Physical Review Letters
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Spectroscopy of $^{4}\mathrm{He}$ at 0.25 ppt Uncertainty and Improved Alpha-Helion Charge-Radius Difference Determination
Author(s): K. Steinebach, J. C. J. Koelemeij, H. L. Bethlem, and K. S. E. Eikema
Improved measurements of an electronic transition in helium-4 atoms constrain the size difference between helium-4 and helium-3 nuclei. [Phys. Rev. Lett. 136, 243001] Published Thu Jun 18, 2026
