Delbrück scattering

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The Delbrück scattering is the scattering of a photon to the electromagnetic field of an atomic nucleus and after Biophysiker Max Delbruck named. It is described by quantum electrodynamics as a non-linear electromagnetic effect due to the vacuum polarization and could be proven experimentally .

description

In classical electrodynamics, an electromagnetic wave cannot be scattered by a Coulomb field because the electromagnetic fields are linearly superimposed. It is different in quantum electrodynamics, where the creation and annihilation of virtual particles turns the vacuum into a non-linearly polarizable electromagnetic medium (vacuum polarization). Therefore, in quantum electrodynamics, photons can be scattered by an electromagnetic field, as Max Delbrück first pointed out qualitatively in 1933 using the example of the electromagnetic field of atomic nuclei. At the time, Delbrück was Lise Meitner's assistant , who carried out the relevant experiments.

Feynman diagram for the second order vacuum polarization tensor

The first theoretical estimate of what they called the Delbrück scattering was carried out in 1952 by Hans Bethe and Fritz Rohrlich . In the lowest order of perturbation theory , Delbrück scattering is described by the second order vacuum polarization tensor with two real and two virtual photons. Its complete calculation was published by V. Costantini, B. de Tollis and G. Pistoni 1971. The associated Feynman diagram with a closed loop of four electron propagators describes the Delbrück scattering (two real and two virtual photons) as well as the photon splitting ( three real and one virtual photon) and the photon-photon scattering (four real photons), both of which could not be proven experimentally because of their small size. There are now very good indications for photon-photon scattering in the data from the Atlas experiment at CERN .

In the case of heavy nuclei with a large atomic number Z, perturbation theory does not provide a good approximation for Delbrück scattering because the vacuum polarization in a strong electromagnetic field is no longer well described by low approximations of perturbation theory. This case was investigated by Hung Cheng and TT Wu in 1969 .

The Delbrück scattering was first observed in 1953 by Robert R. Wilson in the scattering of gamma radiation with 1.33 MeV energy on lead atomic nuclei. U. Stierlin, W. Scholz and Bogdan Povh in 1962 presented a measurement on several atomic nuclei with different atomic numbers Z. A more recent measurement at higher energies from 1973 at DESY is compatible with the theoretical predictions of Cheng and Wu.

  • Addendum:

This measurement was carried out at DESY (Hamburg). It corresponds to the case of extreme forward scattering, in which only the imaginary part of the scattering amplitude makes a contribution (shadow scattering). Cheng and Wu's calculation corresponds to an approximation that was later verified by Milstein and Strakhovenko. These authors take a quasi-classical approach that differs significantly from that of Cheng and Wu. However, it could be shown that both approaches are equivalent and lead to the same numerical result. The final proof of the Delbrück scattering took place 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 atomic and nuclear Rayleigh scattering . In this experiment, the exact calculation based on the Feynman graph was verified for the first time. The high precision achieved in both the theoretical prediction and the experiment made it possible to prove that, in addition to the lowest order (see the Feynman graph shown), there is also a smaller amount of the next higher order. In 1979 in Göttingen, for the first time, purely dispersive Delbrück scattering, i.e. H. Delbrück scattering below the generation threshold for electron-positron pairs can be detected. A comprehensive presentation of the current state of research into Delbrück scattering can be found in. At present, precise investigations into high-energy Delbrück scattering are taking place at the Budker Institute for Nuclear Physics in Novosibirsk (Russia). With the ROKK-1M device of the VEPP-4 M, photon splitting was detected for the first time , in which one of the two virtual photons exchanged with the nucleus during Delbrück scattering is emitted as a real photon.

literature

  • Josef-Maria Jauch , Fritz Rohrlich: The theory of photons and electrons. The relativistic quantum field theory of charged particles with spin one-half. 2nd Edition. Springer, Berlin 1976, ISBN 3-540-07295-0 . (Reprinted from London 1955 edition)

Web links

Individual evidence

  1. ↑ L. Meitner, H. Kösters on the scattering of short-wave gamma rays , Zeitschrift für Physik, Volume 84, 1933, pp. 137-144, with the addition of Delbrück
  2. ↑ Bethe, Rohrlich Small angle scattering of light by a Coulomb field , Physical Review, Volume 86, 1952, pp. 10-16
  3. ↑ Nuovo Cimento A2, 1971, p. 733
  4. ↑ The virtual photons stand for the Coulomb interaction with the nucleus, the real photons for the photon scattering process
  5. ↑ Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC , The ATLAS Collaboration, https://arxiv.org/pdf/1702.01625.pdf
  6. ↑ a b c Physical Review Letters 22, 1969, p. 666
  7. ↑ a b c Physical Review 182, 1969, p. 1873
  8. ^ Wilson Scattering of 1.33 MeV gamma-rays by an electric field , Physical Review, Volume 90, 1953, pp. 720-721
  9. ↑ Zeitschrift für Physik A, Volume 170, Number 1, p. 47
  10. ^ A b G. Jarlskog, L. Jonsson, S. Prunster, HD Schulz, HJ Willutzki, GG Winter, Physical Review D8, 1973, p. 3813
  11. ↑ a b A.I. Milstein, VM Strakhovenko, Phys. Lett. A 95 (1983) 135; So V. Phys. - JETP 58 (1983) 8.
  12. ↑ M. Schumacher , I. Borchert, F. Smend, P. Rullhusen Delbrück scattering of 2.75 MeV Photons by Lead , Phys. Lett. 58 B (1975) 134.
  13. ↑ P. Papatzacos, K. Mork, Phys. Rev. D 12 (1975) 206; Phys. Rep. 21 (1975) 81.
  14. ↑ H. Falkenberg et al., Atomic Data and Nucl. Data Tables 50 (1992) 1.
  15. ↑ Wolfgang Mückenheim , Martin Schumacher Delbrück and Rayleigh scattering by uranium investigated at photon energies between 0.1 and 1.5 MeV , J. Phys. G: Nucl. Phys. 6 (1980) 1237
  16. ^ AI Milstein, M. Schumacher The present status of Delbrück Scattering , Phys. Rep. 234 (1994) 183-214.
  17. ↑ M. Schumacher Delbrück Scattering , Rad. Phys. Chem. 56 (1999) 101.
  18. ↑ SZ Akhmadalev, et al., Phys. Rev. C 58 (1998) 2844.
  19. ↑ SZ Akhmadalev, et al. Phys. Rev. Lett. 89 (2002) 061802
  20. ^ RN Lee. et al., Phys. Reports 373 (2003) 213.