Improved sum rules for light mesons and thermal hadronic threshold

The thermal properties of light vector and scalar mesons are investigated in the framework of QCD sum rules. The phenomenological side of the correlation function can be calculated using either the quark-hadron duality approach or in terms of two-pion continuum contributions. In the quark-hadron duality approach, a free parameter (hadronic threshold) arises in the phenomenological part and it is necessary to know the temperature dependence of the hadronic threshold. A comparison of above mentioned approaches gives us additional information about the temperature dependence of the hadronic threshold. Taking into account the thermal spectral density and additional operators appearing at finite temperature the thermal QCD sum rules are improved. The decay constants of ρ and σ mesons are calculated and our investigations show that the above mentioned methods give us the same results.