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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We investigate np scattering in the pseudoscalar (TT) and axial-vector (Ai) channel. Our model based on s-as well as t-channel meson exchange can resolve apparent discrepancies occurring in the analysis of the axial-vector Ai meson. Furthermore, it leads to an appreciable softening of the TTNN form factor in agreement with information from other sources. This strong contribution in the pionic channel substantiates earlier claims that inclusion of correlated Trp exchange might allow the use of a soft TTNN form factor in NN potential models and thus could resolve a long-standing puzzle.PACS numbers: 2L30.+y, 13.75.Gx, 13.75.Lb The form factor at a meson-baryon vertex is a welldefined physical concept, which takes into account the effects of the extension of the vertex. It effectively describes not only mesonic processes but also the inner quark-gluon structure of the hadrons involved.The structure of the TTNN vertex is of outstanding importance for a variety of problems in medium-energy physics. For example, the TTNN form factor modifies the one-pion-exchange NN potential (OPEP) and in this way influences the important tensor interaction between two nucleons. When both nucleons are kept on the mass shell this form factor can be suitably represented, in a phenomenological way, by a monopole parametrizationwhich fulfills the normalization condition r^^NNi'^ = mj) = 1. Here t is the four-momentum-transfer squared between two nucleons and A^^NN is a parameter, the socalled cutoff mass.In meson exchange NN interaction models this cutoff mass is usually adjusted to produce the best fit of the empirical NN data. The resulting values for A^^NN are rather large (> 1.3 GeV) leading to a mild suppression in the inner region (r < 1 fm, r being the distance between the two nucleons) only, and thus to a hard form factor. This is necessary in order to have a sufficiently strong tensor force to reproduce the deuteron properties, especially the asymptotic D-to 5-wave ratio and the quadrupole moment [1].There is a long-standing discrepancy between the rather large value seemingly required by the NN (deuteron) data and information from other sources; the latter consistently points to a much smaller value for AT^NNI around 0.8 GeV [2], i.e., to a rather "soft" form factor, and thus would lead to a much stronger suppression of the OPEP tensor force, in apparent disagreement with the deuteron data.A possible way out of this puzzle would be the existence of further (short-ranged) tensor contributions, strong enough to cancel the effect of the soft form factor. Indeed, in a recent work, Holinde and Thomas [3] were able to demonstrate that a comparably good fit of A^A^ data can be obtained if a soft TTNN form factor is used (ATTTVAT = 0.8 GeV), provided an additional shortranged tensor force is introduced, parametrized in terms of an exchange of a heavy pion TT' with mass 1.2 GeV and phenomenologically adjusted strength.The decisive question is of course whether microscopic strong interaction mechanisms can be identified, which stand be...