For almost a century, measurements have indicated that a significant proportion of matter in the universe consists of unknown dark matter, which interacts with visible matter through gravitation. It is unclear whether there are also new, so-called "dark forces" that can "communicate" with the visible and the dark matter.
Such forces should also act on atoms, which can be analyzed with high precision nowadays. "Measuring the shift in electronic resonances in isotopes is a particularly powerful method for shedding light on the interaction between nuclear and electron structure", Tanja Mehlstäubler explains. Isotopes are different variants of an atomic element that differ only in the number of neutrons in the nucleus.
In 2020, a team at the Massachusetts Institute of Technology (MIT) discovered a deviation from the expected result – a nonlinearity – while examining such isotope shifts in the element ytterbium. This sent the world of atomic physicists into a spin: Could this anomaly be the first evidence of a new "dark force" or is it due to properties of the atomic nucleus? Did atomic physicists enter nuclear physics through the back door by comparing the transition frequencies of electrons measured in different isotopes?
Driven by this question, Tanja Mehlstäubler from PTB in Braunschweig and Klaus Blaum from MPIK in Heidelberg set out to investigate the ytterbium isotope shifts. Their research teams carried out high-precision measurements of atomic transition frequencies and isotope mass ratios of ytterbium isotopes. Linear high-frequency ion traps and ultra-stable laser systems were used for optical spectroscopy at PTB. At MPIK, the isotope mass ratios were determined in the PENTATRAP Penning trap mass spectrometer. Both measurements were up to a hundred times more accurate than previous measurements of this kind.
The researchers confirmed the anomaly, and the team was able to provide an explanation with the help of new nuclear theory calculations by Achim Schwenk's group at TU Darmstadt. In collaboration with theoretical atomic physicists from MPIK in Heidelberg and the University of New South Wales in Sydney as well as particle physicists from Leibniz University Hannover, they were able to establish a new limit for the existence of dark forces.
The international collaboration team was even able to use this data to obtain direct information about the deformation of the atomic nucleus along the ytterbium isotope chain. This can provide new insights into the structure of heavy atomic nuclei and into the physics of neutron-rich matter, which forms the basis for understanding neutron stars.
This research opens up new opportunities for atomic, nuclear and particle physics to collaborate in the search for new physics and to obtain a better understanding of the complex phenomena that determine the structure of matter.
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