Abstract
Photoproduction of mesons plays a key role for the investigation of the excitation spectrum of the nucleon and thus for our understanding of the strong interaction in the non-perturbative regime. In this contribution we discuss recent results from the experiments at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz. They include the measurement of cross sections and (double) polarization observables for single meson production and production of meson pairs off free protons as well as of quasi-free nucleons bound in light nuclei (in particular the deuteron).
Highlights
The excitation spectrum of a bound system of particles is expected to reflect the properties of the underlying interaction
In this contribution we discuss recent results from the experiments at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz. They include the measurement of cross sections and polarization observables for single meson production and production of meson pairs off free protons as well as of quasifree nucleons bound in light nuclei
The previously available data base had been analyzed with different reaction models and partial wave analyses like e.g. MAID, SAID, and the Bonn-Gatchina model (BnGn) [16,17,18]
Summary
The excitation spectrum of a bound system of particles is expected to reflect the properties of the underlying interaction. A similar approach for the relation between the excited states of nucleons and the fundamental properties of Quantum Chromodynamics (QCD) had so far much more limited success. Relativistic quark model approaches have been developed [2], and the application of the Dyson-Schwinger approach to QCD has led to promising results [3, 4]. The advances in lattice gauge calculations allowed first predictions of the excitation spectrum [5]. They “re-discovered” the SU(6) ⊗ O(3) excitation structure of the nucleon with a level counting consistent with the standard non-relativistic quark model
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