QCD S Parameter from Inhomogeneous Bethe-Salpeter Equation

We calculate the low-energy parameter S in QCD, which is also known as L_{10}, and the pion decay constant f_\pi using inhomogeneous Bethe-Salpeter equation in improved ladder approximation. To extract these quantities we calculate the ``V-A'' two-point function, \Pi_{VV}(q^2) - \Pi_{AA}(q^2), in space-like region. We obtain S = 0.43 \sim 0.48, which is about 30% larger than the experimental value. The calculated f_\pi is well consistent with the result by solving the homogeneous Bethe-Salpeter equation for pion. We also evaluate $S$ parameter in SU(3) gauge theory with N_D doublets of fermions in connection with walking technicolor model, and find that the value of S/N_D hardly depends on N_D.