Krauth, Julian J. and Schuhmann, Karsten and Ahmed, Marwan Abdou and Amaro, Fernando D. and Amaro, Pedro and Biraben, François and Chen, Tzu-Ling and Covita, Daniel S. and Dax, Andreas J. and Diepold, Marc and Fernandes, Luis M. P. and Franke, Beatrice and Galtier, Sandrine and Gouvea, Andrea L. and Götzfried, Johannes and Graf, Thomas and Hänsch, Theodor W. and Hartmann, Jens and Hildebrandt, Malte and Indelicato, Paul and Julien, Lucile and Kirch, Klaus and Knecht, Andreas and Liu, Yi-Wei and Machado, Jorge and Monteiro, Cristina M. B. and Mulhauser, Françoise and Naar, Boris and Nebel, Tobias and Nez, François and dos Santos, Joaquim M. F. and Santos, José Paulo and Szabo, Csilla I. and Taqqu, David and Veloso, João F. C. A. and Vogelsang, Jan and Voss, Andreas and Weichelt, Birgit and Pohl, Randolf and Antognini, Aldo and Kottmann, Franz (2021) Measuring the α-particle charge radius with muonic helium-4 ions. Nature, 589 (7843). pp. 527-531. ISSN 0028-0836 - 1476-4687

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The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S–2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering1, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle2–5, in line with recent determinations of the proton charge radius6–9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.

Item Type: Article
Subjects: Science and mathematics > Physics
Divisions: Faculty of Mathematics and Science > Institute of Physics (IfP)
Date Deposited: 16 Feb 2022 12:58
Last Modified: 16 Feb 2022 12:58
URN: urn:nbn:de:gbv:715-oops-53873
DOI: 10.1038/s41586-021-03183-1

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