Two-photon interactions with baryons in a Faddeev approach

The main objective of this proposal is to understand the interactions of baryons with two photons, i.e., the Compton scattering process off a baryon, from its underlying substructure in terms of quarks and gluons. These are the elementary degrees of freedom in Quantum Chromodynamics (QCD) which is the fundamental theory of strong interactions. The Compton scattering vertex appears in many interesting physical contexts. One prominent example is the issue of two-photon corrections in the determination of the proton`s electric over magnetic form factor ratio. Such corrections are presently believed to explain the discrepancy in results obtained from two different measurement methods. On the other hand, the Compton amplitude at large photon virtuality can be represented by the convolution of a hard scattering process with generalized parton distributions (GPDs) that include the nucleon`s non-perturbative substructure. GPDs provide access to a wealth of interesting hadron structure properties and their determination from experimental data, models and lattice QCD has initiated a fertile area of research. Finally, via crossing symmetry of the Compton amplitude one obtains the proton-antiproton annihilation process which will play a prominent role in future experiments at FAIR/GSI. The Compton amplitude shall be described within a covariant Faddeev approach which is based on the Dyson- Schwinger equations (DSEs) of QCD and generalizes previous quark-diquark studies towards systems of three genuine valence quarks. The binding of a baryon is described by a pairwise and iterated dressed gluon exchange between two dressed quarks which, in related meson studies, is consistent with aspects of chiral symmetry and realizes its spontaneous and dynamical breaking. The corresponding nucleon amplitude as a solution of the covariant Faddeev equation defines the subsequent input to the Compton scattering vertex. A calculation of this vertex allows to study the processes described above. The proposed program aims at a computation of two-photon corrections to form factors, the description of proton-antiproton annihilation and the investigation of GPDs in a non-perturbative continuum description of strongly interacting QCD. It is closely related to current and future experimental efforts and bears many interesting future opportunities in hadron physics.