Coherent Contributions of Nuclear Mesons to Electroproduction and the   HERMES Effect
  We show that nuclear sigma, omega, and pi mesons can contribute coherently to enhance the electroproduction cross section on nuclei for longitudinal virtual photons at low Q^2 while depleting the cross section for transverse photons. We are able to describe recent HERMES inelastic lepton-nucleus scattering data at low Q^2 and small x using photon-meson and meson-nucleus couplings which are consistent with (but not determined by) existing constraints from meson decay widths, nuclear structure, deep inelastic scattering, and lepton pair production data. We find that while nuclear-coherent pion currents are not important for the present data, they could be observed at different kinematics. Our model for coherent meson electroproduction requires the assumption of mesonic currents and couplings which can be verified in separate experiments. The observation of nuclear-coherent mesons in the final state would verify our theory and allow the identification of a specific dynamical mechanism for higher-twist processes. 
G. A. Miller, S. J. Brodsky, and M. Karliner