Analysis of precision measurements of unexplored interactions between electrons and atomic nuclei give information of new particles

Dark matter particles could be mediators of the interaction between electrons and atomic nuclei, as shown by a study conducted by junior group leader, Dr. Konstantin Gaul, Dr. Lei Cong, and Professor Dr. Dmitry Budker, of Johannes Gutenberg University Mainz (JGU), Helmoltz Institute Mainz (HIM) and the PRISMA⁺⁺ Cluster of Excellence. Their work, published last week in the renowned journal Physical Review Letters, presents new constraints on previously unexplored candidates for dark matter and more generally, some hypothetical particles that are not included in the Standard Model of particle physics (SM).

Using results from precision measurements on barium monofluoride (BaF) molecules, the team constrained these interactions mediated by Z’ bosons for the first time. Z’ bosons are hypothetical mediators of the weak interaction and possible dark matter particles in several SM extensions. “These results address a significant blind spot in physics: a regime of forces between electrons and nuclei that had remained unexplored by both laboratory experiments and cosmological data,” explained Gaul. Our universe is made up of about four percent of visible, or ordinary, matter. This includes planets, stars, and life on Earth. The remaining 96 percent of the universe are invisible and consist of dark matter and dark energy, with dark matter making up about 23 percent. Astrophysical observations confirm its presence throughout the cosmos, where it, for example, plays an important part on the structure of galaxies. However, we don’t know what particles make up dark matter. Many theories and ongoing experiments are looking for an answer to this open question.

An interdisciplinary approach to a fundamental question in particle physics

To determine the contribution of Z’ bosons to the interaction between electrons and nuclei, which gives rise to the so-called hyperfine structure of atoms, the authors used the supercomputer MOGON 2 at JGU to reinterpret existing results of precision measurements in BaF molecules. This study did not only require good knowledge of the weak interaction and the properties of these beyond SM bosons, but also a solid foundation of atomic, molecular and nuclear physics, making this a truly interdisciplinary project. “Konstantin Gaul and Lei Cong are new-generation theorists working at the interface of atomic, molecular, and optical physics, particle, and nuclear physics,” said Budker. “Having them embedded in a mostly experimental group within HIM and PRISMA⁺⁺ has led to highly productive collaborations and very interesting and important results, of which this work is just one example.”

In the search for “new physics” such an approach might be able to shed light on long-standing questions. As Gaul explained: “Because the dense internal environment of polar molecules naturally amplifies subtle physical effects, they act as powerful laboratories for detecting new forces that are otherwise invisible to science.”

The study also found similar bounds by analyzing the parity-violation experiment with the atom cesium 133, which is a more traditional method of studying the interactions between electrons and atomic nuclei. However, in contrast to studies of experiments with atoms, the analysis of diatomic molecules, such as BaF, currently does not depend on nuclear theory. This means that, since they are not affected by uncertainties related to nuclear physics, the results can be more precise. “The current study proves that measurements of molecular physics are an emerging tool for new physics, rivaling traditional atomic methods. Our findings demonstrate that future experiments with heavy diatomic species like RaF will boost sensitivity by 100-fold, pushing deeper into unexplored territory to hunt for the hidden forces of the universe,” concluded Gaul.