## Transmission of γ-radiation through matter

The physics of elementary particles originally has dealt with electrons and γ-radiation, so that here the greatest amount, both of the key ideas and of technical methods, have been stored. This and the next sections will be devoted to the technical side of the QED, the calculations of particular processes. For today the QED lost the status of the precise theory in the sense that it is already not a complete theory. It, together with the weak interaction theory, forms the electroweak interaction theory. If one considers some pure electrodynamic process (there are only charged leptons and photons in the initial and final states) in the electroweak theory, then, as a rule, diagrams, additionally in relation to the QED, appear with virtual gauge bosons (W± and Z), neutral leptons and quarks appear. For example, in the second order of the perturbation theory, every time while the Feynman diagram has an internal photon line, then the Z-boson contribution is also possible. Such processes as

e−e+ → e−e+, µ+µ−, τ+τ−,

having the s-channel Feynman diagram will feel the Z-boson effect more strongly as the energy in the center-of-mass frame is approached in the value mZ . There are, however, restricted numbers of processes that proceed at the cost only of the electromagnetic interaction in the second order of the perturbation theory. Under transition to the third and higher orders of the perturbation theory all electrodynamic processes include contributions of the weak interaction. However, the QED is the most simple example of a gauge theory of interacting fields. Understanding its structure will help us to further advance.