In this paper we examine the theoretical foundations underlying the testing of quantum electrodynamics. We show that for the photon propagator (together with the contiguous vertices) it is not necessary to introduce ad hoc modifications in sufficiently accurate scattering experiments. Energy, momentum transfer, and accuracy determine the tested length in a model-independent way. The situation is quite different with the electron propagator. If gauge invariance is taken for granted, the electron propagator cannot be tested with processes where diagrams with open (...) electron lines are important in the lowest order of perturbation theory. These processes can only give limits for anomalous moment and multiphoton parts of the vertices. On the other hand, processes with closed electron loops (vacuum polarization), such as photon-photon and Delbrück scattering, as well as photon splitting or corresponding low-energy, high-precision experiments can give limits also for the electron propagator. But in these cases only less accurate limits can be obtained, which depend on the modification model. Hence testing of the electron propagator, i.e., roughly speaking, the Dirac equation, is much more difficult than testing of the photon propagator, i.e, Maxwell's equations. (shrink)

In this paper we consider the effect of unitarity bounds sb⩾s≡(E1+E2) cms 2 for the recently proposed types of nonderivative 4-fermion contact interactions. To this purpose we decompose the helicity amplitudes at c.m.s. into partial waves. The bounds are defined to hold for all reaction channels due to the same type of contact interaction. We find sb=τ4π/κ. Here κ is the coupling constant. The factor τ depends on the type of coupling and on the different cases to identify the fermions. (...) It ranges from 1/3 to 4. (shrink)

In this work we consider the energy states of muonic atoms which are predominantly influenced by vacuum polarization. This fact is used for testing the electron propagator of QED with the modification $S(p) = (\not p - me)^{ - 1} + f(\not p - M)^{ - 1}$ . The data of some well analyzed transitions in muonic He, Si, Ba, and Pb yield the limit M>29 MeV for f=1.Similarly the presence of a Higgs boson would cause a shift of the (...) energy levels which can be measured easier in muonic atoms since the coupling grows with the fermion mass. The analysis of several transitions in heavy muonic atoms shows, that the mass of the Higgs boson is larger than 12.8 MeV.Further reductions of the present-day uncertainties concerning the experimental data and especially of the nuclear structure effects could improve these limits. (shrink)