Abstract
In order to gain a better understanding of the dynamics inside the nucleon and of the non-perturbative regime of QCD, the nucleon excitation spectra and the properties of nucleon resonances are investigated. An essential experimental tool to achieve this goal is the study of different photoproduction reactions. Partial wave analyses are performed in order to obtain information about the contributing resonances. A complete experiment is needed to extract the underlying amplitudes unambiguously, which requires the measurement of carefully chosen single and double polarization observables in addition to the unpolarized cross section. The CBELSA/TAPS experiment in Bonn offers the possibility to measure several polarization observables using a linearly or circularly polarized photon beam and with a longitudinally or transversely polarized target. This contribution gives an overview of recently measured polarization observables in different final states. The impact of the new data is discussed.
Highlights
Comparisons of the nucleon or delta excitation spectra to either quark model predictions [1] or latest lattice QCD calculations [2] reveal that the dynamics inside the nucleons are not well understood in the non-perturbative regime of QCD
For more than a decade the main focus of the CBELSA/TAPS experiment in Bonn has been dedicated to acquiring polarization observables for different final states
Since the CBELSA/TAPS experiment is equipped with two electromagnetic calorimeters, it is ideally suited for detecting photons in the final state
Summary
Comparisons of the nucleon or delta excitation spectra to either quark model predictions [1] or latest lattice QCD calculations [2] reveal that the dynamics inside the nucleons are not well understood in the non-perturbative regime of QCD. More states are predicted than there have been experimentally observed so far. This feature is known as the missing resonances problem. One possible explanation for this could be that most of the states, which were found in the past, were observed in πN scattering experiments. Partial wave analyses (PWA) are necessary to disentangle the contributing resonances and their parameters. For more than a decade the main focus of the CBELSA/TAPS experiment in Bonn has been dedicated to acquiring polarization observables for different final states. This contribution reports on the latest measured data and their impact
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