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Register a new userWalking and conformal dynamics in many-flavor QCD
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In the search for a realistic walking technicolor model, QCD with many flavors is an attractive candidate. From the series of studies by the LatKMI collaboration, we present updated results of the scaling properties of various hadron spectra, including the (pseudo)scalar, vector, and baryon channels, for $N_f=8$ QCD analyzed with the HISQ action. By comparing these with $N_f=12$ QCD, which has properties consistent with conformality, possible signals of walking dynamics are discussed. We also present a preliminary result of the flavor-singlet pseudoscalar mass in many-flavor QCD.
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We present the first report of the LatKMI collaboration on the the lattice QCD simulation performed at the KMI computer, $varphi$, for the cases of 4 flavors and 8 flavors, the latter being expected to be a candidate for the walking technicolor having an approximate scale invariance near the infrared fixed point. The simulation was carried out based on the highly improved staggered quark (HISQ) action. In this proceedings, we report preliminary results on the spectrum, analyzed through the chiral perturbation theory and the finite-size hyperscaling. We observe qualitatively different behavior of the 8-flavor case in contrast to the 4-flavor case which shows clear indication of the hadronic phase as in the usual QCD.
Information of the phase structure of many flavor SU(3) gauge theory is of great interest for finding a theory which dynamically breaks the electro-weak symmetry. We study the SU(3) gauge theory with fermions for $N_f=12$ and 16 in fundamental representation. Both of them, through perturbation theory, reside in the conformal phase. We try to determine the phase of each theory non-perturbatively with lattice simulation and to find the characteristic behavior of the physical quantities in the phase. HISQ type staggered fermions are used to reduce the discretization error which could compromise the behavior of the physical quantity to determine the phase structure at non-zero lattice spacings. Spectral quantities such as bound state masses of meson channel and meson decay constants are investigated with careful finite volume analysis. Our data favor the conformal over chiral symmetry breaking scenario for both $N_f=12$ and 16.
Based on the highly improved staggered quark action, we perform lattice simulations of $N_f=8$ QCD and confirm our previous observation of a flavor-singlet scalar meson (denoted as $sigma$) as light as the pion and various walking signals through low-lying spectra, with higher statistics, smaller fermion masses $m_f$, and larger volumes. We measure $M_pi$, $F_pi$, $M_rho$, $M_{a_0}$, $M_{a_1}$, $M_{b_1}$, $M_N$, $M_sigma$, $F_sigma$, $langle bar{psi} psirangle$ (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. The hyperscaling relation holds with roughly a universal value of the anomalous dimension, $gamma_m simeq 1$, with a notable exception of $M_pi$ with $gamma_m simeq 0.6$ as in the previous results. 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. 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 $N_f=8$ one-family model.
Motivated by recent progress on many flavor QCD on a lattice, we investigate conformal/walking dynamics by using Schwinger-Dyson (SD) equation within an improved ladder approximation for two-loop running coupling. By numerically solving the SD equation, we obtain a pole mass $m_{p}$, pion decay constant $f_{pi}$, and investigate the chiral symmetry breaking and mass anomalous dimension $gamma_{m}$ in the presence of IR cutoffs $Lambda_{mathrm{IR}}$. We find that the chiral symmetry breaking is suppressed if IR cutoff $Lambda_{mathrm{IR}}$ becomes larger than the critical value near the dynamical mass ($Lambda_{mathrm{IR}}$ $simeq m_{D}$) In the conformal phase the $gamma_{m}$ is strongly suppressed by IR cutoffs for $Lambda _{mathrm{IR}}$ $simeq m_{p}$. We, then, obtain finite size hyperscaling (FSS) relation by adapting a linearized approximation for the SD equation, and examine the $gamma_{m}$ The results offer valuable insight and suggestion for analyses in lattice gauge theories.
LatKMI Collaboration discusses the topological insights in many-flavor QCD on the lattice. We explore walking/conformal/confining phase in $N_mathrm{f}$ = 4, 8 and 12 (in particular $N_mathrm{f}$ = 8) lattice QCD via the topological charge and susceptibility, eigenvalues and anomalous dimension.
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