Structure of the scalar mesonsf0(980) anda0(980)

We investigate the structure of the scalar mesons ${\mathit{f}}_0$(980) and ${\mathit{a}}_0$(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 with existing models, a somewhat large mass for the ${\mathit{f}}_0$(980) (${\mathit{m}}_{\mathit{f}0}$=1015 MeV, ${\mathrm{\Gamma }}_{\mathit{f}0}$=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 ${\mathit{a}}_0$(980) as a dynamically generated threshold effect (which is therefore neither a conventional qq¯ state nor a KK¯ bound state). Exploring the corresponding pole position the ${\mathit{a}}_0$(980) is found to be rather broad (${\mathit{m}}_{\mathit{a}0}$=991 MeV, ${\mathrm{\Gamma }}_{\mathit{a}0}$=202 MeV). The experimentally observed smaller width results from the influence of the nearby KK¯ threshold on this pole.