The helion charge radius from laser spectroscopy of muonic helium-3 ions
Karsten Schuhmann, L. M. P. Fernandes, F. Nez, Marwan Abdou Ahmed, F. D. Amaro, Pedro Amaro, F. Biraben, Tzu-Ling Chen, D. S. Covita, A. Dax, Marc Diepold, B. Franke, Sandrine Galtier, Andrea L. Gouvea, Johannes Götzfried, Thomas Graf, T. W. Hänsch, M. Hildebrandt, P. Indelicato, L. Julién, K. Kirch, A. Knecht, F. Kottmann, Julian J. Krauth, Yi-Wei Liu, Jorge Machado, C. M. B. Monteiro, F. Mulhauser, Boris Naar, Tobias Nebel, J.M.F. dos Santos, J. P. Santos, Csilla I. Szabo, D. Taqqu, J.F.C.A. Veloso, Andreas Voß, Birgit Weichelt, Aldo Antognini, Randolf Pohl
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 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.