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We investigate the structure of the scalar mesons f 0 (975) and a 0 (980) within realistic meson-exchange models of the ππ and πη interactions. Starting from a modified version of the Jülich model for ππ scattering we perform an analysis of the pole structure of the resulting scattering amplitude and find, in contrast to existing models, a somewhat large mass for the f 0 (975) (m f 0 = 1015 MeV, Γ f 0 = 30 MeV). It is shown that our model provides a description of J/ψ → φππ/φKK data comparable in quality with those of alternative models. Furthermore, the formalism developed for the ππ system is consistently extended to the πη interaction leading to a description of the a 0 (980) as a dynamically generated threshold effect (which is therefore neither a conventional qq state nor a KK bound state). Exploring the corresponding pole 1 position the a 0 (980) is found to be rather broad (m a 0 = 991 MeV, Γ a 0 = 202 MeV). The experimentally observed smaller width results from the influence of the nearby KK threshold on this pole.

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A dynamical model for pseudoscalar-pseudoscalar meson scattering based on meson exchange, suitable for use in a variety of low-and intermediate-energy mesonic interactions, has been constructed and applied to rr and Krr scattering with good quantitative results. The model includes both sand t-channel exchange, and is found to require pseudoscalar-pseudoscalar coupling to a scalar octet to fit the high energy s-wave phases in the I = 0 r~ channel and in the I =f KQT channel. Coupling of the rr and KI? channels is found to play a crucial role in explaining the S*(975) resonance.

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