Diffractive Two-Photon Exchange and Beam Normal-Spin Asymmetries for Elastic Electron Scattering on Nuclei

Abstract

We developed a theoretical approach to elastic electron scattering on nuclei that includes a two-photon-exchange mechanism responsible for parity-conserving single-spin beam asymmetries. The two-photon-exchange amplitude at small scattering angles is treated in a diffractive framework similar to that of pion-nucleus elastic scattering applied to the nuclei $^{12}_{6}$C, $^{40}_{20}$Ca and $^{208}_{82}$Pb. The predicted kinematic features of the beam polarization asymmetries for different nuclei may reconcile Jefferson Lab's experimental results at finite scattering angles with a forward limit given by an optical theorem, potentially resolving the so-called ``PREX Puzzle" for $^{208}_{82}$Pb.