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

Koki Ono, Yugo Saito, Taiki Ishiyama, Toshiya Higomoto, Tetsushi Takano, Yosuke Takasu, Yasuhiro Yamamoto*, Minoru Tanaka, and Yoshiro Takahashi
Phys. Rev. X 12, 021033 (2022) [Featured in Physics]
DOI: https://doi.org/10.1103/PhysRevX.12.021033

Weakly interacting new force is an attractive candidate to explain various results of astrophysical observations and low energy experiments. Precision atomic spectroscopy with optical lattice is a powerful tool to find out its effect. While there is a large uncertainty in the theoretical calculation of atomic spectrum, the effect may appear as nonlinearity of the King plot among isotope dependences of distinct transitions. 
In Yb elements, it had been observed such a nonlinearity, which cannot be consistent with other experiments if it originates from a new force beyond the Standard Model.
In this work, new precision isotope shifts of Yb are measured by the quantum optics group in Kyoto University. With this data, the nonlinearity observed before is suppressed by the generalized King plot. It is, however, found that the precision Yb data include at least two source of nonlinearity, which cannot be attributed to the new force. While the known method does not work for the new force search in this situation, it is succeeded to obtain state of the art constraint to new force with the isotope shifts by a new analysis method.