arXiv subjects

Koichi Yamawaki

Publications and source records attributed to Koichi Yamawaki.

At least 37 records · Page 2Linked to original sources

arXiv2016

Walking and conformal dynamics in many-flavor QCD

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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arXiv2015

SU(3) gauge theory with four degenerate fundamental fermions on the lattice

As a part of the project studying large $N_f$ QCD, the LatKMI Collaboration has been investigating the SU(3) gauge theory with four fundamental fermions (four-flavor QCD). The main purpose of studying four-flavor QCD is to provide a qualitative comparison to $N_f= 8$, $12$, $16$ QCD; however, a quantitative comparison to real-world QCD is also interesting. To make such comparisons more meaningful, it is desirable to use the same kind of lattice action consistently, so that qualitative difference of different theories are less affected by artifacts of lattice discretization. Here, we adopt the highly-improved staggered quark action with the tree-level Symanzik gauge action (HISQ/tree), which is exactly the same as the setup for our simulations for $SU(3)$ gauge theories with $N_f=8$, $12$ and $16$ fundamental fermions~\cite{Aoki:2013xza, Aoki:2012eq, Aoki:2014oma}. In the next section, we show the fermion mass dependence of $F_π$, $\langle\barψψ\rangle$, $M_π$, $M_ρ$, $M_N$ and their chiral extrapolations. In section 3, preliminary results of the measurement of the mass of the flavor-singlet scalar bound state will be reported.

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arXiv2015

Old Wine in a New Bottle: Technidilaton as the 125 GeV Higgs -- Dedicated to the late Professor Yoichiro Nambu

We show that the Standard Model (SM) Higgs Lagrangian is identical to the nonlinear realization of both the scale and chiral symmetries ("scale-invariant nonlinear sigma model"), and is further gauge equivalent to the "scale-invariant Hidden Local Symmetry (HLS) model" having possible new vector bosons as the HLS gauge bosons with scale-invariant mass: SM Higgs is nothing but a (pseudo) dilaton. The effective theory of the walking technicolor has precisely the same type of the scale-invariant nonlinear sigma model, thus further having the scale-invariant HLS gauge bosons (technirho's, etc.). The technidilaton mass M_ϕcomes from the trace anomaly, which yields M_ϕ^2 F_ϕ^2 \simeq (2.5)^2 [(8/N_F)(4/N_C)] v^4 via PCDC, in the underlying walking SU(N_C) gauge theory with N_F massless flavors, where F_ϕis the decay constant and v=246 GeV. This implies F_ϕ\simeq 5 v for M_ϕ\simeq 125 GeV \simeq v/2 in the one-family walking technicolor model (N_F=8, N_C=4), in good agreement with the current LHC Higgs data. In the anti-Veneziano limit, N_C \rightarrow \infty, with N_C α= fixed and N_F/N_C= fixed (\gg 1), we have a result: M_ϕ^2/v^2\sim M_ϕ^2/F_ϕ^2 \sim 1/(N_F N_C) \rightarrow 0. Then the technidilaton is a naturally light composite Higgs out of the strongly coupled conformal dynamics, with its couplings even weaker than the SM Higgs.

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arXiv2015

Lattice study of the scalar and baryon spectra in many-flavor QCD

In the search for a composite Higgs boson in walking technicolor models, many flavor QCD, in particular with $N_f=8$, is an attractive candidate, and has been found to have a composite flavor-singlet scalar as light as the pion. Based on lattice simulations of this theory with the HISQ action, we will present our preliminary results on the scalar decay constant using the fermionic bilinear operator, and on the mass of the lightest baryon state which could be a dark matter candidate. Combining these two results, implications for dark matter direct detection are also discussed.

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arXiv2015

Topological insights in many-flavor QCD on the lattice

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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arXiv2015

2 TeV Walking Technirho at LHC?

The ATLAS collaboration has recently reported an excess of about 2.5 $σ$ global significance at around 2 TeV in the diboson channel with the boson-tagged fat dijets, which may imply a new resonance beyond the standard model. We provide a possible explanation of the excess as the isospin-triplet technivector mesons (technirhos, denoted as $ρ_Π^{\pm,3}$) of the walking technicolor in the case of the one-family model as a benchmark. As the effective theory for the walking technicolor at the scales relevant to the LHC experiment, we take a scale-invariant version of the hidden local symmetry model so constructed as to accommodate technipions, technivector mesons, and the technidilaton in such a way that the model respects spontaneously broken chiral and scale symmetries of the underlying walking technicolor. In particular, the technidilaton, a (pseudo) Nambu-Goldstone boson of the (approximate) scale symmetry predicted in the walking technicolor, has been shown to be successfully identified with the 125 GeV Higgs. Currently available LHC limits on those technihadrons are used to fix the couplings of technivector mesons to the standard-model fermions and weak gauge bosons. We find that the technirho's are mainly produced through the Drell-Yan process and predominantly decay to the dibosons, which accounts for the currently reported excess at around 2 TeV. The consistency with the electroweak precision test and other possible discovery channels of the 2 TeV technirhos are also addressed.

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arXiv2015

Conformality in twelve-flavour QCD

The spectrum of twelve-flavor QCD has been studied in details by the LatKMI collaboration. In this proceeding we present our updated results for the spectrum obtained with the HISQ action at two lattice spacings, several volumes and fermion masses. In particular, we emphasize the existence of a flavor-singlet scalar state parametrically light with respect to the rest of the spectrum, first reported in our paper. This feature is expected to be present for theories in the conformal window, but the lattice calculation of such a state is difficult and requires noise-reduction techniques together with large statistics, in order to evaluate disconnected diagrams. Being able to provide a robust observed connection between a light flavor-singlet scalar and (near-)conformality is an important step towards observing a light composite Higgs boson in walking technicolor theories on the lattice. We also show updated results for the mass anomalous dimension $γ_m$ obtained from various spectral quantities, including the string tension, under the assumption that the theory is inside the conformal window.

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arXiv2015

Determination of SU(4)$_{TC}$ Technicolor Gauge Group from Embedding in Extended Technicolor

In technicolor theories using an SU($N_{TC}$) gauge group, the value of $N_{TC}$ is not, {\it a priori}, determined and is typically chosen by phenomenological criteria. Here we present a novel way to determine $N_{TC}$ from the embedding of a one-family technicolor model, with fermions in the fundamental represention of SU($N_{TC}$), in an extended technicolor theory, and use it to deduce that $N_{TC}=4$ in this framework.

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arXiv2014

Walking Technipions in a Holographic Model

We calculate masses of the technipions in the walking technicolor model with the anomalous dimension gamma_m =1, based on a holographic model which has a naturally light technidilaton phi as a composite Higgs with mass m_phi simeq 125 GeV. The one-family model (with 4 weak-doublets) is taken as a concrete example in such a framework, with the inputs being F_pi=v/2 simeq 123 GeV and m_phi simeq 125 GeV as well as gamma_m=1. It is shown that technipion masses are enhanced by the large anomalous dimension to typically O(1) TeV. We find a correlation between the technipion masses and S^{(TC)}, the S parameter arising only from the technicolor sector. The current LHC data on the technipion mass limit thus constrains S^{(TC)} to be not as large as O(1), giving a direct constraint on the technicolor model building. This is a new constraint on the technicolor sector alone quite independent of other sector connected by the extended-technicolor-type interactions, in sharp contrast to the conventional S parameter constraint from the precision electroweak measurements.

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arXiv2014

Discovering walking technirho mesons at the LHC

We formulate a scale-invariant hidden local symmetry (HLS) as a low-energy effective theory of walking technicolor (WTC) which includes the technidilaton, technipions, and technirho mesons as the low-lying spectra. As a benchmark for LHC phenomenology, we in particular focus on the one-family model of WTC having eight technifermion flavors, which can be, at energy scales relevant to the reach of the LHC, described by the scale-invariant HLS based on the manifold $[SU(8)_L \times SU(8)_R]_{\rm global} \times SU(8)_{\rm local}/SU(8)_V$, where $SU(8)_{\rm local}$ is the HLS and the global $SU(8)_L \times SU(8)_R$ symmetry is partially gauged by $SU(3) \times SU(2)_L \times U(1)_Y$ of the standard model. Based on the scale-invariant HLS, we evaluate the coupling properties of the technirho mesons and place limits on the masses from the current LHC data. Then, implications for future LHC phenomenology are discussed by focusing on the technirho mesons produced through the Drell-Yan process. We find that the color-octet technirho decaying to the technidilaton along with the gluon is of interest as the discovery channel at the LHC, which would provide a characteristic signature to probe the one-family WTC.

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arXiv2014

Higgs as a Top-Mode Pseudo

In the spirit of the top quark condensation, we propose a model which has a naturally light composite Higgs boson, "tHiggs", to be identified with the 126 GeV Higgs discovered at the LHC. The tHiggs, a bound state of the top quark and its flavor (vector-like) partner, emerges as a pseudo Nambu-Goldstone boson (NGB), "Top-Mode Pseudo", together with the exact NGBs to be absorbed into the W and Z bosons as well as another (heavier) Top-Mode Pseudo (CP-odd composite scalar). Those five composite (exact/pseudo) NGBs are dynamically produced simultaneously by a single supercritical four-fermion interaction having U(3) x U(1) symmetry which includes the electroweak symmetry, where the vacuum is aligned by small explicit breaking term so as to break the symmetry down to a subgroup, U(2) x U(1)', in a way not to retain the electroweak symmetry, in sharp contrast to the little Higgs models. The explicit breaking term for the vacuum alignment gives rise to a mass of the tHiggs, which is protected by the symmetry and hence naturally controlled against radiative corrections. Realistic top quark mass is easily realized similarly to the top-seesaw mechanism by introducing an extra (subcritical) four-fermion coupling which explicitly breaks the residual U(2) x U(1)' symmetry. We present a phenomenological Lagrangian of the Top-Mode Pseudos along with the standard model particles, which will be useful for the study of the collider phenomenology. The coupling property of the tHiggs is shown to be consistent with the currently available data reported from the LHC. Several phenomenological consequences and constraints from experiments are also addressed.

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arXiv2014

Dilaton Chiral Perturbation Theory - Determining Mass and Decay Constant of Technidilaton on the Lattice

We propose a scale-invariant chiral perturbation theory of the pseudo-Nambu-Goldstone bosons of the chiral symmetry (pion "pi") as well as the scale symmetry (dilaton "phi") for the large N_f QCD. The resultant dilaton mass M_phi reads M_phi^2= m_phi^2 + (3-gamma_m)(1+gamma_m)/4 (2 N_f F_pi^2/F_phi^2) m_pi^2 + (chiral log corrections), where m_phi, m_pi, gamma_m, F_pi and F_phi are the dilaton mass in the chiral limit, the pion mass, mass anomalous dimension, decay constants of pi and phi, respectively. The chiral extrapolation of the lattice data on M_phi^2 vs m_pi^2 plot then simultaneously determines (m_phi, F_phi) of the technidilaton in the walking technicolor with gamma_m =~1. The chiral logarithmic corrections are explicitly given.

hep-lat↗

arXiv2014

Light composite scalar in eight-flavor QCD on the lattice

We present the first observation of a flavor-singlet scalar meson as light as the pion in $N_f=8$ QCD on the lattice, using the Highly Improved Staggered Quark action. Such a light scalar meson can be regarded as a composite Higgs with mass 125 GeV. In accord with our previous lattice results showing that the theory exhibits walking behavior, the light scalar may be a technidilaton, a pseudo Nambu-Goldstone boson of the approximate scale symmetry in walking technicolor.

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arXiv2013

Composite flavor-singlet scalar in twelve-flavor QCD

We report the calculation of the flavor-singlet scalar in the SU(3) gauge theory with the degenerate twelve fermions in the fundamental representation using a HISQ-type action at a fixed $β$. In order to reduce the large statistical error coming from the vacuum-subtracted disconnected correlator, we employ a noise reduction method and a large number of configurations. We observe that the flavor-singlet scalar is lighter than the pion in this theory from the calculations with the fermion bilinear and gluonic operators. This peculiar feature is considered to be due to the infrared conformality of this theory, and it is a promissing signal for a walking technicolor, where a light composite Higgs boson is expected to emerge by approximate conformal dynamics.

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arXiv2013

Gluonic observables and the scalar spectrum of twelve-flavour QCD

We measure glueball masses and the string tension in twelve-flavour QCD, aiming at comparing the emerging gluonic spectrum to the mesonic one. When approaching the critical surface at zero quark mass, the hierarchy of masses in the different sectors of the spectrum gives a new handle to determine the existence of an infrared fixed point. We describe the details of our gluonic measurements and the results obtained on a large number of gauge configurations generated with the HISQ action. In particular, we focus on the scalar glueball and its mixing with a flavour-singlet fermionic state, which is lighter than the pseudoscalar (would-be pion) state. The results are interesting in view of a light composite Higgs boson in walking technicolor theories.

hep-lat↗

arXiv2013

Light composite scalar in twelve-flavor QCD on the lattice

Based on lattice simulations using highly improved staggered quarks for twelve-flavor QCD with several bare fermion masses, we observe a flavor-singlet scalar state lighter than the pion in the correlators of fermionic interpolating operators. The same state is also investigated using correlators of gluonic interpolating operators. Combined with our previous study, that showed twelve-flavor QCD to be consistent with being in the conformal window, we infer that the lightness of the scalar state is due to infrared conformality. This result shed some light on the possibility of a light composite Higgs boson ("technidilaton") in walking technicolor theories.

hep-lat↗

arXiv2013

A light composite scalar in eight-flavor QCD on the lattice

In search for a composite Higgs boson (techni-dilaton) in the walking technicolor, we present our preliminary results on the first observation of a light flavor-singlet scalar in a candidate theory for the walking technicolor, the Nf=8 QCD, which was found in our previous paper to have spontaneous chiral symmetry breaking together with remnants of the conformality. Based on simulations with the HISQ-type action on several lattice sizes with various fermion masses, we find evidence of a flavor-singlet scalar meson with mass comparable to that of the Nambu-Goldstone pion in both the small fermion-mass region, where chiral perturbation theory works, and the intermediate fermion-mass region where the hyperscaling relation holds. We further discuss its chiral limit extrapolation in comparison with other states studied in our previous paper: the scalar has a mass much smaller than that of the vector meson, which is compared to the Nambu-Goldstone pion having a vanishing mass in that limit.

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arXiv2013

Gluonic Effects on g-2: Holographic View

We study "gluonic effects" (gluon condensation effects) on the hadronic leading order (HLO) contributions to the anomalous magnetic moment (g-2) of leptons, based on a holographic model having explicit gluonic mode introduced for consistency with the operator product expansion of QCD. We find gluonic enhancement of HLO contributions to the muon g-2 by about 6%, which nicely fills in the gap between the holographic estimate without gluonic effects and the phenomenological one using the experimental data as inputs. Similar calculations including the gluonic effects for the electron and the tau lepton g-2 are also carried out in good agreement with the phenomenological estimates. We then apply our holographic estimate to the Walking Technicolor (WTC) where large techni-gluonic effects were shown to be vital for the Technidilaton, (pseudo) Nambu-Goldstone boson of the (approximate) scale symmetry of WTC, to be naturally as light as 125 GeV. It is shown that the value of the techni-HLO contributions to the muon g-2 is 10-100 times enhanced by inclusion of the same amount of the gluonic effects as that realizing the 125 GeV Technidilaton, although such an enhanced techni-HLO contribution is still negligibly small compared with the current deviation of the Standard Model prediction of the muon g-2 from the experiments. The techni-HLO contributions to the tau lepton g-2 is also discussed, suggesting a possible phenomenological relevance to be tested by the future experiments.

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