Delbrück scattering
The Delbruck scattering is called the scattering of a photon in the electromagnetic field of an atomic nucleus and after the biophysicist Max Delbrück. She is described as a nonlinear electromagnetic effect due to the vacuum polarization of quantum electrodynamics and could be detected experimentally.
Description
In classical electrodynamics can not be scattered by a Coulomb field of an electromagnetic wave because electromagnetic fields are superimposed linearly. Unlike in quantum electrodynamics, where by creation and annihilation of virtual particles, the vacuum to a nonlinearly polarizable electromagnetic medium ( vacuum polarization). Therefore, photons can be scattered by an electromagnetic field in quantum electrodynamics, followed in 1933 first Max Delbrück has qualitatively noted the example of the electromagnetic field of atomic nuclei. Delbrück was then assistant to Lise Meitner, who carried out such experiments.
The first theoretical estimation of of them so named Delbrück scattering was carried out in 1952 by Hans Bethe and Fritz Rohrlich In the lowest order of perturbation theory, the Delbrück scattering is described by the Vakuumpolarisationstensor second stage with two real and two virtual photons. Its full calculation published V. Costantini, B. de Tollis and G. Pistoni 1971. Allocated Feynman diagram with a closed loop of four electron propagators describes not only the Delbrück scattering ( two real and two virtual photons) and photocleavage ( three real and a virtual photon ) and the photon-photon scattering ( four real photons), both of which previously could not be verified experimentally because of their smallness.
For heavy nuclei with large atomic number Z perturbation theory does not provide a good approximation for the Delbrück scattering, because the vacuum polarization is described not good in a strong electromagnetic field by low approximations of perturbation theory. This case was examined by Hung Cheng and TT Wu 1969.
Delbrück scattering was first observed in 1953 by Robert R. Wilson in the scattering of gamma-rays of 1.33 MeV for lead - nuclei. A measurement at several nuclei with different atomic numbers Z laid U. Stierlin, W. Scholz and Bogdan Povh 1962, .. A recent measurement at higher energies in 1973 at DESY with the theoretical predictions of Cheng and Wu tolerated.
- Addendum:
This measurement was performed at DESY (Hamburg). It corresponds to the case of the extreme forward scattering, wherein only the imaginary component of the scattering amplitude provides a contribution (shadow scattering). The calculation of Cheng and Wu corresponds to an approximation, which was later verified by Milstein and Strakhovenko. These authors assume a quasi-classical approach, which is made considerably from that of Cheng and Wuunterscheidet. However, it could be shown that both approaches are equivalent and lead to the same numerical result. The final proof of Delbrück scattering in 1975 in Göttingen at an energy of 2.754 MeV. At this energy, the differential cross section is dominated by the real part of the Delbrück scattering amplitude which interferes with smaller contributions from the atomic and nuclear Rayleigh scattering. In this experiment, for the first time based on the Feynman graphs exact invoice has been verified. The scoring high precision both the theoretical prediction and the experiment made it possible to demonstrate that in addition to the lowest order (see the Feynman graphs shown ), a smaller amount of the next higher order is available. 1979 was the first time in Göttingen even purely dispersive Delbruck scattering, ie Delbrück scattering are shown below the production threshold for electron-positron pairs. A comprehensive review of the current state of research into the Delbruck scattering is in. Currently find precise studies on the high-energy Delbrück scattering at the Budker Institute of Nuclear Physics in Novosibirsk ( Russia) instead. To the 1M ROKK means of VEPP -4M there Photon cleavage was first identified in which one of the two in the Delbrück scattering exchanged with the core virtual photon is emitted as a real photon.