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Pohl, Randolf; Nez, Francois; Fernandes, Luis M. P.; Amaro, Fernando D.; Biraben, Francois; Cardoso, Joao M. R.; Covita, Daniel S.; Dax, Andreas; Dhawan, Satish; Diepold, Marc; Giesen, Adolf; Gouvea, Andrea L.; Graf, Thomas; Hänsch, Theodor W.; Indelicato, Paul; Julien, Lucile; Knowles, Paul; Kottmann, Franz; Le Bigot, Eric-Olivier; Liu, Yi-Wei; Lopes, Jose A. M.; Ludhova, Livia; Monteiro, Cristina M. B.; Mulhauser, Francoise; Nebel, Tobias; Rabinowitz, Paul; dos Santos, Joaquim M. F.; Schaller, Lukas A.; Schuhmann, Karsten; Schwob, Catherine; Taqqu, David; Veloso, Joao F. C. A.; Antognini, Aldo (2016): Laser spectroscopy of muonic deuterium. In: Science, Vol. 353, No. 6300: pp. 669-673
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The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius r(d) and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium mu d is the exotic atom formed by a deuteron and a negative muon mu-. We measured three 2S-2P transitions in md and obtain r(d) = 2.12562(78) fm, which is 2.7 times more accurate but 7.5 sigma smaller than the CODATA-2010 value r(d) = 2.1424(21) fm. The md value is also 3.5 sigma smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a "small" proton radius r(p), similar to the one from muonic hydrogen, amplifying the proton radius puzzle.