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 d p → 3 He η , 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 3 H e η scattering length, i.e., a very strong 3 H e η 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 d p → 3 He η 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 3 H e η final state momenta p f and the associated beam momenta p d with highest precision.