Fifth-force search with isotope-shift spectroscopy in Yb⁺

We present our latest results in a spectroscopic search for new physics by measuring isotope shifts in ytterbium ions. Isotope shifts, when measured on at least two atomic transitions can be displayed in a “King plot”. The presence of nonlinearities in such a plot indicates the existence of effects beyond the expected first-order standard model (SM) contributions to the isotope shifts. We have measured isotope shifts on a highly forbidden 467nm octupole transition in five spinless isotopes of Yb⁺ and, by combining our data with our previous measurements on quadrupole transitions of the same ion and with recent measurements by other groups on a further two transitions in neutral Yb, we find a King nonlinearity with up to 240σ confidence. Furthermore, we determine, with 4.3σ confidence, that this nonlinearity originates from two distinct physical effects. We identify the main source of nonlinearity as originating from as differences in the 4^th nuclear charge moment between isotopes, a higher-order nuclear effect that had not previously been measured with high precision. We discuss possible sources of the second nonlinearity and find that it likely cannot be explained by the expected next largest SM contribution.