Mass renormalization in low-energy photon scattering

The amplitude for non-resonant photon scattering by an atom or molecule in its ground state is considered in the context of non-relativistic quantum electrodynamics. The forward scattering amplitude has applications to the refractive index of dilute isotropic media, including optically active media. The scattering amplitude, however, exhibits an apparent discontinuity in the forward direction. It is shown that the discontinuity is removed by non-relativistic renormalization of the electron mass.