Results from EDDA@COSY: Spin Observables in Proton‐Proton Elastic Scattering

Elastic proton‐proton scattering as one of the fundamental hadronic reactions has been studied with the internal target experiment EDDA at the Cooler‐Synchrotron COSY/Jülich. A precise measurement of differential cross section, analyzing power and three spin‐correlation parameters over a large angular (θ_c.m. ≈ 35° – 90°) and energy (T_p ≈ 0.5 – 2.5 GeV) range has been carried out in the past years. By taking scattering data during the acceleration of the COSY beam, excitation functions were measured in small energy steps and consistent normalization with respect to luminosity and polarization. The experiment uses internal fiber targets and a polarized hydrogen atomic‐beam target in conjunction with a double‐layered, cylindrical scintillator hodoscope for particle detection. The results on differential cross sections and analyzing powers have been published and helped to improve phase shift solutions. Recently data taking with polarized beam and target has been completed. Preliminary results for the spin‐correlation parameters A _NN, A_SS, and A_SL are presented. The observable A_SS has been measured the first time above 800 MeV and our results are in sharp contrast to phase‐shift predictions at higher energies. Our analysis shows that some of the ambiguities in the direct reconstruction of scattering amplitudes which also show up as differences between available phase‐shift solutions, will be reduced by these new measurements.