Polarization in electron-proton scattering

Thesis



This thesis is concerned with polarization effects in elastic electron-proton scattering. The principal results are summarized below.

A general polarization analysis of high-energy electron-proton scattering is first presented, and the problem of a 'complete set' of experiments, which allow a unique determination of the elastic scattering matrix at one energy and scattering angle, is discussed. A straightforward reconstruction procedure is shown to be inadequate when an expansion of the strength of interaction in powers of andalpha;, the fine structure constant, is reasonable: an alternative procedure for this case involves using both electron and positron scattering experiments. Although the experimental evidence for the one-photon-exchange approximation in electron-proton scattering is generally accepted to be very good, from a strictly logical point of view the analysis is incomplete and does not permit an unambiguous determination of the two-photon-exchange amplitudes. It is shown that measurement of various polarization tensors for both electron and positron scattering could eliminate the ambiguity.

Some interesting polarization experiments are then considered, in the one-photon-exchange approximation. A specific experiment is suggested to measure the electric form factor of the proton G_E, and explicit expressions are derived for the proton depolarization tensor. In the time-like region, the form factors are not restricted to real values and a 'generalized' polarization-asymmetry relation for the two crossed-channel processes, PP andrarr; e⁺e^andminus; and e⁺e^andminus; andrarr; PP, may be derived. Violation of this relation would represent a violation of time-reversal-invariance and JCP-invariance.

The polarization of the recoil proton in elastic electron-proton scattering is strictly zero in the one-photon-exchange approximation: any non-zero polarization must, to lowest order in andalpha;, arise from interference of the one-photon-exchange amplitudes with two-photon-exchange amplitudes. The contribution to the polarization of the two-photon-exchange amplitude with an elastic intermediate state, is calculated for arbitrary form factors for the proton. Numerical results are displayed for two interesting parameterizations of the form factors. This elastic effect is expected to be a good approximation to the polarization up to energies where the production of inelastic states becomes important. A rigorous upper bound is derived for the contribution to the polarization, of all two-photon amplitudes involving multi-particle intermediate states: the bound involves measurable inelastic electron-proton cross sections. However, a rough numerical estimate shows the bound has very limited usefulness, since it leads to very large bounds at large momentum-transfers.

Authors: Hey, A; Hey, Anthony J.G.

• Publication date: 1970
• DOI: 10.5287/ora-azeyx7kbd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English