Light scalar mesons in QCD sum rules with inclusion of instantons

The light scalar meson nonet above 1 GeV ({\it i.e.}the $a_0$, $K_0^{\ast }$ and $f_0$) are studied within the framework of QCD sum rules. In conventional QCD sum rules, the calculated masses of this nonet are degenerate, and the mass of $K_0^{\ast }$ is always larger than the $a_0$ in contradiction with the observed spectrum. After improving the correlation function by including instanton effects, the masses are well separated from each other. In particular, our results shows glueball content plays an important role in the underlying structure of $f_0(1500)$. The decay constants are also discussed.