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CREMA Report Summary

Project ID: 279765
Funded under: FP7-IDEAS-ERC
Country: Germany

Final Report Summary - CREMA (Charge radius experiment with muonic atoms)

The “proton radius puzzle”, i.e. the 7 standard deviation discrepancy between the proton rms charge radii obtained using electrons (hydrogen spectroscopy and elastic electron scattering) and muons (our experiment in muonic hydrogen) has been one of the biggest discrepancies of the Standard Model.
The measurements and analyses performed in the past 5 years with the help of the ERC Starting Grant CREMA have brought us closer to a possible solution:
The recently published results on muonic deuterium [Pohl et al., (CREMA Collab.), Science 353, 671 (2016)] have confirmed the smaller proton radius from muonic hydrogen. This result excludes some scenarios proposed to solve the proton radius puzzle, such as problems in theory of muonic atoms, or certain types of physics beyond the Standard Model (BSM).
The muonic helium-3 and -4 measurements performed within the CREMA Collaboration have been extremely successful: We have measured both 2S-2P transitions in muonic helium-4 ions, both with an accuracy of better than 20 GHz. We have also measured three 2S-2P transitions in muonic helium-3, with accuracies around 20 GHz. These measurements will yield charge radii with nearly tenfold higher precision than the current best values from elastic electron scattering and isotope shift measurements in regular helium atoms. Our uncertainty is limited by theoretical uncertainties; the experimental precision achieved in CREMA is another factor of about 5 better.
A lot of theory is required to extract the charge radii from our measurements in muonic helium. As expected, several groups have recently revisited the various theory contributions to the Lamb shift in light muonic atoms. In particular, the nuclear structure contributions have been calculated which much higher precision now, as anticipated in the ERC proposal.
As the theory is being actively developed, the numbers have become a moving target, but the results seem to be stabilizing now. As for muonic hydrogen and muonic deuterium, we have also started an effort to summarize all known theory contributions in muonic helium-4 and -3.
The muonic helium-4 theory paper (arxiv 1606.05231) has been submitted and received generally positive referee reports, but refinement is needed. The first paper on experimental results in muonic helium-4 has been drafted, but the final numbers depend of course on the exact theory result. We expect to (re-)submit the muonic helium-4 papers in Q1/2017.
For muonic helium-3, the data is analyzed, the theory paper is being drafted and will be sent to the theory colleagues for comments in early 2017. A paper on the experimental results will also be submitted in the 1st half of 2017.
What we can say already today, despite of the remaining uncertainties of the muonic helium theory, is that there seems to be no discrepancy for the helium-4 case! This excludes more BSM scenarios, and brings into focus again the accuracy of the proton and deuteron charge radii measured with electrons. On the atomic physics side, a check of the proton radius in electronic hydrogen requires
a new determination of the Rydberg constant, and therefore the Garching hydrogen group (which I belong to, as well) has performed a measurement of the 2S-4P transitions in electronic hydrogen. This yields a Rydberg constant, and hence a proton radius from hydrogen, with an accuracy that matches the previous world average of 15 measurements in hydrogen. The surprising results have already been submitted.
To summarize, we have established laser spectroscopy of muonic atoms as a reliable tool to improve our knowledge on the light nuclei by an order of magnitude. We have corrected the value of the Rydberg constant, and improved its value by a factor of 6. The novel laser system we have developed for CREMA has yielded industrial spinoffs, and will be used for our measurement of the magnetic properties of the proton and helium-3 nucleus. Our unique muon beam line will will be used by the muX collaboration (of which I am a member) for x-ray spectroscopy of e.g. radioactive nuclei such as radium. The “proton radius puzzle” has stimulated a huge amount of theoretical works and many new experiments in atomic physics and scattering. This will further improve our knowledge on nuclear radii and structure, as well as the accuracy of fundamental constants.

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