Energy dependence of the form factor for elastic electron scattering fromC12

A static analysis of measurements of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ elastic electron-scattering cross sections demonstrates unambiguously the existence of a form-factor energy dependence beyond that due to Coulomb distortion effects. This energy dependence increases smoothly as a function of momentum transfer and incident energy in the region covered by the experiments (1.0<${\mathit{q}}_{\mathrm{eff}}$<2.3 ${\mathrm{fm}}^{\mathrm{-}1}$, 240<${\mathit{E}}_0$<690 MeV). Outside the first diffraction minimum the discrepancies between the form factors deduced from data obtained at different energies are as large as 5%; in the minimum discrepancies as large as 18% have been observed. Including dispersion corrections which are reasonably compatible with this observed energy dependence in the analysis of the data increases the rms charge radius deduced for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ by 0.007 to 2.478(9), fm, which is in excellent agreement with the value from muonic x-ray studies.