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.