$S$ Parameter in the Holographic Walking/Conformal Technicolor

Abstract

We explicitly calculate the $S$ parameter in entire parameter space of the holographic walking/conformal technicolor (W/C TC), based on the deformation of the holographic QCD by varying the anomalous dimension from $\gamma_m \simeq 0$ through $\gamma_m \simeq 1$ continuously. The $S$ parameter is given as a positive monotonic function of $\xi$ which is fairly insensitive to $\gamma_m$ and continuously vanishes as $S \sim \xi^2 \to 0$ when $\xi \to 0$, where $\xi$ is the vacuum expectation value of the bulk scalar field at the infrared boundary of the 5th dimension $z=z_m$ and is related to the mass of (techni-) $\rho$ meson ($M_\rho$) and the decay constant ($f_\pi$) as $\xi \sim f_\pi z_m \sim f_\pi/M_\rho$ for $\xi \ll 1$. However, although $\xi$ is related to the techni-fermion condensate $\condense$, we find no particular suppression of $\xi$ and hence of $S$ due to large $\gamma_m$, based on the correct identification of the renormalization-point dependence of $\condense$ in contrast to the literature. Then we argue possible behaviors of $f_\pi/M_\rho$ as $\condense \to 0$ near the conformal window characterized by the Banks-Zaks infrared fixed point in more explicit dynamics with $\gamma_m \simeq 1$. It is a curious coincidence that the result from ladder Schwinger-Dyson and Bethe-Salpeter equations well fits in the parameter space obtained in this paper. When $f_\pi/M_\rho \to 0$ is realized, the holography suggests a novel possibility that $f_\pi$ vanishes much faster than the dynamical mass $m$ does.