A group of theoretical physicists at Johannes Gutenberg University Mainz (JGU) has once again succeeded in significantly improving their calculations of the electric charge radius of the proton published in 2021. For the first time they obtained a sufficiently precise result completely without the use of experimental data.
With respect to the size of the proton, these new calculations also favor the smaller value. Concurrently, the physicists for the first time have published a stable theory prediction for the magnetic charge radius of the proton. All new findings can be found in three preprints published on the arXiv server.
All known atomic nuclei consist of protons and neutrons, yet many of the characteristics of these ubiquitous nucleons remain to be understood. Specifically, despite several years of effort, scientists have been unable to pin down the radius of the proton. In 2010, the result of a new proton radius measurement technique involving laser spectroscopy of muonic hydrogen caused a stir—in this ‘special’ kind of hydrogen, the electron in the shell of the atom was replaced by its heavier relative, the muon, which is a much more sensitive probe for the proton’s size.
The experimentalists came up with a significantly smaller value than that found following corresponding measurements of ‘normal’ hydrogen as well as the traditional method of determining the proton radius using electron-proton scattering. The big question that physicists have been asking ever since is whether this deviation could be evidence for new physics beyond the Standard Model or ‘simply’ reflects systematic uncertainties inherent to the different measuring methods.
In recent years, there has been increasing evidence that the smaller experimental value is the correct one, i.e. that there is no new physics behind the proton radius puzzle. Theoretical calculations make a significant contribution to answering this question definitively. Already in 2021, researchers led by Prof. Dr. Hartmut Wittig of the Mainz Cluster of Excellence PRISMA+ succeeded in performing so-called lattice calculations with sufficient precision to provide another reliable clue to the smaller proton radius.
2023-10-07 05:00:04
Link from phys.org