Speaker: Mark Paris (T-16)

Electromagnetic response of confined Dirac particle

The quark-parton model has been successful in describing perturbative aspects of hadronic structure in terms of a single particle model. We are beginning to appreciate however that the single particle model has fundamental limitations. For example, it cannot describe the spectral function of constituents in a hadron. The quark-parton model neglects interactions of the struck constituent with the spectators in the target. However, it has recently been shown that one must take into account final state interactions when relating structure functions measured in deep inelastic scattering to parton probabilities. Single spin asymmetries in semi-inclusive deep inelastic scattering also receive contributions from final state interactions of the struck constituent with the spectators. In this talk I will present a simple, non-perturbative model of a single confined Dirac particle in a potential well. The response is calculated exactly and compared with quark-parton model approximations where one treats the constituents as being on their mass shell. The exact response in is seen to have about 10% of its strength in the timelike region. This contribution is precluded in the quark-parton model and is therefore omitted from the quark-parton model sum rules. We calculate deviations from the Callan-Gross relation which are observed experimentally to be 10-15%.