The standard procedure of extracting the proton root-mean-square radii from models for the Sachs form factors $G_e\left(q\right)$ and $G_m\left(q\right)$ fitted to elastic electron-proton scattering data is more uncertain than traditionally assumed. The extrapolation of $G\left(q\right)$, 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.

• Received 13 November 2013

DOI:https://doi.org/10.1103/PhysRevC.89.012201