Abstract
The excitation spectrum of the proton consists of resonances with substancial width which are often strongly overlapping and are therefore difficult to disentangle. To determine the exact contributions and identify these resonances, a partial wave analysis solution has to be found. For a complete experiment, which leads to an unambiguous solution, several single and double polarization observables are needed. With the Crystal Barrel/TAPS experiment at ELSA, the measurement of double polarization observables in different reactions is possible by using a circularly or linearly polarized photon beam on a transversely or longitudinally polarized butanol target.
Highlights
For the nucleon excitation spectrum several theoretical predictions are available
The excitation spectrum of the proton consists of resonances with substancial width which are often strongly overlapping and are difficult to disentangle
For a complete experiment, which leads to an unambiguous solution, several single and double polarization observables are needed
Summary
Aside from constituent quark models, first lattice QCD calculations have been conducted. Even if the lattice QCD calculations are performed at a non-physical pion mass, the correct level counting of the states is already visible. In both models states have been predicted, which were not confirmed so far and the most promising search can be done using photoproduction. The resonant states have a large width and overlap, see Fig. 1. For the extraction of these resonances from the data, partial wave analyses have to be performed, which need a preferably complete database of polarization observables as input
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