New aspects of loop diagrams in strong-field QED
In the absence of background fields the effect of loop corrections is usually small in comparison with tree-level predictions. In QED with an external field, however, the dressed one-particle states are unstable. Therefore, unitarity requires to take the imaginary part of loop diagrams into account for a self-consistent description. To this end the Dyson-Schwinger equations and the optical theorem are discussed. It is shown that this approach also reveals the birefringence of the vacuum (for photons and electrons) without the need to derive an effective (classical) action. Another qualitatively new aspect of loop diagrams in the presence of an oscillatory electromagnetic field is the prediction of recollision processes, where an electron-positron pair is accelerated over a distance of the order of the (macroscopic) laser wavelength before annihilation. By analysing the polarization operator, the differences between recollision processes and vacuum fluctuations on the Compton-wave length scale are investigated.