Abstract
The main goal of the CBELSA/TAPS experiment is the investigation of the nucleon excitation spectrum, which consists of many overlapping resonances. In order to disentangle the different resonance contributions, a partial wave analysis is necessary. In the field of photoproduction of single pseudoscalar mesons, the measurement of a well chosen set of at least eight single and double polarization observables allows for the determination of an unambiguous solution. Of particular interest is the η ′ meson since it couples only to resonances with isospin , thus reducing the number of overlapping resonances. Additionally, its comparatively high mass gives access to the poorly understood regime of high-lying resonances.With the CBELSA/TAPS experiment at the electron stretcher accelerator ELSA, double polarization observables such as E can be obtained by studying photoproduction reactions using a circularly polarized photon beam in combination with a longitudinally polarized butanol target. The decay mode η′ → γγ was analyzed for a beam photon energy range of 1447-2350 MeV. The preliminary results for the double polarization observable E are shown.
Highlights
Current baryon models predict more resonance states than experimentally observed in the excitation spectrum, especially in the high mass region [1]
The main goal of the CBELSA/TAPS experiment is the investigation of the nucleon excitation spectrum, which consists of many overlapping resonances
In the field of photoproduction of single pseudoscalar mesons, the measurement of a well chosen set of at least eight single and double polarization observables allows for the determination of an unambiguous solution
Summary
Current baryon models predict more resonance states than experimentally observed in the excitation spectrum, especially in the high mass region [1]. An effective Langrangian [4] and an isobar model [5] were recently used to fit the high precision data of CLAS and CBELSA/TAPS. The latter η -MAID model takes t-channel vector meson exchange in combination with a set of resonances into account [5]. One of the fit solutions considers the four resonances S 11(1904), P13(1926), P11(2083) and D13(2100) to play an important role in the photoproduction of η [5]. The former two resonances are not listed in the Particle Data Tables and could contribute to the missing resonances, if they exist.
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