Narrow Results

With the new stage of the MAMI accelerator, the physics program on hadron physics at MAMI will be extended up to an end point energy of 1.5 GeV. A few selected experiments on electro production are presented to show examples on nucleon structure and baryon resonance structure studies. The photo production program at MAMI was described in a separate talk in this conference.

Transverse single spin asymmetries in inclusive deep-inelastic lepton-nucleon scattering can be generated through multi-photon exchange between the leptonic and the hadronic part of the process. Here we consider two-photon exchange, and mainly focus on the transverse target spin asymmetry. In particular, we investigate the case where two photons couple to different quarks. Such a contribution involves a quark-photon-quark correlator in the nucleon, which has a (model-dependent) relation to the Efremov-Teryaev-Qiu-Sterman quark-gluon-quark correlator T_F. Using different parameterizations for T_F we compute the transverse target spin asymmetry for both a proton and a neutron target, and compare the results to recent experimental data. Potential implications on our general understanding of single spin asymmetries in hard scattering processes are discussed as well.

The single-spin asymmetry of unpolarized leptons scattering deep-inelastically off transversely polarized nucleons is discussed. This observable is generated by a two-photon exchange between lepton and nucleon. In a partonic description of the asymmetry the non-perturbative part is given in terms of multiparton correlations: quark-gluon correlation functions and quark-photon correlation functions. Using input from models and data fits for these twist-3 correlation functions, estimates for the single-spin asymmetries for a proton and a neutron are presented.

Recent theoretical developments in the studies of two-photon exchange effects in elastic electron-proton scattering are reviewed. Two-photon exchange mechanism is considered a likely source of discrepancy between polarized and un-polarized experimental measurements of the proton electric form factor at momentum transfers of several GeV². This mechanism predicts measurable effects that are currently studied experimentally.