Evidence for nonlinear isotope shift in Yb+ search for new boson

Abstract

We measure isotope shifts for five Yb+ isotopes with zero nuclear spin on two narrow optical quadrupole transitions S21/2→D23/2, S21/2→D25/2 with an accuracy of ∼300 Hz. The corresponding King plot shows a 3×10-7 deviation from linearity at the 3σ 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.