Nonlinear calcium King plot constrains new bosons and nuclear properties

Abstract

Nonlinearities in King plots (KP) of isotope shifts (IS) can reveal the existence of beyond-Standard-Model (BSM) interactions that couple electrons and neutrons. However, it is crucial to distinguish higher-order Standard Model (SM) effects from BSM physics. We measure the IS of the transitions ${}^3{P}_0\to {}^3{P}_1$ in ${\mathrm{Ca}}^{14+}$ and ${}^2{S}_{1/2}\to {}^2{D}_{5/2}$ in ${\mathrm{Ca}}^{+}$ with sub-Hz precision as well as the nuclear mass ratios with relative uncertainties below $4\times 10^{-11}$ for the five stable, even isotopes of calcium (${}^{40,42,44,46,48}\mathrm{Ca}$). Combined, these measurements yield a calcium KP nonlinearity with a significance of $\sim 900\ddot{I}ƒ$. Precision calculations show that the nonlinearity cannot be fully accounted for by the expected largest higher-order SM effect, the second-order mass shift, and identify the little-studied nuclear polarization as the only remaining SM contribution that may be large enough to explain it. Despite the observed nonlinearity, we improve existing KP-based constraints on a hypothetical Yukawa interaction for most of the new boson masses between $10eV/{\mathrm{c}}^2$ and $10^{7}eV/{\mathrm{c}}^2$.