Physical Review Letters ( IF 8.385 ) Pub Date : 2020-09-15 , DOI: 10.1103/physrevlett.125.123002
Ian Counts; Joonseok Hur; Diana P. L. Aude Craik; Honggi Jeon; Calvin Leung; Julian C. Berengut; Amy Geddes; Akio Kawasaki; Wonho Jhe; Vladan Vuletić

We measure isotope shifts for five ${\mathrm{Yb}}^{+}$ isotopes with zero nuclear spin on two narrow optical quadrupole transitions ${{}^{2}S}_{1/2}\to {{}^{2}D}_{3/2}$, ${{}^{2}S}_{1/2}\to {{}^{2}D}_{5/2}$ with an accuracy of $\sim 300\text{\hspace{0.17em}}\text{\hspace{0.17em}}\mathrm{Hz}$. The corresponding King plot shows a $3×{10}^{-7}$ deviation from linearity at the $3\sigma$ uncertainty level. Such a nonlinearity can indicate physics beyond the Standard Model (SM) in the form of a new bosonic force carrier, or arise from higher-order nuclear effects within the SM. We identify the quadratic field shift as a possible nuclear contributor to the nonlinearity at the observed scale, and show how the nonlinearity pattern can be used in future, more accurate measurements to separate a new-boson signal from nuclear effects.

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