Observation of non-linearity of generalized King plot in the search for new boson

TL;DR

This study measures isotope shifts in ytterbium with high precision, revealing nonlinearities in King plots that suggest potential new physics beyond the Standard Model, and sets bounds on hypothetical new bosons.

Contribution

It demonstrates the first observation of non-linearity in generalized King plots in Yb isotopes, indicating higher-order effects and constraining new boson couplings.

Findings

Observed large nonlinearity in King plots with $ ext{chi}^2$ on the order of 10^4.

Detected deviation from linearity at 3$\sigma$ level in generalized King plot.

Set upper bounds on couplings of a hypothetical new boson mediating forces.

Abstract

We measure isotope shifts for neutral Yb isotopes on an ultranarrow optical clock transition $^{1}\text{S}_{0}-^{3}\text{P}_{0}$ with an accuracy of a few Hz. The part-per-billion precise measurement was possible by loading the ultracold atoms into a three-dimensional magic-wavelength optical lattice and alternately interrogating the isotope pairs, thus minimizing the effects due to the optical lattice light-shift and inter-atomic interaction as well as the drifts of a clock laser frequency and a magnetic field. The determined isotope shifts, combined with one of the recently reported isotope-shift measurements of Yb$^+$ on two optical transitions, allow us to construct the King plots. Extremely large nonlinearity with the corresponding $\chi^2$ on the order of $10^4$ is revealed, and is not explained by a quadratic field shift. We further carry out the generalized King plot for three optical transitions so that we can eliminate the contribution arising from a higher-order effect within the Standard Model which might explain the observed nonlinearity of King plots for two transitions. Our analysis of the generalized King plot shows a deviation from linearity at the 3$\sigma$ level, indicating that there exist at least two higher order contributions in the measured isotope shifts. Then, under the reasonable assumption to attribute them to higher-order field shifts within the Standard Model, we obtain the upper bound of the product of the couplings for a new boson mediating a force between electrons, and neutrons $|y_ey_n|/(\hbar c)< 1\times10^{-10}$ for the mass less than 1 keV with the 95% confidence level is derived, providing an important step towards probing new physics via isotope-shift spectroscopy.