Narrow Results

We study the sigma meson in the QCD sum rule. We use the linear combination of five independent currents. We find that for some cases the Borel mass stability and threshold value stability are both weak, while for some cases not. To know the reason for this we ought to consider the π-π continuum contribution in the phenomenological side of the QCD sum rule. Then we study the stability of the extracted mass against the Borel mass and the threshold value and find the σ(600) mass at 530 MeV ± 40 MeV.

In the framework of two flavor quark-meson model we study the effect of mixing between effective quarkonium and tetraquark fields on chiral phase transition. Depending on the values of different parameters we explore different scenarios. The common feature among all of them is that the transition for quarkonium and tetraquark happening at the same temperature for all densities. We find a sufficiently strong negative cubic self interaction coupling constant of the tetraquark field can drive the chiral phase transition to first order even at zero quark chemical potential.