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

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 ${}_6^{12}$C, ${}_{20}^{40}$Ca and ${}_{82}^{208}$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 ${}_{82}^{208}$Pb.