The helion charge radius from laser spectroscopy of muonic helium-3 ions.

Schuhmann, Karsten; Fernandes, Luis M P; Nez, François; Abdou Ahmed, Marwan; Amaro, Fernando D; Amaro, Pedro; Biraben, François; Chen, Tzu-Ling et al. · Science · 2025

basic_science · Level V

Where this comes from

Abstract

Hydrogen-like light muonic ions, in which one negative muon replaces all of the electrons, are extremely sensitive probes of nuclear structure. Using pulsed laser spectroscopy, we have measured three 2<i>S</i>-2<i>P</i> transitions in the muonic helium-3 (μ<sup>3</sup>He<sup>+</sup>) ion, an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift, the 2<i>P</i> fine structure splitting, and the 2<i>S</i>-hyperfine splitting in μ<sup>3</sup>He<sup>+</sup>. Comparing these measurements with theory, we determined the root-mean-square charge radius of the helion (<sup>3</sup>He nucleus) to be <i>r<sub>h</sub></i> = 1.97007(94) fm, in good agreement with the value from elastic electron scattering but a factor 15 more accurate. Our results represent benchmarks for few-nucleon theories and open the way for precision quantum electrodynamics tests in He atoms and ions.