P.Moskal für die COSY-11-Kollaboration, Institute of Physics, Jagiellonian University, Cracow, Poland
Cross sections of production of the eta'(958)-meson, which is the heaviest meson of the pseudoscalar meson nonet, have not been measured at threshold in the pp scattering so far. Data will be presented at excitation energies in the range up to 4.1 MeV. They were taken at the internal facility COSY-11 [1] using a cluster target in front of a regular COSY dipole. For the two exit protons in the reaction pp->pp eta' the four-momentum vectors were determined by means of drift chambers and scintillators whereas the eta'-meson was identified exclusively via the missing mass method based on energy and momentum conservation laws. The absolute normalisation of the total cross section was obtained by comparing the simultaneously measured rate of elastically scattered protons to data recently published by the EDDA collaboration [2]. For excitation energies between 1.5 MeV and 4.1 MeV the cross sections range from 2.5 nb to 25 nb, being about a factor of 50 smaller than the ones for the eta and pi0 production processes. On the other hand, plotting the cross section as a function of the eta-parameter (eta = maximum meson momentum normalized by its mass) the eta'-meson data are on the same curve as the pi0 production rate, in contrast to the eta-meson cross section which is much larger. It might be speculated that the eta'-meson production mechanism - as known for the pi0 - is not mediated via a baryon resonance whereas the eta-meson production is dominated by the large N*(S11(1535)) resonance - eta-meson interaction. The experimental data will be compared with theoretical model predictions.
[1] S. Brauksiepe et al., Nucl. Instr. and Meth. A 376 (1996) 397
[2] D. Alberts et al., Phys. Rev. Lett. 78 (1997) 1652