Abstract

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive pi ^0 and eta photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the gamma p rightarrow pi ^0 p and gamma p rightarrow eta p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the eta n system at W = 1.68 textrm{GeV}. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the S_{11}(1535) and S_{11}(1650) resonances within the S_{11}-partial wave.

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