@article {Krauth:2021gr,
	title = {{Measuring the $\alpha$-particle charge radius with muonic helium-4 ions}},
	journal = {Nature},
	volume = {589},
	number = {7843},
	year = {2021},
	note = {n/a},
	month = {jan},
	pages = {527{\textendash}531},
	publisher = {Nature Publishing Group},
	abstract = {<p>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{\textendash}2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the $\alpha$ 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{\textendash}5, in line with recent determinations of the proton charge radius6{\textendash}9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties. The 2S{\textendash}2P transitions in muonic helium-4 ions are measured using laser spectroscopy and used to obtain an $\alpha$-particle charge-radius value five times more precise than that from electron scattering.</p>
},
	doi = {10.1038/s41586-021-03183-1},
	url = {http://www.nature.com/articles/s41586-021-03183-1},
	author = {Krauth, Julian J and Schuhmann, Karsten and Ahmed, Marwan Abdou and Amaro, Fernando D. and Amaro, Pedro and Biraben, Fran{\c c}ois 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 G{\"o}tzfried, Johannes and Graf, Thomas and H{\"a}nsch, Theodor W. and Jens Hartmann 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, Fran{\c c}oise and Naar, Boris and Nebel, Tobias and Nez, Fran{\c c}ois and dos Santos, Joaquim M. F. and Jos{\'e} Paulo Santos and Szabo, Csilla I and Taqqu, David and Veloso, Jo{\~a}o F. C. A. and Jan Vogelsang and Voss, Andreas and Weichelt, Birgit and Pohl, Randolf and Antognini, Aldo and Kottmann, Franz}
}
