Higher Order (Dispersive) Corrections To The Born Approximation In Elastic Electron-Nucleas Scattering In The Intermediate Energy Regime*

Two-photon exchange contributions have become a necessary ingredient in theoretical calculations trying to precisely calculate hydrogen elastic scattering cross sections. This correction typically modifies the cross section at the few percent level. In contrast, dispersive effects can cause significantly larger changes from the Born approximation. The LEDEX Collaboration at Jefferson Lab measures the ¹²C elastic cross section in Hall A at beam energies of 362 MeV and 685 MeV around the 1^stdiffractive minimum, where the Born term contributions to the cross section are small to maximize the sensitivity to dispersive effects. The results are in very good agreement with previous world data and the average deviation from a static charge distribution expected from linear and quadratic fits indicate a 38.8% contribution of dispersive effects to the cross section at 1 GeV. Their magnitude has been confirmed to be large with a strong energy dependence and could account for a large fraction of the observed quenching of the longitudinal nuclear response. They could also be important for nucleon radii extracted from parity violating asymmetries measured near a diffractive minimum.