Isotope shift spectroscopy in neutral strontium

We present isotope shift measurements between ⁸⁸Sr, ⁸⁷Sr, ⁸⁶Sr, and ⁸⁴Sr, on the ¹S₀ -> ³P₀ clock transition and the ¹S₀ -> ³P₁ intercombination line. The two main contributions to isotope shifts come from the change in mass (the mass shift) and the change in nuclear charge radius (the field shift). Comparing bosonic measurements in a King plot analysis allow us to extract the difference in mass shift constants and the ratio of field shift constants between the two transitions without knowledge of the nuclear charge radius. Previous results have found discrepancies between measured and theoretical values for the field shift constants from a similar analysis in Ca⁺ [Shi, C., Gebert, F., Gorges, C. et al. Appl. Phys. B (2017) 123: 2], which this work could help resolve. In addition, recent proposals have suggested looking for nonlinearities in King plots as a signature of new physics [J.C. Berengut et al., Phys. Rev. Lett. 120, 091801 (2018)]. Along with proper incorporation of the hyperfine structure of the fermionic isotope, or with new measurements in one of the unstable isotopes of Sr, this work could help provide limits on these theories.