Importance of Coulomb Effects in Half-Shell Scattering

The half-shell cross section for charged-particle scattering is discontinuous at the on-shell point. Moreover, its limits for $p\to k^{-}$ and $p\to k^{+}$ not only differ from each other, they also differ from the corresponding on-shell cross section, often by appreciable energy-dependent factors. This has important consequences for the theoretical description of bremsstrahlung and quasifree data.