Although photons couple only to electric charge and magnetic moment, they serve as important probes of hadrons—particles characterized primarily by their nuclear interactions. That’s true for several basic reasons:
For linearly polarized photon beams, the polarization direction gives a simple and revealing dependence of the differential scattering cross section dσ/dω on the azimuthal angle φ between the polarization direction and the production plane:
The production plane is defined by the beam and a specified particle emerging from the scattering event and ϑ is the scattering angle between that particle and the beam.
The functions A and B have simple expressions when individual electromagnetic multipole transition-matrix elements dominate the scattering process. That simplification lets the experimenter discriminate among different conjectured transition mechanisms. A famous example...
