Baryon spectroscopy with polarized photon beams at ELSA

Abstract

The excitation spectrum of the nucleon appears as a composition of broad and overlapping resonances, which are not specifiable in the total cross section. The measurements of polarization observables, which increase the sensitivity to small resonance contributions, will be essential to reduce the ambiguities in the existing descriptions of the spectrum. This is a prerequisite to get a hand on the properties of individual states. At the electron stretcher ring ELSA of the University of Bonn, with maximum beam energy of 3.5 GeV, real photons are used to perform single-and double polarization experiments with the Crystal Barrel/TAPS experiment. The experimental setup includes a phi-symmetric calorimeter system covering almost the complete solid angle, surrounding a longitudinally or transversally polarized butanol target. The system is ideally suited to detect single and multiple neutral meson final states. Linearly polarized photon beams provide the basis for the measurement of azimuthal beam asymmetries, such as Σ (unpolarized target) and G (longitudinally polarized target), while circularly polarized photon beams allow the extraction of the beam-target asymmetry E. First results for the double polarization observables in single meson photoproduction are presented.