Laser spectroscopy of light muonic atoms and the nuclear charge radii

Laser spectroscopy of light muonic atoms, starting with the pristine muonic hydrogen system and expanding to muonic deuterium and Helium ions has been a scientific highlight of the PSI research program. It allows for precision measurements which are very difficult to match by any other method, as the tightly bound muon probes the electromagnetic structure of the nucleon and light nuclei with a few million times larger overlap than the electrons of ordinary atoms. The surprising disagreement with previous measurements on the proton charge radius, dubbed the proton radius puzzle, caught the interest of the physics community at large, including atomic physics as well as nuclear and particle physics. It stimulated a rich research program both in theoretical developments and a new generation of precision measurements and changed the previously accepted value of the Rydberg Constant. After years of debate a largely consistent picture is emerging with the muonic atom data not only holding up, but defining a new precision standard in this field.

This paper presents a comprehensive overview of the scientific questions, the experimental method, which is based on several remarkable technical achievements, and the impact of the results on various fields of physics. It is formulated well and includes extensive references for more detailed questions. The paper can be published as it, as it clearly exceeds all the required criteria for publication.

Below I add some minor remarks for the author's consideration.
Fig 21.3: In the way the CAD of the cavity is cut, the two sides look the same. So it is hard to comprehend the difference between the cylindrical mirror side and the flat mirror with the ears side. Perhaps this figure could be improved.
Line 147. It would be helpful to state in the preceding paragraph that the Michel electrons are detected by the LAAPDs.
Fig. 21.4: Define the lighter/darker bands and colors early on in the figure caption. Are they consistently used? Carboni is colored in blue, though this was a muonic experiment, Sick in magenta, though probably this was an e-scattering evaluation.
3He measurements are mentioned, but no results presented. Maybe comment on the status of this analysis.

We (the editors Cy Hoffman, Klaus Kirch, Adrian Signer) had the
opportunity to review an earlier draft of the article and were in
communication with the authors before the submission. All our comments
and suggestions have been taken into account. Hence, we think the
paper can now be published in the current form.