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Title: Measuring the α-particle charge radius with muonic helium-4 ions
Authors: Krauth, Julian J.
Schuhmann, Karsten 
Ahmed, Marwan Abdou
Amaro, Fernando D. 
Amaro, Pedro 
Biraben, François 
Chen, Tzu-Ling
Covita, Daniel S. 
Dax, Andreas J. 
Diepold, Marc 
Fernandes, Luis M. P. 
Franke, Beatrice
Galtier, Sandrine
Gouvea, Andrea L. 
Götzfried, Johannes
Graf, Thomas 
Hänsch, Theodor W. 
Hartmann, Jens
Hildebrandt, Malte
Indelicato, Paul 
Julien, Lucile 
Kirch, Klaus
Knecht, Andreas
Liu, Yi-Wei 
Machado, Jorge
Monteiro, Cristina M. B. 
Mulhauser, Françoise 
Naar, Boris
Nebel, Tobias 
Nez, François 
Santos, Joaquim M. F. dos 
Santos, José Paulo
Szabo, Csilla I.
Taqqu, David 
Veloso, João F. C. A. 
Vogelsang, Jan 
Voss, Andreas
Weichelt, Birgit
Pohl, Randolf 
Antognini, Aldo 
Kottmann, Franz 
Issue Date: 2021
Publisher: Nature
Project: PTDC/FIS-NUC/0843/2012 
UID/04559/2020 (LIBPhys) 
Serial title, monograph or event: Nature
Volume: 589
Issue: 7843
Abstract: 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 puzzle2,3,4,5, in line with recent determinations of the proton charge radius6,7,8,9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.
ISSN: 0028-0836
DOI: 10.1038/s41586-021-03183-1
Rights: embargoedAccess
Appears in Collections:LIBPhys - Artigos em Revistas Internacionais

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