Proton Radius Puzzle

Organizers: Carl Carlson (College of William and Mary), Richard Hill (Chicago Univ.), Savely Karshenboim (MPI for Quantum Optics Munich and Pulkovo Observatory St. Petersburg), Marc Vanderhaeghen (JGU Mainz)

June 2 - 6, 2014, Waldthausen Castle near Mainz

The size of the proton is one of the most fundamental observables in hadron physics. It is measured through an electromagnetic form factor. The latter is directly related to the distribution of charge and magnetization of the baryon and through provides the basis of nearly all studies of the hadron structure. In very recent years, a very precise knowledge of nucleon form factors has become more and more important as input for precision experiments in several fields of physics. The recent PSI measurement of the proton charge radius from the Lamb shift in muonic hydrogen has given a value that is 5 standard deviations lower than the values obtained from energy level shifts in electronic hydrogen and from electron-proton scattering experiments. These puzzling findings that the proton radius appears to be different when measured with muons vs. electrons, motivates this workshop dedicated to this topic, bringing together atomic, nuclear, and beyond standard model physicists. It remains unknown if the solution lies in subtle experimental effects, unexpected behavior of QCD, new physics beyond the standard model, or something not yet discussed.