Bounds to the coupling of bosons beyond the Standard Model to atoms through precise isotope shift measurements

By combining high-resolution spectroscopy of the 3d ²D_3/2 − 3d ²D_5/2 interval with an accuracy of ∼20 Hz using direct frequency-comb Raman spectroscopy with isotope shift measurements of the 4s ²S_1/2 ↔ 3d ²D_5/2 transition (~2 kHz accuracy) in all stable even isotopes of ^ACa⁺ (A = 40, 42, 44, 46, and 48), we have been able to carry out a King plot analysis with unprecedented sensitivity to coupling between electrons and neutrons by bosons beyond the Standard Model. We estimate, furthermore, that by improved spectroscopic techniques available, King plots based on data from spectroscopy on either Ca⁺, Ba⁺ and Yb⁺ ions should be able to produce sensitivity to such potentially new bosons, which surpass all other current methods in a broad mass range of 10 to 10⁸  eV/c².