Spectroscopy of ${}^4$He at 0.25 ppt Uncertainty and Improved Alpha-Helion Charge-Radius Difference Determination

K. Steinebach, J. C. J. Koelemeij, H. L. Bethlem +1

High-precision spectroscopy of simple atomic systems can be used to advance the theory of atomic energy levels but can also serve as a sensitive probe of nuclear charge radii. For…