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Precision measurements in simple atomic systems offer powerful probes for physics beyond the Standard Model. One promising approach is based on high-precision measurements of the bound-electron g factor in hydrogen-like ions. The exchange of a hypothetical scalar boson would produce a small additional contribution to the ground-state g factor. By calculating this effect and comparing it with experimental results, constraints on new scalar interactions can be derived. To enhance sensitivity, we employ nuclide shifts—differences between isotopes with different proton or neutron numbers—which help isolate potential new-physics contributions [1]. Combining existing measurements for several ions with current theoretical precision allows constraints on the electron–proton coupling strength that largely improve present limits from atomic data.
A complementary method investigates possible spin-dependent forces mediated by axion-like particles or other hypothetical pseudoscalar and vector bosons. Such particles could slightly modify the hyperfine splitting in ions through interactions with electrons and nucleons [2]. Hyperfine splittings in hydrogen- and lithium-like charge states are particularly sensitive to this effect. By forming normalized differences between these splittings, uncertainties related to nuclear structure can be strongly suppressed, enabling more precise tests. Existing measurements in Be-9 already provide competitive bounds for boson masses above 100~keV, confirming or improving present constraints on pseudoscalar couplings by up to a factor of two depending on the nuclear model. Future measurements in Cs-133 ions could further enhance the discovery reach by factors of about 2–2.5 for pseudoscalar interactions and by an order of magnitude for new vector bosons.
[1] M. Moretti, C. H. Keitel, and Z. Harman, Phys. Rev. Lett. 136, 011803 (2026)
[2] C. Quint, F. Heiße, J. Jaeckel, L. Leimenstoll, C. H. Keitel, and Z. Harman, Phys. Rev. Lett., in print; arXiv:2506.03274