Observation of Nonlinearity of Generalized King Plot in the Search for New Boson
PHYSICAL REVIEW X(2022)
摘要
We measure isotope shifts for neutral Yb isotopes on an ultranarrow optical clock transition S-1(0) - P-3(0) with an accuracy of a few hertz. Combined with one of the recently reported isotope-shift measurements of Yb+ on two optical transitions, the result allows us to construct the King plots-a set of scaled isotope shifts data on two different optical transitions plotted in two-dimensional plane. When only the leadingorder terms of isotope shifts are taken into account, a King plot should exhibit a linear relation as a result of elimination of the leading nuclear-size dependence. Extremely large nonlinearity unexplainable by a quadratic field shift is revealed, which was proposed previously as a source of the observed nonlinearity of the King plot. We further construct the generalized King plot with three optical transitions so that we can eliminate the contribution arising from a higher-order effect within the standard model. 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. Under reasonable assumptions, we obtain the upper bound of the product of the couplings for a new boson, mediating a force between electrons and neutrons-vertical bar YeYn vertical bar/((h) over barc) < 1 x 10(-10) for the mass less than 1 keV-with the 95% confidence level, providing an important step toward probing new physics via isotope-shift spectroscopy.
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