QCD S parameter from inhomogeneous Bethe-Salpeter equation.

We calculate the low-energy parameter S in QCD, which is also known as L10, and the pion decay constant fπ using inhomogeneous Bethe-Salpeter equation in improved ladder approximation. To extract these quantities we calculate the “V − A” two-point function, ΠV V (q 2) − ΠAA(q 2), in space-like region. We obtain S = 0.43 ∼ 0.48, which is about 30% larger than the experimental value. The calculated fπ 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 ND doublets of fermions in connection with walking technicolor model, and find that the value of S/ND hardly depends on ND. Fellow of the Japan Society for the Promotion of Science for Japanese Junior Scientists. e-mail address:[email protected] e-mail address:[email protected]