Finite temperature world-line formalism and analytic continuation

In vacuum, the world-line formalism is an efficient tool for calculating observables in the presence of arbitrary constant external fields. The natural frame of this formalism is the Euclidean space. At finite temperature the analytic continuation to Minkowski space is a subtle task. We study the two point function in scalar QED, and we figure out the problem of analytic continuation giving a possible solution for it. We also show, in contrast to what was claimed in the literature, that a translationally invariant world-line Green's function could be used at finite temperature.