The helion charge radius from laser spectroscopy of muonic helium-3 ions
- Karsten Schuhmann
- Luis M. P. Fernandes
- François Nez
- Marwan Abdou Ahmed
- Fernando D. Amaro
- Pedro Amaro
- François Biraben
- Tzu-Ling Chen
- Daniel S. Covita
- Andreas J. Dax
- Marc Diepold
- Beatrice Franke
- Sandrine Galtier
- Andrea L. Gouvea
- Johannes Götzfried
- Thomas Graf
- Theodor W. Hänsch
- Malte Hildebrandt
- Paul Indelicato
- Lucile Julien
- Klaus Kirch
- Andreas Knecht
- Franz Kottmann
- Julian J. Krauth
- Yi-Wei Liu
- Jorge Machado
- Cristina M. B. Monteiro
- Françoise Mulhauser
- Boris Naar
- Tobias Nebel
- Joaquim M. F. dos Santos
- José Paulo Santos
- Csilla I. Szabo
- David Taqqu
- João F. C. A. Veloso
- Andreas Voss
- Birgit Weichelt
- Aldo Antognini
- Randolf Pohl
2025-05-23
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 S -2 P transitions in the muonic helium-3 (μ 3 He + ) ion, an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift, the 2 P fine structure splitting, and the 2 S -hyperfine splitting in μ 3 He + . Comparing these measurements with theory, we determined the root-mean-square charge radius of the helion ( 3 He nucleus) to be r h = 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.