Meson Exchange Versus Quark Models in Exclusive Meson Production

As an example for exclusive near threshold meson production, we investigate the production of the pseudo-scalar η meson in a microscopic meson exchange (MECM) and in a relativistic quark model, based on a gluon or instanton qbar q pair creation (RQM). We describe the baryons as covariant quark - scalar diquark systems with harmonic confinement; the excitation of intermediate baryon resonances is accounted for by meson or by colorless 2-gluon (Pomeron) exchange in the MECM and RQM, respectively. We find that both models account for the energy dependence of the total cross section near the η threshold, reflecting the dominance of the S₁₁(1535) baryon resonance. As a more stringent test we propose the recoilless excitation of the η meson in nuclei.