Sensitivity to New Physics of Isotope Shift Studies using the Coronal Lines of Highly Charged Calcium Ions

TL;DR

This paper explores isotope-shift spectroscopy in highly charged calcium ions to enhance the sensitivity of detecting hypothetical fifth forces beyond the Standard Model, with implications for dark matter research.

Contribution

It identifies suitable transitions in highly charged calcium ions for high-precision spectroscopy and extends the King plot method to improve searches for new physics.

Findings

Six transitions in highly charged Ca ions are experimentally and theoretically studied.

The results enable application of the generalized King plot method with Ca ions.

Future measurements can improve sensitivity to unknown forces and particles.

Abstract

Promising searches for new physics beyond the current Standard Model (SM) of particle physics are feasible through isotope-shift spectroscopy, which is sensitive to a hypothetical fifth force between the neutrons of the nucleus and the electrons of the shell. Such an interaction would be mediated by a new particle which could in principle be associated with dark matter. In so-called King plots, the mass-scaled frequency shifts of two optical transitions are plotted against each other for a series of isotopes. Subtle deviations from the expected linearity could reveal such a fifth force. Here, we study experimentally and theoretically six transitions in highly charged ions of Ca, an element with five stable isotopes of zero nuclear spin. Some of the transitions are suitable for upcoming high-precision coherent laser spectroscopy and optical clocks. Our results provide a sufficient number of clock transitions for -- in combination with those of singly charged Ca$^+$ -- application of the generalized King plot method. This will allow future high-precision measurements to remove higher-order SM-related nonlinearities and open a new door to yet more sensitive searches for unknown forces and particles.