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 Gₑ(q) and Gₘ(q) fitted to elastic electron-proton scattering data is more uncertain than traditionally assumed. The extrapolation of $G(q)$, from the region qₘᵢₙ<q<qₘₐₓ 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.