Joint effort in the search for new physics

Despite being extraordinarily successful in describing elementary particles as well as the fundamental forces acting between them, the standard model of particle physics is incomplete: it does not explain the matter-antimatter asymmetry in the universe, nor neutrino oscillations and apparent evidence for dark matter. Testing the limits of the standard model and exploring possible alternatives and extensions to it is performed in different fields if current research including high-energy experiments at accelerators, the quest for rare processes, cosmic observations and high-precision low-energy experiments. Among the latter, precision isotope shift spectroscopy has developed into one of the most powerful methods particularly in the search for a hypothetical new force between electrons and neutrons.

Isotopes are variants of an atom of a specific chemical element that differ only in the number of neutrons in the nucleus. The corresponding mass difference of the nuclei leads to a tiny shift in the frequencies of electronic transitions. A classical tool for the analysis of these isotope shifts is the so-called King plot: taking into account well-known effects of nuclear mass and finite nuclear size it obtains a linear relationship for isotopic shifts of different transitions plotted against each other. Due to the enormous progress in the accuracy of atomic spectroscopy, even tiny nonlinearities in a King plot may indicate new interactions beyond the standard model description.

In a recent study on isotope shifts in calcium ions, physicists from the Physikalisch-Technische Bundesanstalt (PTB), the ETH Zurich, the University of New South Wales (UNSW) and MPIK joint their effort and specific expertise in experiment and theory in the search for new physics. The isotope shifts in highly charged Ca¹⁴⁺ ions were measured at PTB using an apparatus developed within a collaboration of MPIK (Pfeifer division) and PTP. The measurements for singly charged Ca⁺ ions were performed at ETH using a linear Paul trap. A further essential ingredient for the analysis is the isotope mass ratios which were determined at MPIK by high-precision mass spectroscopy (Blaum division) in combination with atomic structure calculations (Keitel division).

Combining all results for five stable calcium isotopes, the researchers revealed a highly significant nonlinearity in the calcium King plot. This can be explained only partly by higher-order contributions to the isotope mass shift. Another effect is the little-studied nuclear polarisation which could explain the remaining discrepancy. Including the current theoretical uncertainties within the standard model stricter limits for the properties of a hypothetical new type of interaction can be set. This could be further improved by including isotope shifts for a third transition which is currently studied at ETH.

Original publication

Nonlinear calcium King plot constrains new bosons and nuclear properties
Alexander Wilzewski, Lukas J. Spieß, Malte Wehrheim, Shuying Chen, Steven A. King, Peter Micke, Melina Filzinger, Martin R. Steinel, Nils Huntemann, Erik Benkler, Piet O. Schmidt, Luca I. Huber, Jeremy Flannery, Roland Matt, Martin Stadler, Robin Oswald, Fabian Schmid, Daniel Kienzler, Jonathan Home, Diana P.L. Aude Craik, Menno Door, Sergey Eliseev, Pavel Filianin, Jost Herkenhoff, Kathrin Kromer, Klaus Blaum, Vladimir A. Yerokhin, Igor A. Valuev, Natalia S. Oreshkina, Chunhai Lyu, Sreya Banerjee, Christoph H. Keitel, Zoltán Harman, Julian C. Berengut, Anna Viatkina , Jan Gilles , and Andrey Surzhykov, Michael K. Rosner and José R. Crespo López-Urrutia, Jan Richter, Agnese Mariotti and Elina Fuchs
Phys. Rev. Lett. 134, 233002, (2025), DOI: 10.1103/PhysRevLett.134.233002

Weblinks:

Group 'Dynamik hochgeladener Ionen' am MPIK

PENTATRAP Experiment at MPIK

Group 'Ionic Quantum Dynamics and High-Precision Theory' at MPIK

Group 'Exotic Quantum Systems ' at MPIK

Press release of the ETH Zurich