arXiv subjects

Masafumi Kurachi

Publications and source records attributed to Masafumi Kurachi.

At least 55 records · Page 3Linked to original sources

arXiv2012

Many flavor QCD as exploration of the walking behavior with the approximate IR fixed point

We present the first report of the LatKMI collaboration on the the lattice QCD simulation performed at the KMI computer, "$φ$", 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.

hep-lat↗

arXiv2012

Study of the conformal hyperscaling relation through the Schwinger-Dyson equation

We study corrections to the conformal hyperscaling relation in the conformal window of the large Nf QCD by using the ladder Schwinger-Dyson (SD) equation as a concrete dynamical model. From the analytical expression of the solution of the ladder SD equation, we identify the form of the leading mass correction to the hyperscaling relation. We find that the anomalous dimension, when identified through the hyperscaling relation neglecting these corrections, yields a value substantially lower than the one at the fixed point γ_m^* for large mass region. We further study finite-volume effects on the hyperscaling relation, based on the ladder SD equation in a finite space-time with the periodic boundary condition. We find that the finite-volume corrections on the hyperscaling relation are negligible compared with the mass correction. The anomalous dimension, when identified through the finite-size hyperscaling relation neglecting the mass corrections as is often done in the lattice analyses, yields almost the same value as that in the case of the infinite space-time neglecting the mass correction, i.e., a substantially lower value than γ_m^* for large mass. We also apply the finite-volume SD equation to the chiral-symmetry-breaking phase and find that when the theory is close to the critical point such that the dynamically generated mass is much smaller than the explicit breaking mass, the finite-size hyperscaling relation is still operative. We also suggest a concrete form of the modification of the finite-size hyperscaling relation by including the mass correction, which may be useful to analyze the lattice data.

hep-lat↗

arXiv2011

Spectral-Function Sum Rules in Supersymmetry Breaking Models

The technique of Weinberg's spectral-function sum rule is a powerful tool for a study of models in which global symmetry is dynamically broken. It enables us to convert information on the short-distance behavior of a theory to relations among physical quantities which appear in the low-energy picture of the theory. We apply such technique to general supersymmetry breaking models to derive new sum rules.

hep-ph↗

arXiv2011

The Infrared behavior of SU(3) Nf=12 gauge theory -about the existence of conformal fixed point-

Incorporated with twisted boundary condition, Polyakov loop correlators can give a definition of the renormalized coupling. We employ this scheme for the step scaling method (with step size s = 2) in the search of conformal fixed point of SU(3) gauge theory with 12 massless flavors. Staggered fermion and plaquette gauge action are used in the lattice simulation with six different lattice sizes, L/a = 20, 16, 12, 10, 8 and 6. For the largest lattice size, L/a = 20, we used a large number of Graphics Processing Units (GPUs) and accumulated 3,000,000 trajectories in total. We found that the step scaling function sigma (u) is consistent with u in the low-energy region. This means the existence of conformal fixed point. Some details of our analysis and simulations will also be presented.

hep-lat↗

arXiv2010

Search for the IR fixed point in the Twisted Polyakov Loop scheme (II)

We measure the renormalized coupling in the Twisted Polyakov loop scheme for SU(3) gauge theory coupled with $N_f=12$ fundamental fermions. To find the infrared fixed point of this theory, we focus on the step scaling function for the renormalized coupling. We take the continuum limit using the linear function of $(a/L)^2$ and a constant fit function. We find that there is a sizeable systematic error due to the choice of the continuum extrapolation function in the low energy region. We will give some directions to reduce the systematic errors.

hep-lat↗

arXiv2010

Study of the scaling properties in SU(2) gauge theory with eight flavors

We present our preliminary study of the SU(2) gauge theory with 8 flavors of fermions in fundamental representation. This theory could be a candidate of the gauge theory with conformal fixed point. By using Wilson/Polyakov loop in a finite volume with twisted boundary conditions, we study the renormalization group flow of the gauge coupling constant. Our calculation gives consistent result with the perturbative prediction of the running coupling in the weak coupling region. We investigate a possible signal for conformal behavior in the strong coupling region.

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arXiv2010

Flavor Structure of the Three Site Higgsless Model

We study the flavor structure in the three site Higgsless model. In this model, the gauge bosons and fermions have heavy partners, coming from the Kaluza-Klein excitation in the dimensional deconstruction picture. The yukawa couplings are introduced in a way to minimize the flavor chaning neutral current in the light sector at the tree level. Due to the flavor mixing between the light and the heavy partner fields, new effects on FCNC's appear at one-loop level. As an example of such FCNC processes, we calculate the contribution to the b -> s gamma amplitude in the three site Higgsless model. Interestingly, heavy particles which exist in the three site Higgsless model do not completely decouple in the heavy-mass limit. One-loop level b -> s gamma amplitude is calculated by considering all possible combinations of particles in the loop, then it is compared to the experiment. The result shows that the central value of the B -> X_s gamma branching ratio in the three site Higgsless model takes closer value to its experimental central value as one takes the larger value of a free parameter, \varepsilon_{tR}, within a range allowed by the precision electroweak measurement.

hep-ph↗

arXiv2010

Identifying Better Effective Higgsless Theories via W_L W_L Scattering

The three site Higgsless model has been offered as a benchmark for studying the collider phenomenology of Higgsless models. In this talk, we present how well the three site Higgsless model performs as a general representative of Higgsless models in describing W_L W_L scattering, and which modifications can make it more representative. We employ general sum rules relating the masses and couplings of the Kaluza-Klein (KK) modes of the gauge fields in continuum and deconstructed Higgsless models as a way to compare the different theories. After comparing the three site Higgsless model to flat and warped continuum Higgsless models, we analyze an extensions of the three site Higgsless model, namely, the Hidden Local Symmetry (HLS) Higgsless model. We demonstrate that W_LW_L scattering in the HLS Higgsless model can very closely approximate scattering in the continuum models, provided that the parameter `a' is chosen to mimic rho-meson dominance of pi-pi scattering in QCD.

hep-ph↗

arXiv2010

A new scheme for the running coupling constant in gauge theories using Wilson loops

We propose a new renormalization scheme of the running coupling constant in general gauge theories using the Wilson loops. The renormalized coupling constant is obtained from the Creutz ratio in lattice simulations and the corresponding perturbative coefficient at the leading order. The latter can be calculated by adopting the zeta-function resummation techniques. We perform a benchmark test of our scheme in quenched QCD with the plaquette gauge action. The running of the coupling constant is determined by applying the step-scaling procedure. Using several methods to improve the statistical accuracy, we show that the running coupling constant can be determined in a wide range of energy scales with relatively small number of gauge configurations.

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arXiv2009

Higgsinoless Supersymmetry and Hidden Gravity

We present a simple formulation of non-linear supersymmetry where superfields and partnerless fields can coexist. Using this formalism, we propose a supersymmetric Standard Model without the Higgsino as an effective model for the TeV-scale supersymmetry breaking scenario. We also consider an application of the Hidden Local Symmetry in non-linear supersymmetry, where we can naturally incorporate a spin-two resonance into the theory in a manifestly supersymmetric way. Possible signatures at the LHC experiments are discussed.

hep-ph↗

arXiv2009

Search for the IR fixed point in the twisted Polyakov loop scheme

We present a non-perturbative study of the running coupling constant in the Twisted Polyakov Loop (TPL) scheme. We investigate how the systematic and statistical errors can be controlled {\it via} a feasibility study in SU(3) pure Yang-Mills theory. We show that our method reproduces the perturbative determination of the running coupling in the UV. In addition, our numerical result agrees with the theoretical prediction of this coupling constant in the IR. We also present our preliminary results for $N_f=12$ QCD, where an IR fixed point may be present.

hep-lat↗

arXiv2009

W_L W_L Scattering in Higgsless Models: Identifying Better Effective Theories

The three-site model has been offered as a benchmark for studying the collider phenomenology of Higgsless models. In this paper we analyze how well the three-site model performs as a general exemplar of Higgsless models in describing W_L W_L scattering, and which modifications can make it more representative. We employ general sum rules relating the masses and couplings of the Kaluza-Klein (KK) modes of the gauge fields in continuum and deconstructed Higgsless models as a way to compare the different theories. We show that the size of the four-point vertex for the (unphysical) Nambu-Goldstone modes and the degree to which the sum rules are saturated by contributions from the lowest-lying KK resonances both provide good measures of the extent to which a highly-deconstructed theory can accurately describe the low-energy physics of a continuum 5d Higgsless model. After comparing the three-site model to flat and warped continuum models, we analyze extensions of the three-site model to a longer open linear moose with an additional U(1) group and to a ring ("BESS' or "hidden local symmetry") model with three sites and three links. Both cases may be readily analyzed in the framework of the general sum rules. We demonstrate that W_L W_L scattering in the ring model can very closely approximate scattering in the continuum models, provided that the hidden local symmetry parameter `a' is chosen to mimic rho-meson dominance of pion-pion scattering in QCD. The hadron and lepton collider phenomenology of both extended models is briefly discussed, with a focus on the complementary information to be gained from precision measurements of the Z' line shape and ZWW coupling at a high-energy lepton collider.

hep-ph↗

arXiv2008

A New Method of Calculating the Running Coupling Constant

We propose a new method to compute the running coupling constant of gauge theories on the lattice. We first give the definition of the running coupling in the new scheme using the Wilson loops in a finite volume, and explain how the running of the coupling constant is extracted from the measurement of the volume dependence. The perturbative calculation of the renormalization constant to define the scheme is also given at the leading order. As a benchmark test of the new scheme we apply the method the case of the quenched QCD. We show the preliminary result from our numerical simulations which are carried out with plaquette gauge action for various lattice sizes and bare lattice couplings. With techniques to improve the statistical accuracy, we show that we can determine the non-perturbative running of the coupling constant in a wide range of the energy scale with relatively small number of gauge configurations in our scheme. We compare our lattice data of the running coupling constant with perturbative renormalization group evolution at one- and two-loop order, and confirm the consistency between them at high energy.

hep-lat↗

arXiv2008

General Sum Rules for WW Scattering in Higgsless Models: Equivalence Theorem and Deconstruction Identities

We analyze inelastic 2 to 2 scattering amplitudes for gauge bosons and Nambu-Goldstone bosons in deconstructed Higgsless models. Using the (KK) Equivalence Theorem in 4D (5D), we derive a set of general sum rules among the boson masses and multi-boson couplings that are valid for arbitrary deconstructed models. Taking the continuum limit, our results naturally include the 5D Higgsless model sum rules for arbitrary 5D geometry and boundary conditions; they also reduce to the elastic sum rules when applied to the special case of elastic scattering. For the case of linear deconstructed Higgsless models, we demonstrate that the sum rules can also be derived from a set of general deconstruction identities and completeness relations. We apply these sum rules to the deconstructed 3-site Higgsless model and its extensions; we show that in 5D ignoring all higher KK modes (n>1) is inconsistent once the inelastic channels become important. Finally, we discuss how our results generalize beyond the case of linear Higgsless models.

hep-ph↗

arXiv2007

Z Boson Propagator Correction in Technicolor Theories with ETC Effects Included

We calculate the Z boson propagator correction, as described by the S parameter, in technicolor theories with extended technicolor interactions included. Our method is to solve the Bethe-Salpeter equation for the requisite current-current correlation functions. Our results suggest that the inclusion of extended technicolor interactions has a relatively small effect on S.

hep-ph↗

arXiv2007

Study of the Change from Walking to Non-Walking Behavior in a Vectorial Gauge Theory as a Function of N_f

Based on recent works [1,2], we present the results of calculations for several physical quantities (meson masses, the S parameter, etc.) in a vectorial gauge theory, as a function of the number of fermions, N_f. Solutions of the Schwinger-Dyson and the Bethe-Salpeter equations with the improved ladder approximation are used for the calculations. We focus on how the values of physical quantities change as one moves from the QCD-like (non-walking) to walking regimes.

hep-ph↗

arXiv2007

Deconstruction and Elastic pi pi Scattering in Higgsless Models

We study elastic pion-pion scattering in global linear moose models and apply the results to a variety of Higgsless models in flat and AdS space using the Equivalence Theorem. In order to connect the global moose to Higgsless models, we first introduce a block-spin transformation which corresponds, in the continuum, to the freedom to perform coordinate transformations in the Higgsless model. We show that it is possible to make an "f-flat" deconstruction in which all of the f-constants f_j of the linear moose model are identical; the phenomenologically relevant f-flat models are those in which the coupling constants of the groups at either end of the moose are small - corresponding to the global linear moose. In studying pion-pion scattering, we derive various sum rules, including one analogous to the KSRF relation, and use them in evaluating the low-energy and high-energy forms of the leading elastic partial wave scattering amplitudes. We obtain elastic unitarity bounds as a function of the mass of the lightest KK mode and discuss their physical significance.

hep-ph↗

arXiv2006

A Three Site Higgsless Model

We analyze the spectrum and properties of a highly-deconstructed Higgsless model with only three sites. Such a model contains sufficient complexity to incorporate interesting physics issues related to fermion masses and electroweak observables, yet remains simple enough that it could be encoded in a Matrix Element Generator program for use with Monte Carlo simulations. The gauge sector of this model is equivalent to that of the BESS model; the new physics of interest here lies in the fermion sector. We analyze the form of the fermion Yukawa couplings required to produce the ideal fermion delocalization that causes tree-level precision electroweak corrections to vanish. We discuss the size of one-loop corrections to b \to s γ, the weak-isospin violating parameter αT and the decay Z \to b \bar{b}. We find that the new fermiophobic vector states (the analogs of the gauge-boson KK modes in a continuum model) can be reasonably light, with a mass as low as 380 GeV, while the extra (approximately vectorial) quark and lepton states (the analogs of the fermion KK modes) must be heavier than 1.8 TeV.

hep-ph↗