Proton root-mean-square radii and electron scattering

The standard procedure of extracting the proton root-mean-square radii from models for the Sachs form factors G_e(q) and G_m(q) fitted to elastic electron-proton scattering data is more uncertain than traditionally assumed. The extrapolation of G (q), from the region q_min<q<q_max covered by data to momentum transfer q =0 where the rms radius is obtained, often depends on uncontrolled properties of the parametrization used. Only when ensuring that the corresponding densities have a physical behavior at large radii r can reliable rms radii be determined.