February 11 is the International Day of Women and Girls in Science: A good occasion to introduce our new junior research group leader Dr. Franziska Hagelstein and her research. Franziska Hagelstein studied physics at the Johannes Gutenberg University Mainz (JGU) and received her PhD in theoretical nuclear physics there in 2017 under the supervision of Prof. Marc Vanderhaeghen and Dr. Vladimir Pascalutsa. After several years of research at the University of Bern and the Paul Scherrer Institute in Switzerland, she returned to JGU in 2022 and has since been leading the Emmy Noether Young Investigator Group "Hadronic Contributions to Precision Observables and the Search for New Physics" at the Institute of Nuclear Physics. She currently supervises two PhD students and is supported in her research by a postdoc - Dr. Vadim Lensky.
According to her own statement, JGU is an almost optimal place for her research, because on the one hand she finds here an inspiring exchange with the colleagues in the large theory group and at the same time has the proximity to collaborations from experimental nuclear and atomic physics, such as in the experiments for proton form factor (A1 collaboration , JGU) and proton polarizability measurements (A2 collaboration , JGU) at the electron accelerator MAMI, or the spectroscopy experiments on normal and muonic atoms (group around Prof. Randolf Pohl , JGU). Particularly exciting is that these experiments play a central role in the so-called "proton radius puzzle".
The proton radius puzzle describes the fact that more than 100 years after its discovery, the size of the proton is still not fully understood. In 2010, the CREMA collaboration was able to spectroscopically determine the radius of the proton for the first time from the Lamb shift in muonic hydrogen. Muonic atoms, such as muonic hydrogen, are atoms in which an electron has been replaced by a muon particle. Since muons are about two hundred times heavier, they are much closer to the nucleus. Consequently, the spectra of muonic atoms are more sensitive to nuclear properties such as their radii or polarizabilities.
The 2010 measurement on muonic hydrogen had a previously unattained precision and determined the proton radius to be rp= 0.84087(39) fm [1,2] Surprisingly, however, the measured radius was significantly smaller than it had been previously determined using conventional methods, such as spectroscopy of normal hydrogen and scattering experiments with electrons. For comparison, a precise measurement of proton shape factors by the A1 collaboration at the MAMI accelerator yielded a radius rp= 0.879(8) fm . Therefore, after 2010, many new spectroscopy and scattering experiments were performed to solve the proton radius puzzle and until today some questions are still open. But thanks to intensive research, the research field of muonic atoms is moving “from puzzle to precision”!
In the future, therefore, further experiments with muonic atoms are planned, which will provide much information on the properties of protons, deuterons, helium nuclei, as well as heavier nuclei [4,5,6]. Of particular note is the planned measurement of the hyperfine structure of muonic hydrogen in the ground state. However, for the evaluation of these experiments - e.g., to extract nuclear radii- a precise theory prediction for the spectra of the hydrogen-like atoms must always be available. That is, a truly precise result can only be obtained if both theory and experiment are equally precise and maximally precise. According to Franziska Hagelstein, theory still has some catching up to do here in the case of muonic atoms. That is why she and her junior research group are devoting themselves, among other things, to making the most precise predictions possible for myonic hydrogen or myonic deuterium. The key factor in increasing precision is determining the influence of the atomic nucleus on the atomic properties. For this purpose, so-called two-photon exchange contributions are relevant, which describe, for example, the effect of the polarizability of the nucleons or nuclei.
Assuming a successful interim evaluation, the junior research group is expected to be financially supported by the German Research Foundation DFG within the framework of the Emmy Noether Program until February 2028.
More detailed information for experts can be found in the references given as well as in the two reviews [7,8]:
- Pohl, et al. [CREMA Kollaboration], The Size of the Proton, Nature 466 (2010) 213-216
- Antognini, et al. [CREMA Kollaboration], Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen, Science 339 (2013) 417-420
- Bernauer, et al. [A1 Kollaboration], High-precision determination of the electric and magnetic form factors of the proton, Phys. Rev. Lett. 105 (2010) 242001
- Pohl, et al. [CREMA Kollaboration], Laser spectroscopy of muonic deuterium, Science 353 (2016) 6300, 669-673
- Krauth, et al. [CREMA Kollaboration], Measuring the -particle charge radius with muonic helium-4 ions, Nature 589 (2021) 7843, 527-531
- Antognini, S. Bacca, A. Fleischmann, L. Gastaldo, F. Hagelstein, P. Indelicato, A. Knecht, V. Lensky, B. Ohayon, V. Pascalutsa, N. Paul, R. Pohl, and F. Wauters, Muonic-Atom Spectroscopy and Impact on Nuclear Structure and Precision QED Theory, 2210.16929.
- A. Antognini, F. Hagelstein, and V. Pascalutsa, The proton structure in and out of muonic hydrogen, Ann. Rev. Nucl. Part. Sci. 72 (2022) 389
- K. Pachucki, V. Lensky, F. Hagelstein, S. S. Li Muli, S. Bacca, and R. Pohl, Comprehensive theory of the Lamb shift in in μH, μD, μ3He+,and μ4He+, 2212.13782