Nuclear-structure origin of King-plot nonlinearity in Yb and Ca
In plain words
Comparing the frequency shifts between isotopes of the same element for two transitions should give a straight line, called a King plot, unless a new force between electrons and neutrons bends it. In ytterbium and calcium the line is bent, and the cause appears to be nuclear structure, but that has to be predicted quantitatively before the method can find new forces.
Precise statement
$\mathrm{Yb}+$ and neutral Yb isotope shifts (Door et al 2025; Kyoto 2026) and $\mathrm{Ca}+$ isotope shifts (2024) show King-plot nonlinearity at many sigma. Compute the second-order field shift and nuclear-deformation contributions (changes in $r^4$ moments) to the Hz level, so that any residual nonlinearity bounds a Yukawa coupling $y_{e} y_{n}$ of a boson of mass $1\ \mathrm{keV}\ \text{to}\ 10\ \mathrm{MeV}$.
What would settle it
Ab initio nuclear and atomic calculations that reproduce the measured nonlinearity patterns across several transitions within the experimental uncertainties.
Status in the literature
Unverified note
A January 2026 measurement of the Yb $1S0-3P2$ isotope shift excludes a light-boson interpretation of the dominant Yb nonlinearity; the nuclear-structure part is not yet computed to the needed precision.