Session DA: Insights into Nucleon Structure

The 3D pictures of the nucleon: status and prospects

We propose an overview of the nucleon tomography in both mixed position-momentum space and pure momentum space. Three-dimensional pictures of the nucleon congregate a lot of information on Quantum ChromoDynamics in its non-perturbative regime, in particular about how the partons are distributed inside the nucleon and how initial and final-state interactions affect the system. After a short presentation of how such information can be accessed experimentally, we present selected recent developments and results. The last part of the presentation will be devoted to prospects.   

Baryon Structure and Spectroscopy at 6 and 12

Baryon spectroscopy can establish more completely and more sensitively, and in an almost model-independent way, nucleon excitations and non-reso\-nant reaction amplitudes by complete measurements of pseudo-scalar meson photoproduction off nucleons. However, beyond baryon spectroscopy at the real photon point $Q^2=0\,(GeV/c)^2$, electron scattering experiments can also investigate the internal hadronic structure at various distance scales by tuning the four-momentum transfer from $Q^2 \approx 0\,(GeV/c)^2$, where the meson cloud contributes significantly to the baryon structure, over intermediate $Q^2$, where the three constituent-quark core starts to dominate, to $Q^2$ up to $12\,(GeV/c)^2$, attainable after the 12-GeV upgrade at JLab, where the constituent quark gets more and more undressed towards the bare current quark. After establishing unprecedented high-precision data, the immanent next challenge are high-quality analyses to extract the baryon spectrum and the relevant electrocoupling parameters for various well-known resonances that then can be compared to state-of-the-art models and QCD-based calculations. Recent results will demonstrate the status of the analysis and of theoretical descriptions.   

Beyond the Born Approximation: Two Photon Exchange in Nucleon Structure Studies

Precision measurements of the electric form factor of the proton at Jefferson Lab have revealed a striking difference between Rosenbluth separation and polarization transfer data. Much of this discrepancy has been attributed to two-photon exchange, previously ignored in nucleon structure studies due to lack of precision in the data. Recently, experiments were conducted at Jefferson Lab where unpolarized electrons were scattered from neutrons polarized normal to the electron scattering plane. The polarized neutrons were produced by polarizing $^3$He nuclei using spin-exchange optical pumping. The single spin asymmetry for inclusive scattering, formed by flipping the target spin direction, is predicted to be exactly zero in the Born approximation by the Christ-Lee theorem. A non-zero asymmetry is an indication of contributions from two-photon exchange. This asymmetry provides a new tool to study the structure of the nucleon and to directly test models of two-photon exchange contributions. Results will be presented from measurements in the quasi-elastic and deep-inelastic regions.