Evidence for Nonlinear Isotope Shift in Yb+ Search for New Boson

We measure isotope shifts for five Yb+ isotopes with zero nuclear spin on two narrow optical quadrupole transitions S-2(1/2) -> D-2(3/2), S-2(1/2) -> E-2(4/2) with an accuracy of similar to 300 Hz. The corresponding King plot shows a 3 x 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.