η meson production in deuteron-proton collisions at COSY-ANKE

The ANKE spectrometer (Apparatus for studies of Nucleon and Kaon Ejectiles) situated at the COSY of the Froschungszentrum Jülich is well suited to investigate η meson production in deuteron-proton collisions, especially close to threshold. By studying the reaction dp→3Heη, differential and total cross sections as well as the η meson mass have been determined with high accuracy. The very rapid rise of the total cross section to its maximum value within the first 0.5 MeV above the threshold implies a very large 3Heη scattering length, i.e., a very strong 3Heη final state interaction, and might indicate the presence of a quasi-bound state extremely close to threshold. In order to clarify the unambiguous η meson mass situation, a new measurement investigating the dp→3Heη reaction at 12 fixed energies at COSY-ANKE was conducted. Using this two-body reaction, the η meson mass can be extracted from pure kinematics through the determination of the production threshold. This requires to measure the 3Heη final state momenta pf and the associated beam momenta pd with highest precision.