High-energy photoproduction and scattering of leptons in the field of extended nuclei

We consider the deviations from electromagnetic high-energy scattering and photoproduction of leptons in the field of pointlike nuclei, generated by a nuclear charge distribution. The concept of the eikonal form factor for the description of scattering is briefly reviewed. In the region of small momentum transfers a coherence phenomenon of multiphoton exchange is discussed. The strong dependence of the non-Coulomb corrections to the photoproduction cross section of light lepton pairs upon the nuclear charge and the lepton mass can be traced back to this coherence effect. The higher-order corrections found for photoproduction are used to estimate the higher-order corrections to lepton pair production in ultrarelativistic heavy-ion collisions in logarithmic accuracy.