arXiv subjects

Masafumi Kurachi

Publications and source records attributed to Masafumi Kurachi.

At least 37 records · Page 2Linked to original sources

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

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.

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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.

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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.

hep-ph↗

arXiv2013

Walking signals in Nf=8 QCD on the lattice

We investigate chiral and conformal properties of the lattice QCD with eight flavors (Nf=8) through meson spectrum using the Highly Improved Staggered Quark (HISQ) action. We also compare our results with those of Nf=12 and Nf=4 which we study on the same systematics. We find that the decay constant F_pi of the pseudoscalar meson "pion" is non-zero, with its mass M_pi consistent with zero, both in the chiral limit extrapolation of the chiral perturbation theory (ChPT). We also measure other quantities which we find are in accord with the pi data results: The rho meson mass is consistent with non-zero in the chiral limit, and so is the chiral condensate, with its value neatly coinciding with that from the Gell-Mann-Oakes-Renner relation in the chiral limit. Thus our data for the Nf=8 QCD are consistent with the spontaneously broken chiral symmetry. Remarkably enough, while the Nf=8 data near the chiral limit are well described by the ChPT, those for the relatively large fermion bare mass m_f away from the chiral limit actually exhibit a finite-size hyperscaling relation, suggesting a large anomalous dimension gamma_m ~ 1. This implies that there exists a remnant of the infrared conformality, and suggests that a typical technicolor ("one-family model") as modeled by the Nf=8 QCD can be a walking technicolor theory having an approximate scale invariance with large anomalous dimension gamma_m ~ 1.

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arXiv2013

The scalar spectrum of many-flavour QCD

The LatKMI collaboration is studying systematically the dynamical properties of N_f = 4,8,12,16 SU(3) gauge theories using lattice simulations with (HISQ) staggered fermions. Exploring the spectrum of many-flavour QCD, and its scaling near the chiral limit, is mandatory in order to establish if one of these models realises the Walking Technicolor scenario. Although lattice technologies to study the mesonic spectrum are well developed, scalar flavour-singlet states still require extra effort to be determined. In addition, gluonic observables usually require large-statistic simulations and powerful noise-reduction techniques. In the following, we present useful spectroscopic methods to investigate scalar glueballs and scalar flavour-singlet mesons, together with the current status of the scalar spectrum in N_f = 12 QCD from the LatKMI collaboration.

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arXiv2013

Exploring walking behavior in SU(3) gauge theory with 4 and 8 HISQ quarks

We present the report of the LatKMI collaboration on the lattice QCD simulation for the cases of 4 and 8 flavors. The Nf=8 in particular is interesting from the model-building point of view: The typical walking technicolor model with the large anomalous dimension is the so-called one-family model (Farhi-Susskind model). Thus we explore the walking behavior in LQCD with 8 HISQ quarks by comparing with the 4-flavor case (in which the chiral symmetry is spontaneously broken). We report preliminary results on the spectrum, analyzed through the chiral perturbation theory and the finite-size hyperscaling, and we discuss the availability of the Nf=8 QCD to the phenomenology.

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arXiv2012

Low energy spectra in many flavor QCD with Nf=12 and 16

We present our result of the many-flavor QCD. Information of the phase structure of many-flavor SU(3) gauge theory is of great interest, since the gauge theories with the walking behavior near the infrared fixed point are candidates of new physics for the origin of the dynamical electroweak symmetry breaking. We study the SU(3) gauge theories with 12 and 16 fundamental fermions. Utilizing the HISQ type action which is useful to study the continuum physics, we analyze the lattice data of the mass and the decay constant of the pseudoscalar meson and the mass of the vector meson as well at several values of lattice spacing and fermion mass. The finite size scaling test in the conformal hypothesis is also performed. Our data is consistent with the conformal scenario for Nf=12. We obtain the mass anomalous dimension $γ_m \sim 0.4-0.5$. An update of $N_f=16$ study is also shown.

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arXiv2012

Conformal fixed point of SU(3) gauge theory with 12 fundamental fermions

We study the infrared properties of SU(3) gauge theory coupled to 12 massless Dirac fermions in the fundamental representation. The renormalized running coupling constant is calculated in the Twisted Polyakov loop scheme on the lattice. From the step-scaling analysis, we find that the infrared behavior of the theory is governed by a non-trivial fixed point.

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arXiv2012

Lattice study of conformality in twelve-flavor QCD

We study infrared conformality of the twelve-flavor QCD on the lattice. Utilizing the highly improved staggered quarks (HISQ) type action which is useful to study the continuum physics, we analyze the lattice data of the mass and the decay constant of a pseudoscalar meson and the mass of a vector meson as well at several values of lattice spacing and fermion mass. Our result is consistent with the conformal hypothesis for the mass anomalous dimension $γ_m \sim 0.4-0.5$.

hep-lat↗

arXiv2012

Many flavor QCD with N_f=12 and 16

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.

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