Evidence of two-source King nonlinearity in spectroscopic fifth-force search in Yb+​

Isotope-shift spectroscopy has recently been put forward as a table-top method for searching for a hypothetical boson that mediates an interaction between the neutron and the electron. The Yukawa potential generated by this boson would lead to neutron-number-dependent shifts in atomic transition frequencies. When measured on at least two transitions, these shifts can be displayed in a “King plot”, which will exhibit nonlinearities in the presence of effects beyond the expected first-order standard model shifts.

We have measured isotope shifts on three narrow optical transitions, on five spinless isotopes of Yb+. Our latest data, on the highly forbidden 467nm octupole transition between 2S1/2 and 2F7/2, when combined with our previous measurements of quadrupole transitions in Yb+ and recent measurements in neutral Yb, confirm the presence of a King nonlinearity with up 240σ confidence. The data also reveal, with 4.3σ sigma confidence, that this nonlinearity emerges from at least two distinct physical effects.

We identify the main source of nonlinearity as differences in the 4th nuclear charge moment between isotopes, a higher-order nuclear effect that had not previously been probed with high precision. We find that the second source of nonlinearity likely cannot be explained by the expected next largest effect within the standard model, the quadratic field shift. We discuss possible sources for this second nonlinearity and outline how ongoing and future work can elucidate whether it emerges from a new boson.