Volume 11 Issue 4
Proton Charge Radius from Electron Scattering
Ingo Sick
Abstract
The rms-radius R of the proton charge distribution is a fundamental quantity needed for precision physics. This radius, traditionally determined from elastic electron-proton scattering via the slope of the Sachs form factor ๐บ๐(๐2)extrapolated to momentum transfer ๐2=0, shows a large scatter. We discuss the approaches used to analyze the e-p data, partly redo these analyses in order to identify the sources of the discrepancies and explore alternative parameterizations. The problem lies in the model dependence of the parameterized ๐บ(๐) needed for the extrapolation. This shape of ๐บ(๐ < ๐๐๐๐) is closely related to the shape of the charge density ๐(๐) at large radii r, a quantity that is ignored in most analyses. When using our physics knowledge about this large-r density together with the information contained in the high-q data, the model dependence of the extrapolation is reduced, and different parameterizations of the pre-2010 data yield a consistent value for ๐ =0.887ยฑ0.012 fm. This value disagrees with the more precise value 0.8409ยฑ0.0004fm determined from the Lamb shift in muonic hydrogen.