energy levels shift in response to changes in the nuclear structure, scientists can detect subtle influences such as dark forces.
Tanja Mehlstäubler, a lead researcher at PTB, explains the significance of this approach: “Measuring the shift in electronic resonances in isotopes is a particularly powerful method for shedding light on the interaction between nuclear and electron structure.” This technique allows scientists to look beyond the behavior of electrons themselves and probe deeper into the atomic nucleus, unveiling details about how the nucleus may be deformed or influenced by unseen forces.
In particular, the team focused on the element ytterbium, which has several isotopes that offer ideal conditions for these measurements. Ytterbium is already well-known in the atomic physics community for its precise atomic transitions and its potential to help uncover the workings of fundamental forces.
The 2020 MIT Discovery: A Puzzle Unveiled
In 2020, a team of researchers at the Massachusetts Institute of Technology (MIT) observed an unexpected result when examining isotope shifts in ytterbium. They noticed a nonlinearity—a deviation from the expected outcome—that sent ripples through the scientific community. Could this anomaly be the first evidence of a dark force, or was it simply a property of the atomic nucleus that had yet to be fully understood?
The deviation sparked a lively debate in the field of atomic physics: Was this a glimpse of something entirely new, or was it a challenge to the understanding of the atomic nucleus itself? Could atomic physicists have stumbled into nuclear physics territory while comparing the transition frequencies of electrons in different isotopes?
Fueled by this uncertainty, Tanja Mehlstäubler from PTB and Klaus Blaum from MPIK assembled research teams to further investigate the anomaly. Their goal was to confirm the findings and explain what was really happening.
High-Precision Measurements of Isotope Shifts
To delve deeper into the mystery of the ytterbium anomaly, Mehlstäubler, Blaum, and their colleagues conducted high-precision measurements using state-of-the-art tools. At PTB, they used linear high-frequency ion traps and ultra-stable laser systems to perform optical spectroscopy—a technique that allows for the measurement of atomic transition frequencies with exceptional precision.
Meanwhile, at MPIK, the team used the PENTATRAP Penning trap mass spectrometer to determine the isotope mass ratios with incredible accuracy. These measurements were up to a hundred times more precise than previous attempts, allowing the researchers to detect even the faintest shifts and deviations.
The results confirmed the anomaly observed by the MIT team, furthering the mystery of what caused the nonlinearity. But the researchers didn’t stop there—they sought an explanation that would reconcile this finding with known principles of physics.
Nuclear Theory Calculations and the Role of Deformation
To better understand the anomaly, the team turned to theoretical physicists for help. Achim Schwenk’s group at TU Darmstadt played a pivotal role in developing new nuclear theory calculations. These calculations helped the team uncover an important connection between the observed isotope shifts and the deformation of the atomic nucleus. The findings suggested that the deformation of the ytterbium nucleus could be influencing the electron shell in ways not previously understood.
In collaboration with theoretical atomic physicists from MPIK, Leibniz University Hannover, and the University of New South Wales, the team refined the nuclear theory to account for the anomaly. Their work led to the establishment of a new limit on the strength of any potential dark forces that could be acting between neutrons and electrons. By extending existing models of atomic structure, the team was able to place tighter constraints on the possibility that dark forces might be influencing atomic and subatomic systems.
Advancing Our Understanding of Neutron-Rich Matter
One of the most exciting outcomes of this research is its potential to advance our understanding of neutron-rich matter, which is key to understanding the behavior of matter under extreme conditions, such as those found in neutron stars. The research team used their newfound insights into the deformation of the ytterbium nucleus to gain a deeper understanding of the structure of heavy atomic nuclei, particularly those rich in neutrons.
This is crucial because the study of neutron stars—extremely dense remnants of stars that have exploded in supernovae—relies heavily on understanding the properties of neutron-rich matter. The results of this study could pave the way for future research that helps us understand how matter behaves under these extreme conditions, offering insights into the physics of the universe’s most mysterious objects.
The Path Forward: A Collaborative Effort
The successful collaboration between atomic, nuclear, and particle physicists in this research highlights the importance of interdisciplinary cooperation in tackling some of the most fundamental questions in science. The ability to combine different methods of measurement and theoretical models allowed the team to push the boundaries of precision and make critical new discoveries.
Looking ahead, the research paves the way for further exploration into the nature of dark matter and dark forces, as well as the deeper properties of atomic nuclei. By continuing to refine measurement techniques and developing new theoretical models, physicists are now better equipped to explore the complex phenomena that shape the structure of matter and the universe.
This work is not just a breakthrough in understanding the fundamental forces of nature—it is also an important step toward unlocking the mysteries of the universe itself. As atomic, nuclear, and particle physics continue to advance, scientists are likely to uncover even more profound insights into the building blocks of reality.
Reference: Menno Door et al, Probing New Bosons and Nuclear Structure with Ytterbium Isotope Shifts, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.063002