The Peskin-Takeuchi S parameter and vector meson decay constants in Nf=8 QCD
Abstract
We use lattice gauge theory to investigate the Peskin-Takeuchi S parameter in SU(3) gauge theory with eight light fundamental fermions (eight-flavor QCD), a candidate for walking technicolor. We develop a new approach to computing S using the first time moment of current correlators, in addition to the conventional vacuum-polarization method, and obtain consistent results. Finite-volume (FV) corrections are estimated using data at multiple lattice volumes and a formula motivated by chiral perturbation theory. At the smallest fermion mass, the one-doublet contribution is S = 0.281(5)(17), smaller than the value ∼ 0.35 in scaled-up 2+1-flavor QCD (without subtracting the SM contribution). However, contrary to naive expectations, S toward smaller fermion masses is not necessarily suppressed once FV corrections are included. The resulting Gasser-Leutwyler parameter L10r(Mρ) is similar to that in real-world QCD. We also determine the vector and axial-vector meson decay constants, Fρ and Fa1. The ratio Fρ/Fπ is stable against the fermion mass and consistent with the KSRF estimate, implying a ρππ coupling similar to that in real-world QCD. We predict the techni-rho decay width into weak bosons to be Γρ/Mρ≲ 0.24/N, where N is the number of weak doublets. We further find that the rho- and a1-dominated Weinberg spectral-function and Das-Mathur-Okubo sum rules are satisfied, yielding S and L10r(Mρ) consistent with our main results. We discuss implications for BSM model building.
Related
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BibTeX
@misc{aoki2026peskintakeuchi,
title = {{The Peskin-Takeuchi $S$ parameter and vector meson decay constants in $N_f=8$ QCD}},
author = {Yasumichi Aoki and Tatsumi Aoyama and Ed Bennett and Toshihide Maskawaa and Kohtaroh Miura and Hiroshi Ohki and Enrico Rinaldi and Akihiro Shibata and Koichi Yamawaki and Takeshi Yamazaki},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.35371},
eprint = {2609.35371},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.35371},
}
RIS
TY - UNPB
TI - The Peskin-Takeuchi S parameter and vector meson decay constants in N_f=8 QCD
AU - Yasumichi Aoki
AU - Tatsumi Aoyama
AU - Ed Bennett
AU - Toshihide Maskawaa
AU - Kohtaroh Miura
AU - Hiroshi Ohki
AU - Enrico Rinaldi
AU - Akihiro Shibata
AU - Koichi Yamawaki
AU - Takeshi Yamazaki
PY - 2026
DA - 2026/09/28
PB - arXiv
DO - 10.48550/arxiv.2609.35371
UR - https://arxiv.org/abs/2609.35371
AB - We use lattice gauge theory to investigate the Peskin-Takeuchi S parameter in SU(3) gauge theory with eight light fundamental fermions (eight-flavor QCD), a candidate for walking technicolor. We develop a new approach to computing S using the first time moment of current correlators, in addition to the conventional vacuum-polarization method, and obtain consistent results. Finite-volume (FV) corrections are estimated using data at multiple lattice volumes and a formula motivated by chiral perturbation theory. At the smallest fermion mass, the one-doublet contribution is S = 0.281(5)(17), smaller than the value ∼ 0.35 in scaled-up 2+1-flavor QCD (without subtracting the SM contribution). However, contrary to naive expectations, S toward smaller fermion masses is not necessarily suppressed once FV corrections are included. The resulting Gasser-Leutwyler parameter L_10^r(M_ρ) is similar to that in real-world QCD. We also determine the vector and axial-vector meson decay constants, F_ρ and F_a_1. The ratio F_ρ/F_π is stable against the fermion mass and consistent with the KSRF estimate, implying a ρππ coupling similar to that in real-world QCD. We predict the techni-rho decay width into weak bosons to be Γ_ρ/M_ρ≲ 0.24/N, where N is the number of weak doublets. We further find that the rho- and a_1-dominated Weinberg spectral-function and Das-Mathur-Okubo sum rules are satisfied, yielding S and L_10^r(M_ρ) consistent with our main results. We discuss implications for BSM model building.
ER -