CHAPTER X - INTERACTION OF ELECTRONS WITH PHOTONS

Abstract

This chapter discusses the interaction of electrons with photons. It presents the calculation of the cross section for the scattering of polarized photons by an unpolarized electron, summed over polarizations of the final electron. The cross section for scattering of unpolarized photons with detection of a polarized photon has similar properties. It has been shown that the qualitative properties of the polarization effects in the scattering of photons by electrons follow from the general requirements of symmetry. A pseudoscalar cannot be formed from two polar vectors. The parameters of linear polarization are related to the components of the two-dimensional symmetric tensor. Reaction channels differing by an interchange of initial and final photons correspond to the same process—scattering of a photon by an electron. The squared modulus of the amplitude and the scattering cross section must be invariant under a transformation that expresses the change from one of these channels to the other. Owing to the conservation of momentum, the annihilation of the electron and positron in positronium must be accompanied by the emission of at least two photons. The main process determining the lifetime of positronium is two-photon annihilation for parapositronium and three-photon annihilation for orthopositronium. The decay probability can be related to the cross section for annihilation of a free pair.