Observation of Nonlinearity of Generalized King Plot in the Search for New Boson

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