Narrow Results

Enlarging the set of hard exclusive reactions to be studied in the framework of QCD collinear factorization opens new possibilities to access generalized parton distributions (GPDs). We studied the photoproduction of a large invariant mass photon-photon or photon-meson pair in the generalized Bjorken regime which may be accessible both at JLab and at the EIC.

Weeks V and VI of the INT program 2018 were dedicated to physics opportunities at a future Electron-Ion Collider. Discussions were wide ranging and included topics such as short-range correlation, jets physics in deep inelastic scattering, nuclear PDFs, TMDs, and GPDs as well as small-x helicity distributions, non-linear small-x evolution equations, diffraction and particle production and correlations.

Besides a GPD based description, the collinear factorisation theorem of QCD also allows a TDA based description of the hard exclusive pion electroproduction. Both descriptions provide access to different kinematic regions and therefore give us insights into different aspects of the 3 dimensional nucleon structure. To understand the complete reaction mechanism and the validity of the different models, experimental studies over the full kinematic range have to be performed. In this work measurements of the beam-spin asymmetry have been performed over a wide range of kinematics in the deep inelastic regime for the hard exclusive e p → e n π⁺ reaction using the CEBAF Large Acceptance Spectrometer (CLAS) and a 5.5 GeV polarized electron beam at Jefferson Lab (JLAB). The ϕ dependence of the BSA as well as the Q², x_B and t dependence of the extracted moments will be presented. A clear sign change of the BSA can be observed between pions emitted in forward and backward direction with a smooth transition around 90° in CM.

Seven years of experimental efforts at Jefferson Lab on Deeply Virtual Compton Scattering yielded very significant results such as a strong indication for the handbag dominance (i.e. scaling) even et low energy, the first comparisons to models and a large data sample to constrain their free parameters and even a first model-dependent extraction of the total angular momentum carried by quarks. These proceedings try to summarize the information collected so far without going into details, and in a semi-chronological fashion.

Hard exclusive reactions are the tool to learn about generalized parton distributions, which provide a more complete parametrization of the nucleon than the ordinary parton distribution functions. Recent measurements by the HERMES collaboration of the exclusive production of photons, i.e., Deeply-Virtual Compton Scattering, are summarized and compared to model calculations, focusing on the measurements and model comparisons relevant to the extraction of quark orbital angular momentum and on the measurements on heavy nuclei.

The beam spin asymmetries of the reaction in the Bjorken regime were measured over a wide kinematical domain using the CLAS detector and a new lead-tungstate calorimeter. Through the interference of the Bethe-Heitler process with Deeply Virtual Compton Scattering, those asymmetries provide constraints for the nucleon Generalized Parton Distributions models. The observed shapes are in agreement with twist-2 dominance predictions.

Exclusive π⁰ electroproduction from nucleons can be related to transversity, the tensor charge, and other quantities related to transversity. This process isolates C-parity odd and chiral odd combinations of t-channel exchange quantum numbers. In a hadronic picture the meson production amplitudes for intermediate energy and Q² are determined by C–odd Regge exchanges with final state interactions. The helicity structure for this C-odd process relates to the quark helicity flip, or chiral odd generalized parton distributions (GPDs). Various cross sections and asymmetries will be sensitive to the transversity.

Exclusive meson leptoproduction from nucleons in the deeply virtual exchanged boson limit can be described by generalized parton distributions (GPDs). Including spin dependence in the description requires 8 independent quark-parton and gluon-parton functions. The chiral even subset of 4 quark-nucleon GPDs are related to nucleon form factors and to parton distribution functions. The chiral odd set of 4 quark-nucleon GPDs are related to transversity, the tensor charge, and other quantities related to transversity. Different meson or photon production processes access different combinations of GPDs. This is analyzed in terms of t-channel exchange quantum numbers, J^PC and it is shown that pseudoscalar production can isolate chiral odd GPDs. There is a sensitive dependence in various cross sections and asymmetries on the tensor charge of the nucleon and other transversity parameters. In a second section, analyticity and completeness are shown to limit the partonic interpret ation of the GPDs in the ERBL region.