Structure of the scalar mesons f 0 (980) and a 0 (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 \ensuremath{\pi}\ensuremath{\pi} and \ensuremath{\pi}\ensuremath{\eta} interactions. Starting from a modified version of the J\"ulich model for \ensuremath{\pi}\ensuremath{\pi} 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, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{f}0}$=30 MeV). It is shown that our model provides a description of J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\varphi}\ensuremath{\pi}\ensuremath{\pi}/\ensuremath{\varphi}KK data comparable in quality with those of alternative models. Furthermore, the formalism developed for the \ensuremath{\pi}\ensuremath{\pi} system is consistently extended to the \ensuremath{\pi}\ensuremath{\eta} interaction leading to a description of the ${\mathit{a}}_{0}$(980) as a dynamically generated threshold effect (which is therefore neither a conventional qq\ifmmode\bar\else\textasciimacron\fi{} state nor a KK\ifmmode\bar\else\textasciimacron\fi{} 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, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{a}0}$=202 MeV). The experimentally observed smaller width results from the influence of the nearby KK\ifmmode\bar\else\textasciimacron\fi{} threshold on this pole.