eta physics at threshold


The production of eta and eta' mesons in elementary proton-proton collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE. The high quality proton beam with low emittance and small momentum spread permits to study the creation of these mesons very close to the kinematical threshold, where - due to the rapid growth of the phase space volume - the total cross section increases by orders of magnitude over a few MeV range of the excess energy.

The experimental data allows to establish that the dependence of the total cross section on the excess energy is predominantly determined by the final state interaction among the produced particles. In case of the pp->pp eta reaction the achieved precision permits to distinguish the effect of the eta-nucleon interaction from the overwhelming influence of the nucleon-nucleon distortion. Moreover, by the comparison of the eta and eta' production one can deduce the relative strength of the eta-nucleon and eta'-nucleon interactions.

A modification of the phase space occupation due to the eta pp interaction is evidently seen in the high statistics exclusive measurements of the pp->pp eta reaction recently performed at COSY.

Because of the high momentum transfer involved, the production probes the short range (~0.3 fm) of the nucleon-nucleon interaction. At such small distances residual color forces may play a significant role. The relevance of hadronic and quark degrees of freedom will be briefly reported. In case of the production of eta and eta' mesons via the proton-proton collision the dominance of the 3P0->Ss transition is expected up to an excess energy of about 40 MeV and 100 MeV, respectively. This crucially simplifies the study of the meson-nucleon-nucleon interaction and of the primary production mechanism. The latter manifests itself in practice in the magnitude of the total cross section only. Especially interesting is the relative magnitude of the total cross section for the different isospin channels. For the eta production in proton-proton and proton-neutron collisions the ratio of the total cross sections implies that the yield of eta in the isosinglet nucleons configuration exceeds the one in the isospin triplet state by more than an order of magnitude.

The presentation will comprise the description of the experimental methods and the interpretation of the available data in view of the production mechanism and the meson-nucleon interaction. The eta and eta' production in the scattering of free nucleons as well as the quasi free creations off nucleons inside a deuteron will be considered.

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Eur.Phys.Jou. A 18 (2003) 335-338