Analytical tools for investigating strong-field QED processes in tightly focused laser fields

The present paper is the natural continuation of a recent paper [A. Di Piazza, Phys. Rev. Lett. 113, 040402 (2014)], in which the electron wave functions in the presence of a background electromagnetic field of general space-time structure were constructed analytically, assuming that the initial energy of the electron is the largest dynamical energy scale in the problem and having in mind the case of a background tightly focused laser beam. Here, we determine the scalar and the spinor propagators under the same approximations, which are useful tools for calculating, e.g., total probabilities of processes occurring in such complex electromagnetic fields. In addition, we also present a simpler and more general expression of the electron wave functions found in the earlier paper, and we indicate a substitution rule to obtain them starting from the well-known Volkov wave functions in a plane-wave field.