Polarization observables in the eta meson production
  We propose to extend the investigations of the eta meson production in nucleon-nucleon collisions by studying single and double polarization observables. Over the last years, the eta meson has been studied at several different experiments mainly by measurements of unpolarized total and differential cross sections. To reveal the production mechanism, polarization observables help to disentangle the amplitudes of the partial waves and therefore to understand the underlying exchange processes. Analyzing powers have been measured enabling to separate the contributing amount of some of these amplitudes. Double polarization observables will widen the number of accessible amplitudes and hence give further constraints to the theoretical models. Due to the trigonometric symmetries in polarization measurements, a symmetric detector geometry in the azimuthal angle is preferable. The WASA detector with its 4-pi geometry for neutral and charged particles together with the use of a frozen spin target should therefore allow to study double polarization observables not only in vec{p}vec{p} but also in vec{p}vec{d} or vec{d}vec{p} collisions. This first requires the development of a frozen-spin target whereas single polarization observables can be measured directly with the current setup. 
P. Winter, R. Czyzykiewicz, H.-H. Adam, A. Budzanowski, D. Grzonka, M.   Janusz, L. Jarczyk, B. Kamys, A. Khoukaz, K. Kilian...