Eta and eta' production at COSY-Jüich

P.Moskal for the COSY-11-Collaboration

One of the topics investigated at the COSY-Jülich accelerator is the production of the eta and eta' mesons in the proton-proton interaction close to the kinematical threshold. Similar studies, however, by means of different detection methods, are performed at the CELSIUS and SATURNE accelerators.
Recent results concerning both eta and eta' mesons production in the proton-proton collisions enables one besides the investigations of the primary production mechanism also study of the eta-proton and eta'-proton interaction. The available experimantal data made possible to conclude that the eta meson is predominantly produced via the excitation of the S-wave baryonic resonance and that the interaction of eta meson with proton is attractive. In contrary, in the case of the eta' meson, production via excitation of the baryonic resonance seems to be not significant and the eta'-proton interaction seems to be repulsive, however, the latter conclusion is still very uncertain. A plausible production mechanism of the eta'-proton mesons can be a fusion of gluons emitted from the two colliding protons, which is complementary to meson exchange current and would probe the gluonic content of this meson.
According to the SU(3) classification the eta- and eta'-mesons together with pions and kaons belong to the ground pseudoscalar meson nonet. The hadron-quark mass formula, which describes quite well masses of the octet members implies that the eta' meson mass should be about factor of three smaller than the observed one. This discrepancy and the anomalous large branching ratio of the B-mesons decay into eta' suggest other than the quarkonium components within the eta' structure. The structure of the eta' meson enters in the meson exchange models as the effective coupling constant, which is still not well known. The better estimation of this constant and the investigation of the possible production processes of the eta' meson in the collisions of protons constitutes the basic motivation for the performed measurements, which will be presented in this talk.
The discussion of the above issues will be interlaced with the presentation of the experminetal method used by the COSY-11 group for the measurement of the pp->pp eta' and pp->pp eta reactions, which have been performed at the cooler synchrotron COSY in Jülich.