arXiv subjects

Koichi Yamawaki

Publications and source records attributed to Koichi Yamawaki.

At least 19 recordsLinked to original sources

arXiv2026

The Peskin-Takeuchi $S$ parameter and vector meson decay constants in $N_f=8$ QCD

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 $\sim 0.35$ 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 $Γ_ρ/M_ρ\lesssim 0.24/N$, 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.

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arXiv2026

A novel view of the flavor-singlet spectrum from multi-flavor QCD on the lattice

SU(3) gauge theories with increasing number of light fermions are the templates of strongly interacting sectors and studying their low-energy dynamics and spectrum is important, both for understanding the strong dynamics of QCD itself, but also for discovering viable UV completions of beyond the Standard Model physics. In order to contrast many-flavors strongly interacting theories with QCD on a quantitative footing, we use Lattice Field Theory simulations. We focus on the study of the flavor-singlet spectrum in the scalar and pseudoscalar channels: this is an interesting probe of the dynamics of the strongly interacting sector, as reminded by the QCD case with the $f_0(500)$ ($σ$) and $η^\prime$ mesons. The hierarchy of the spectrum of a strongly coupled new gauge sector of the Standard Model defines the potential reach of future colliders for new physics discoveries. In addition to a novel hierarchy with light scalars, introducing many light flavors at fixed number of colors can influence the dynamics of the lightest flavor-singlet pseudoscalar. We present a complete lattice study of both these flavor-singlet channels on high-statistics gauge ensembles generated by the LatKMI collaboration with 4, 8, and 12 copies of light mass-degenerate fermions. We also present other hadron masses on the lightest ensemble for $N_f=8$ generated by the LatKMI collaboration and discuss the chiral extrapolation of the spectrum in this particular theory. We contrast the results to $N_f=4$ simulations and previous results of $N_f=12$ simulations.

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arXiv2023

Proving Rho Meson be a Dynamical Gauge Boson of Hidden Local Symmetry

The rho meson has long been successfully identified with a dynamical gauge boson of the Hidden Local Symmetry (HLS) $H_{\rm local}$ in the nonlinear sigma model $G/H$ gauge equivalent to the model having the symmetry $G_{\rm global}\times H_{\rm local}$, with $G= [SU(2)_L \times SU(2)_R]\simeq O(4), H=SU(2)_{V}\simeq O(3)$, however under a hitherto unproven assumption that its kinetic term is dynamically generated, together with an ad hoc choice of the auxiliary field parameter "$a=2$". We prove this assumption, thereby solving the long-standing mystery: The rho meson kinetic term is generated simply by the large $N$ limit of the Grassmannian model $G/H=O(N)/[O(N-3)\times O(3)] $ gauge equivalent to $O(N)_{\rm global}\times [O(N-3)\times O(3)]_{\rm local}$, extrapolated to $N=4$, $O(4)_{\rm global}\times O(3)_{\rm local}$, with all the phenomenologically successful "$a=2$ results" i.e., $ρ$-universality, KSRF relation and the Vector Meson Dominance, realized {\it independently of the parameter "$a$".} This in turn establishes validity of the large $N$ dynamics at quantitative level directly by the experiments. The relevant cutoff reads $Λ\simeq 4 πF_π$ for $N=4$, which is regarded as a matching scale of the HLS as a "magnetic dual" to QCD. Skyrmion is stabilized by such a dynamically generated rho meson without recourse to the underlying QCD, further signal of the duality. The unbroken phase with massless rho meson may be realized as a novel chiral restored hadronic phase in hot/dense QCD.

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arXiv2023

Anomalous Dimension in QCD

The anomalous dimension $γ_m =1$ in the infrared region near conformal edge in the broken phase of the large $N_f$ QCD has been shown by the ladder Schwinger-Dyson equation and also by the lattice simulation for $N_f=8$ for $ N_c=3$. Recently Zwicky claimed another independent argument (without referring to explicit dynamics) for the same result, $γ_m =1$. We show that this is not justified by explicit evaluation of each matrix element based on the ``dilaton chiral perturbation theory (dChPT)'' : $<π(p_2)| 2\cdot \sum^{N_f}_{i=1} m_f \bar ψ_i ψ_i |π(p_1)>= 2M_π^2 + [(1-γ_m) M_π^2\cdot 2/(1+γ_m)]= 2 M_π^2 \cdot 2/(1+γ_m) \ne 2 M_π^2$ in contradiction with his estimate, which is compared with $<π(p_2)| (1+γ_m) \cdot \sum^{N_f}_{i=1} m_f \bar ψ_i ψ_i |π(p_1)> =(1+γ_m) M_π^2+ [(1-γ_m) M_π^2]=2 M_π^2$ (both up to trace anomaly), where the terms in $[ \,\,]$ are from the $σ$ (pseudo-dilaton) pole contribution. Thus there is no constraint on $γ_m$ when the $σ$ pole contribution is treated consistently for both.

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arXiv2019

Gravitational Waves from Walking Technicolor

We study gravitational waves from the first-order electroweak phase transition in the $SU(N_c)$ gauge theory with $N_f/N_c\gg 1$ ("large $N_f$ QCD") as a candidate for the walking technicolor, which is modeled by the $U(N_f)\times U(N_f)$ linear sigma model with classical scale symmetry (without mass term), particularly for $N_f=8$ ("one-family model"). This model exhibits spontaneous breaking of the scale symmetry as well as the $U(N_f)\times U(N_f)$ radiatively through the Coleman-Weinberg mechanism $\grave{a}$ la Gildener-Weinberg, thus giving rise to a light pseudo dilaton (techni-dilaton) to be identified with the 125 GeV Higgs. This model possess a strong first-order electroweak phase transition due to the resultant Coleman-Weinberg type potential. We estimate the bubble nucleation that exhibits an ultra supercooling and then the signal for a stochastic gravitational wave produced via the strong first-order electroweak phase transition. We show that the amplitude can be reached to the expected sensitivities of the LISA.

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arXiv2018

Dynamical Gauge Boson of Hidden Local Symmetry within the Standard Model

The Standard Model (SM) Higgs Lagrangian is straightforwardly rewritten into the {\it scale-invariant} nonlinear sigma model $G/H=[SU(2)_L \times SU(2)_R]/SU(2)_{V}\simeq O(4)/O(3)$, with the (approximate) scale symmetry realized nonlinearly by the (pseudo) dilaton ($=$ SM Higgs). It is further gauge equivalent to that having the symmetry $O(4)_{\rm global}\times O(3)_{\rm local}$, with $O(3)_{\rm local}$ being the Hidden Local Symmetry (HLS). In the large $N$ limit of the scale-invariant version of the Grassmannian model $G/H=O(N)/[O(N-3)\times O(3)] $ $\simeq O(N)_{\rm global}\times [O(N-3)\times O(3)]_{\rm local}$, identical to the SM for $N\rightarrow 4$, we show that the kinetic term of the HLS gauge bosons ("SM rho") $ρ_μ$ of the $O(3)_{\rm local}\simeq [SU(2)_V]_{\rm local}$ are dynamically generated by the nonperturbative dynamics of the SM itself. The dynamical SM rho stabilizes the skyrmion ("SM skyrmion") $X_s$ as a dark matter candidate within the SM: The mass $M_{X_s} ={\cal O}(10\, {\rm GeV})$ consistent with the direct search experiments implies the induced HLS gauge coupling $g_{_{\rm HLS}}={\cal O}(10^3)$, which realizes the relic abundance, $Ω_{X_s} h^2 ={\cal O}(0.1)$. If instead $g_{_{\rm HLS}}\lesssim 3.5$ ($M_ρ\lesssim 1.2 $ TeV), the SM rho could be detected with "narrow width" $\lesssim 100 \,{\rm GeV}$ at LHC, having all the "$a=2$ results" of the generic HLS Lagrangian ${\cal L}_A+ a {\cal L}_V$, i.e., $ρ$-universality, KSRF relations and the vector meson dominance, independently of "$a$". There exists the second order phase phase transition to the unbroken phase having massless $ρ_μ$ and massive $π$ (no longer NG bosons), both becoming massless free particles just on the transition point (scale-invariant ultraviolet fixed point).The results readily apply to the 2-flavored QCD as well.

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arXiv2017

Flavor-singlet spectrum in multi-flavor QCD

Studying SU(3) gauge theories with increasing number of light fermions is relevant both for understanding the strong dynamics of QCD and for constructing strongly interacting extensions of the Standard Model (e.g. UV completions of composite Higgs models). In order to contrast these many-flavors strongly interacting theories with QCD, we study the flavor-singlet spectrum as an interesting probe. In fact, some composite Higgs models require the Higgs boson to be the lightest flavor-singlet scalar in the spectrum of a strongly interacting new sector with a well defined hierarchy with the rest of the states. Moreover, introducing many light flavors at fixed number of colors can influence the dynamics of the lightest flavor-singlet pseudoscalar. We present the on-going study of these flavor-singlet channels using multiple interpolating operators on high-statistics ensembles generated by the LatKMI collaboration and we compare results with available data obtained by the Lattice Strong Dynamics collaboration. For the theory with 8 flavors, the two collaborations have generated configurations that complement each others with the aim to tackle the massless limit using the largest possible volumes.

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arXiv2017

Light flavor-singlet scalars and walking signals in $N_f=8$ QCD on the lattice

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 $σ$) 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 $M_π$, $F_π$, $M_ρ$, $M_{a_0}$, $M_{a_1}$, $M_{b_1}$, $M_N$, $M_σ$, $F_σ$, $\langle \barψ ψ\rangle$ (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 $M_π$ 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, $γ_m \simeq 1$, with a notable exception of $M_π$ with $γ_m \simeq 0.6$ 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 $σ$ with mass comparable to the pion in the studied $m_f$ region. In a chiral limit extrapolation of the $σ$ 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 $γ_m \simeq 1$ 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.

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arXiv2017

Dark Side of the Standard Model: Dormant New Physics Awaken

We find that the nonperturbative physics of the standard-model Higgs Lagrangian provides a dark matter candidate, "dormant skyrmion in the standard model", the same type of the skyrmion, a soliton, as in the hadron physics. It is stabilized by another nonperturbative object in the standard model, the dynamical gauge boson of the hidden local symmetry, which is also an analogue of the rho meson.

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arXiv2016

Walking on the Ladder: 125 GeV Technidilaton, or Conformal Higgs -Dedicated to the late Professor Yoichiro Nambu-

The walking technicolor based on the ladder Schwinger-Dyson gap equation is studied, with the scale-invariant coupling being an idealization of the Caswell-Banks-Zaks infrared fixed point in the "anti-Veneziano limit", such that $N_C \rightarrow \infty$ with $N_C \cdot α(μ^2)=$ fixed and $N_F/N_C=$ fixed ($\gg 1$), of the $SU(N_C)$ gauge theory with massless $N_F$ flavors near criticality. We show that the 125 GeV Higgs can be naturally identified with the technidilaton (TD) predicted in the walking technicolor, a pseudo Nambu-Goldstone (NG) boson of the spontaneous symmetry breaking of the approximate scale symmetry. Ladder calculations yield the TD mass $M_ϕ$ from the trace anomaly as $M_ϕ^2 F_ϕ^2= -4 \langle θ_μ^μ\rangle = - \frac{β(α(μ^2))}{α(μ^2)}\, \langle G_{λν}^2(μ^2)\rangle \simeq N_C N_F\frac{16}{π^4} m_F^4$, independently of the renormalization point $μ$, where $m_F$ is the dynamical mass of the technifermion, and $F_ϕ={\cal O} (\sqrt{N_F N_C}\, m_F)$ the TD decay constant. It reads $M_ϕ^2\simeq (\frac{v_{\rm EW}}{2} \cdot \frac{5 v_{\rm EW}}{F_ϕ})^2 \cdot [\frac{8}{N_F}\frac{4}{N_C}]$, ($v_{\rm EW}=246$ GeV), which implies $F_ϕ\simeq 5 \,v_{\rm EW} $ for $M_ϕ\simeq 125\, {\rm GeV}\simeq \frac{1}{2} v_{\rm EW}$ in the one-family model ($N_C=4, N_F=8$), in good agreement with the current LHC Higgs data. The result reflects a generic scaling $ M_ϕ^2/v_{\rm EW}^2\sim M_ϕ^2/F_ϕ^2 \sim m_F^2 /F_ϕ^2 \sim 1/(N_F N_C) \rightarrow 0 $ as a vanishing trace anomaly, namely the TD has a mass vanishing in the anti-Veneziano limit, similarly to $η^\prime$ meson as a pseudo-NG boson of the ordinary QCD with vanishing $U(1)_A$ anomaly in the Veneziano limit ($N_F/N_C \ll 1$).

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arXiv2016

Hidden Local Symmetry and Beyond

Gerry Brown was a godfather of our hidden local symmetry (HLS) for the vector meson from the birth of the theory throughout his life. The HLS is originated from very nature of the nonlinear realization of the symmetry G based on the manifold G/H, and thus is universal to any physics based on the nonlinear realization. Here I focus on the Higgs Lagrangian of the Standard Model (SM), which is shown to be equivalent to the nonlinear sigma model based on G/H= SU(2)_L x SU(2)_R/SU(2)_V with additional symmetry, the nonlinearly realized scale symmetry. Then the SM does have a dynamical gauge boson of the SU(2)_V HLS, "SM rho meson", in addition to the Higgs as a pseudo dilaton as well as the NG bosons to be absorbed into the W and Z. Based on the recent work done with S. Matsuzaki and H. Ohki, I discuss a novel possibility that the SM rho meson acquires kinetic term by the SM dynamics itself, which then stabilizes the skyrmion dormant in the SM as a viable candidate for the dark matter, what we call "Dark SM skyrmion (DSMS)".

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arXiv2016

Walking from 750 GeV to 950 GeV in the Technipion Zoo

If the 750 GeV diphoton excess is identified with the color-singlet isosinglet-technipion, $P^0$ (750), in the one-family walking technicolor, as in our previous paper, then there should exist another color-singlet technipion, isotriplet one, $P^{\pm,3}$, definitely predicted at around 950 GeV independently of the dynamical details. The $P^{\pm,3}(950)$ are produced at the LHC via vector boson and photon fusion processes, predominantly decaying to $W γ$, and $γγ$, respectively. Those walking technicolor signals can be explored at the Run 2, or 3, which would further open a way to a plethora of yet other (colored) technipions.

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arXiv2016

Origin of Mass - Horizons Expanding from the Nambu's Theory

Origin of mass may be strong dynamics of matter in the vacuum. Since the initial proposal of Nambu for the origin of the nucleon mass, the dynamical symmetry breaking in the strongly coupled underlying theories has been expanding the horizons in the context of the modern version of the origin of mass beyond the Standard Model (SM). The Nambu-Jona-Lasinio (NJL) model is a typical strong coupling theory with the non-zero critical coupling, in sharp contrast to its precedent model, the Bardeen-Cooper-Schrieffer theory for the superconductor. The non-zero critical coupling is also hidden in the asymptotically free gauge theories including QCD and walking technicolor. As is well known, the NJL model can be cast into the SM Higgs Lagrangian. We show that the SM Higgs Lagrangian is simply rewritten into a form of the (approximately) scale-invariant nonlinear sigma model, with both the chiral symmetry and scale symmetry realized nonlinearly, with the SM Higgs being nothing but the (pseudo-) dilaton. The SM Higgs Lagrangian is further gauge equivalent to the scale-invariant Hidden Local Symmetry (HLS) Lagrangian, s-HLS, having spin 1 bosons hidden in the SM. As the simplest possible underlying theory for the SM Higgs Lagrangian we first discuss the top quark condensate ("top-mode SM"), where the top-mode dilaton is the 125 GeV Higgs. We then discuss the walking technicolor having near infrared conformality, whose effective theory is the s-HLS model precisely the same as the SM Higgs Lagrangian (with larger chiral symmetry), where the 125 GeV Higgs is successfully identified with the techidilaton.

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arXiv2016

750 GeV Diphoton Signal from One-Family Walking Technipion

The ATLAS and CMS groups have recently reported an excess at around 750 GeV with the local significance by about 3 sigma in the diphoton channel at the 13 TeV LHC. We give a possible explanation for the excess by a composite pseudo scalar $(P^0)$ predicted in the one-family model of walking technicolor.

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arXiv2016

Conformal Barrier for New Vector Bosons Decay to the Higgs

The ATLAS collaboration has recently reported excesses about 2.5 sigma at mass around 2 TeV in the diboson channels, which can be identified with new vector bosons as a hint for the new physics. It is shown that spontaneously broken conformal/scale symmetry prohibits new vector bosons decay to the Higgs, which is contrasted to the popular "equivalence theorem" valid only in a special limit not necessarily relevant to the 2 TeV mass. If the decay $V\to WH/ZH$ is not observed in the ongoing Run II of the LHC, then the 125 GeV Higgs can be a dilaton.

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arXiv2016

S-parameter and vector decay constant in QCD with eight fundamental fermions

SU(3) gauge theory with eight massless fundamental fermions seems to be near the conformal boundary, and is a candidate theory of walking technicolor. Along the series of study by LatKMI collaboration using HISQ fermions, S-parameter and vector decay constant, which provide important constraints in the model of electroweak symmetry breaking, are calculated for this theory. Use of various volumes allows a systematic investigation of finite volume effects. A strong sensitivity of the S-parameter to the volume is found.

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arXiv2016

Conformal Barrier and Hidden Local Symmetry Constraints: Walking Technirhos in LHC Diboson Channels

We expand the previous analyses of the conformal barrier on the walking technirho for the 2 TeV diboson excesses reported by the ATLAS collaboration, with a special emphasis on the hidden local symmetry (HLS) constraints. We first show that the Standard Model (SM) Higgs Lagrangian is equivalent to the scale-invariant nonlinear chiral Lagrangian, which is further gauge equivalent to the scale-invariant HLS model, with the scale symmetry realized nonlinearly via SM Higgs as a (pseudo-) dilaton. The scale symmetry forbids the new vector boson decay to the 125 GeV Higgs plus W/Z boson, in sharp contrast to the conventional "equivalence theorem" which is invalidated by the conformality. The HLS forbids mixing between the iso-triplet technirho's, rho_{Pi} and rho_{P}, of the one-family walking technicolor (with four doublets N_D=N_F/2=4), which, without the HLS, would be generated when switching on the standard model gauging. We also present updated analyses of the walking technrho's for the diboson excesses by fully incorporating the constraints from the conformal barrier and the HLS as well as possible higher order effects: still characteristic of the one-family walking technirho is its smallness of the decay width, roughly of order Gamma/M_rho ~ [3/N_C x 1/N_D] x [Gamma/M_rho]_{QCD} ~ 70 GeV/2TeV (N_D= N_C=4), in perfect agreement with the expected diboson resonance with Gamma<100 GeV. The model is so sharply distinguishable from other massive spin 1 models without the conformality and HLS that it is clearly testable at the LHC Run II. If the 2 TeV boson decay to WH/ZH is not observed in the ongoing Run II, then the conformality is operative on the 125 GeV Higgs, strongly suggesting that the 2 TeV excess events are responsible for the walking technirhos and the 125 GeV Higgs is the technidilaton.

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arXiv2016

Topological observables in many-flavour QCD

SU(3) gauge theory with eight massless flavours is believed to be walking, while the corresponding twelve- and four-flavour appear IR-conformal and confining respectively. Looking at the simulations performed by the LatKMI collaboration of these theories, we use the topological susceptibility as an additional probe of the IR dynamics. By drawing a comparison with SU(3) pure gauge theory, we see a dynamical quenching effect emerge at larger number of flavours, which is suggestive of emerging near-conformal and conformal behaviour.

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