Proton charge radius from electron scattering

Understand

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 $G_e(q^2)$ extrapolated to momentum transfer $q^2$=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 $G(q)$ needed for the extrapolation.