New Physics Searches with Highly Charged IonsONLINE ONLY

Highly charged ions (HCI) are promising candidates for novel optical clocks and tests of fundamental physics. Typically, the megakelvin-range temperatures needed to produce HCI hinder high-precision spectroscopy. To overcome this, we extract HCI from an electron beam ion trap and transfer them to a cryogenic linear Paul trap. There, single HCI are sympathetically cooled by laser-cooled Be⁺ ions down to millikelvin temperatures, thus enabling quantum logic state readout. We demonstrated in this way an optical clock based on Ar¹³⁺ and determined its absolute frequency with sub-Hz uncertainty against the Yb⁺ octupole ion clock at PTB. We recently applied the developed techniques to determine the isotope shift of a narrow magnetic-dipole transition in Ca¹⁴⁺ with 150 mHz accuracy for all five even and stable isotopes ^{40,42,44,46,48}Ca. We combine these results with recently improved measurements of isotope shifts in Ca⁺ conducted at ETH Zürich and more precise measurements of nuclear mass ratios performed at MPIK Heidelberg in a so-called King. To first order, the standard model predicts a linear King plot while nonlinearities can arise from higher-order effects or a new force that would couple electrons and neutrons. For the first time, we observe a ∼ 900σ nonlinearity in the King plot of Ca and identify two standard model effects as its potential sources: the second-order recoil shift and the nuclear polarization effect. Despite the observed nonlinearity, we derive improved constraints on a new force using the King plot method and argue that more precise calculations and additional isotope shift measurements could further tighten these constraints in the future.

References

A. Wilzewski, L. I. Huber, M. Door, A. Mariotti, J. Richter et al., PRL 134, 233002 (2025)

https://gsi-fair.zoom.us/j/63856816325
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