The Delbrück experiment 1969-73
During the course of the measurements on the bremsstrahlung spectra
there appeared a series of publications by Cheng and Wu on a systematic
study of two-body elastic scattering amplitudes in quantum
electrodynamics. Among other processes they treated the elastic
scattering of a photon by the static Coulomb field of nuclei via a
virtual electron-positron pair. This process was predicted by Delbrück
already in 1933 but not until the study of Cheng and Wu in 1969 was
there a calculation made based on conventional perturbation theory.
Delbrück scattering is one of the non-linear processes in quantum
electrodynamics which are a direct consequence of vacuum polarization
and thus the detection of this process would be the most direct proof
of the existence of vacuum polarization and would provide a significant
test of perturbation theory. The calculations by Cheng and Wu, however,
were valid only for asymptotic energies and not too small a momentum
transfer thus requiring a photon beam in the GeV region in order to
give reliable results. Earlier measurements ranged from a few MeV to
100 MeV, but these suffered from various problems and the quantitative
evaluation of the results was not clear.
Monte Carlo simulations showed that Delbrück scattering is very much
peaked in the forward direction and dominates over Compton scattering
only in the angular region of a few mrad in the GeV region. This was
an experimental challenge which we solved by using a pair spectrometer
with a ring-shaped converter target which let the main photon beam pass
through its central hole and only converted photons scattered between
1 and 3 mrad.
After preliminary studies at the Lund synchrotron we proposed a
collaboration with DESY to get access to the 7 GeV electron synchrotron
there. This was the first international collaboration by our department.
The experiment successfully measured the differential cross section for
Delbrück scattering for copper, silver, gold and uranium scattering
targets in the photon energy range of 1 to 7 GeV. The results indicated
a difference of a factor of 3 to 5 between the theoretical calculations
and our data points. Cheng and Wu had only considered the lowest order
diagram where two photons are exchanged with the target nucleus, but
when they heard about our results they realized that effects of
multi-photon exchange had to be included. After this correction had
been done very good agreement with experimental data was obtained.