Speaker
Description
The excitation spectrum of the nucleon is an important testing ground for quantum chromodynamics in the regime where it cannot be treated perturbatively. During the last two decades much progress has been made on the theory side, e.g. lattice gauge methods, and in experiments, particularly using energy tagged photon beams at electron accelerators, which has now reached a state where not only differential cross sections but also asymmetries measured with polarized photons and polarized targets allow for detailed partial wave analyses. This provides much more stringent information about the involved reaction multipoles and thus the contributing nucleon resonances.
The present experiment was done at the ELSA accelerator in Bonn with the CBELSA/TAPS detector setup. The incident electron beam of $3.2$ GeV impinged on a diamond radiator where it produced coherent bremsstrahlung photons with linear polarization, which again impinged on a transversely polarized, deuterated butanol target. This allows the simultaneous measurement of the polarization observables $\Sigma$, $T$, $P$, and $H$. Analyzed were the final states $\pi^0 N$ and $\eta N$ with the almost $4\pi$ covering electromagnetic calorimeter CBELSA/TAPS.
One of the main motivations of this experiment was a more detailed investigation of the not yet understood narrow structure in the excitation function of the $\eta n$ reaction at approximately $1$ GeV.
