Abstract
Spectroscopy of baryons and their excited states plays a key role for our under- standing of the strong interaction in the non-perturbative regime. Both, in theory and in experiment, large progress has been made during the last few years. The rapid develop- ments in lattice gauge calculations and the application of the Dyson-Schwinger equation to QCD have opened new perspectives for the interpretation of the excitation spectrum of the nucleon. In parallel, large efforts have been undertaken world-wide, and are still running, to investigate excited nucleon states experimentally, in particular with photon- induced production of mesons. In the present contribution we discuss such experimental programs conducted at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz. These programs are diverse. They include the measurement of cross sections, single- 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 the deuteron (and sometimes other light nuclei).
Highlights
The excitation spectrum of any bound system of particles is expected to reflect the properties of the underlying interaction
In the latter case, such studies have revealed very successfully the relevant effective degrees of freedom that allow us to characterize the different types of excited states like single-particle excitations or collective vibrations and rotations
Polarized target nucleons are available from buthanol targets in frozen-spin mode, the device used at MAMI is described in [14]
Summary
The excitation spectrum of any bound system of particles is expected to reflect the properties of the underlying interaction. Clustering of the quarks into di-quark structures, additional degrees of freedom due to excited flux tubes, and molecule-like states arising from coupled channel dynamics have been suggested [1] Such different effective degrees of freedom are reflected in the predicted excitation patterns, but so far agreement between all model predictions and the experimental data base was not good enough to draw any final conclusions. The Bonn ELSA and Mainz MAMI experiments can deliver linearly and circularly polarized photon beams and are equipped with solid-state cryo-targets that provide longitudinally or transversely polarized protons or deuterons Both experimental programs cover all aspects discussed above: the measurement of many different final states including meson pairs like ππ and πη, the measurement of all single and double polarization observables accessible with polarized beams or/and polarized targets and (in Mainz) the measurement of some observables involving recoil polarization, and the use of protons and neutrons as initial state nucleons. The experiments discussed here are sensitive to final states with two or more neutral mesons even when they are produced off the neutron
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