Laser spectroscopy of light muonic atoms and ions

Simple muonic atoms have been proven to be of particular interest for studies of nuclear properties, such as the charge and Zemach radii, and the nuclear polarizabilities. It has been found that the proton [1] and deuteron [2] charge radii extracted from muonic atom spectroscopy deviate by about 5σ from the present CODATA values. This discrepancy has been coined the proton radius puzzle [3]. The proton and deuteron radius discrepancies could originate from a problem in the determination of the Rydberg constant [4] and indeed a recent measurement of the Rydberg constant by means of the 2S-4P transition in ordinary hydrogen performed in Garching [5] suggested a smaller Rydberg constant and hence a smaller proton radius also in hydrogen. However, the large proton radius has recently been confirmed by a measurement of the 1S-3S transition in Paris [6]. Recently, also 3He and 4He were measured by laser spectroscopy of the muonic ions. Using modern theory summarized in [2], we determine the charge radii of these nuclei with 10 and 6 times higher accuracy than the values from elastic electron scattering. In this talk, we will present ongoing and planned measurements of the CREMA collaboration targeting the (magnetic) Zemach radius of the proton [7], by laser spectroscopy of the hyperfine structure in μp. We will also present plans to extend the Lamb shift measurements to lithium and beryllium nuclei. With the muon as a sensitive probe for the nuclear structure, these measurements have the potential to improve the corresponding charge radii by a factor of 10 and 5, respectively. Comparing these measurements with studies from elastic electron-proton scattering events provides a suitable tool to test nuclear theory. ———————— [1] A. Antognini et al., Science 339, (2013) 417 [2] R. Pohl et al., Science 353, (2016) 669 [3] R. Pohl et al., Annu. Rev. Nucl. Sci 63, (2013) 175 [4] R. Pohl et al., Metrologia 54 (2017) [5] A. Beyer et al., Science 358, (2017) 79 [6] H. Fleurbaey et al., arXiv preprint arXiv:1801.08816 (2018) [7] R. Pohl et al., J. Phys. Soc. Japan Conf. Proc. 18, (2017) 011021