Title:

Polarization in electronproton scattering

This thesis is concerned with polarization effects in elastic electronproton scattering. The principal results are summarized below. A general polarization analysis of highenergy electronproton 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 α, 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 onephotonexchange approximation in electronproton 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 twophotonexchange 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 onephotonexchange 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 timelike region, the form factors are not restricted to real values and a 'generalized' polarizationasymmetry relation for the two crossedchannel processes, P‾P → e^{+}e^{} and e^{+}e^{} → P‾P, may be derived. Violation of this relation would represent a violation of timereversalinvariance and JCPinvariance. The polarization of the recoil proton in elastic electronproton scattering is strictly zero in the onephotonexchange approximation: any nonzero polarization must, to lowest order in α, arise from interference of the onephotonexchange amplitudes with twophotonexchange amplitudes. The contribution to the polarization of the twophotonexchange 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 twophoton amplitudes involving multiparticle intermediate states: the bound involves measurable inelastic electronproton cross sections. However, a rough numerical estimate shows the bound has very limited usefulness, since it leads to very large bounds at large momentumtransfers.
