Speaker
Description
Exploring the excited nucleon spectrum gives insights into the strong interaction dynamics of quantum chromodynamics (QCD) and sheds light on the relevant degrees of freedom for QCD at low energies. In the last two decades, our kowledge of the nucleon excitation spectrum has advanced steadily by studying photoinduced reactions on proton and neutron targets. Measurements of single and double polarization observables using polarized photon beams and polarized targets play a key role for performing unambiguous partial wave analyses and in extracting resonance parameters from the experimental data.
The Crystal Ball experiment, which is located at the accelerator facility MAMI in Mainz, Germany, provides the possibility to measure with a polarized photon beam of up to 1.6 GeV energy, as well as to measure with a polarized target or with a nucleon recoil polarimeter. A large solid angle detector system consisting mainly of two calorimeters enables the measurements of polarization observables with high statistical accuracy and large kinematic angular coverage.
This talk will present recent results from the A2 Collaboration concerning single and double polarization observables, including first measurements of the double polarization observable $E$ using an elliptically polarized photon beam or the measurement of recoil polarization observables like $C_{x'}$.
