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Light flavor-singlet scalars and walking signals in Nf = 8 QCD on the lattice / Yasumichi Aoki; Tatsumi Aoyama; Edward Bennett; Masafumi Kurachi; Toshihide Maskawa; Kohtaroh Miura; Kei-ichi Nagai; Hiroshi Ohki; Enrico Rinaldi; Akihiro Shibata; Koichi Yamawaki; Takeshi Yamazaki

Physical Review D

Swansea University Author: Bennett, Edward

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Abstract

Based on the highly improved staggered quark action, we perform lattice simulations of $N_f=8$ QCD and confirm our previous observations, both of a flavor-singlet scalar meson (denoted as $\sigma$) as light as the pion, and of various “walking signals” through the low-lying spectra, with higher stat...

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Published in: Physical Review D
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Abstract: Based on the highly improved staggered quark action, we perform lattice simulations of $N_f=8$ QCD and confirm our previous observations, both of a flavor-singlet scalar meson (denoted as $\sigma$) as light as the pion, and of various “walking signals” through the low-lying spectra, with higher statistics, smaller fermion masses $m_f$, and larger volumes.We measure $M_\pi$, $F_\pi$, $M_\rho$, $M_{a_0}$, $M_{a_1}$, $M_{b_1}$, $M_N$, $M_\sigma$, $F_\sigma$, $\langle \overline{\psi} \psi\rangle$ (both directly and through the GMOR relation), and the string tension.The data are consistent with the spontaneously broken phase of the chiral symmetry, in agreement with the previous results: ratios of the quantities to $M_\pi$ monotonically increase in the smaller $m_f$ region towards the chiral limit similarly to $N_f=4$ QCD, in sharp contrast to $N_f=12$ QCD where the ratios become flattened.We perform fits to chiral perturbation theory, with the value of $F_\pi$ found in the chiral limit extrapolation reduced dramatically to roughly 2/3 of the previous result, suggesting the theory is much closer to the conformal window.In fact, each quantity obeys the respective hyperscaling relation throughout a more extensive $m_f$ region compared with earlier works.The hyperscaling relation holds with roughly a universal value of the anomalous dimension, $\gamma_m \simeq 1$, with the notable exception of $M_\pi$ with $\gamma_m \simeq 0.6$ as in the previous results, which reflects the above growing up of the ratios towards the chiral limit.This is a salient feature (“walking signal”') of $N_f=8$, unlike either $N_f=4$ which has no hyperscaling relation at all, or $N_f=12$ QCD which exhibits universal hyperscaling.The effective $\gamma_m\equiv\gamma_m(m_f)$ of $M_\pi$ defined for each $m_f$ region has a tendency to grow towards unity near the chiral limit, in conformity with the Nambu-Goldstone boson nature, as opposed to the case of $N_f=12$ QCD where it is almost constant. We further confirm the previous observation of the light $\sigma$ with mass comparable to the pion in the studied $m_f$ region.In a chiral limit extrapolation of the $\sigma$ mass using the dilaton chiral perturbation theory and also using the simple linear fit, we find the value consistent with the 125 GeV Higgs boson within errors.Our results suggest that the theory could be a good candidate for walking technicolor model, having anomalous dimension $\gamma_m \simeq 1$ and a light flavor-singlet scalar meson as a technidilaton, which can be identified with the 125 GeV composite Higgs in the $N_f=8$ one-family model.
College: College of Science