arXiv subjects

Julius Kuti

Publications and source records attributed to Julius Kuti.

At least 37 records · Page 2Linked to original sources

arXiv2015

The running coupling of the minimal sextet composite Higgs model

We compute the renormalized running coupling of SU(3) gauge theory coupled to N_f = 2 flavors of massless Dirac fermions in the 2-index-symmetric (sextet) representation. This model is of particular interest as a minimal realization of the strongly interacting composite Higgs scenario. A recently proposed finite volume gradient flow scheme is used. The calculations are performed at several lattice spacings with two different implementations of the gradient flow allowing for a controlled continuum extrapolation and particular attention is paid to estimating the systematic uncertainties. For small values of the renormalized coupling our results for the beta-function agree with perturbation theory. For moderate couplings we observe a downward deviation relative to the 2-loop beta-function but in the coupling range where the continuum extrapolation is fully under control we do not observe an infrared fixed point. The explored range includes the locations of the zero of the 3-loop and the 4-loop beta-functions in the MSbar scheme. The absence of a non-trivial zero in the beta-function in the explored range of the coupling is consistent with our earlier findings based on hadronic observables, the chiral condensate and the GMOR relation. The present work is the first to report continuum non-perturbative results for the sextet model.

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arXiv2015

The running coupling of 8 flavors and 3 colors

We compute the renormalized running coupling of SU(3) gauge theory coupled to N_f = 8 flavors of massless fundamental Dirac fermions. The recently proposed finite volume gradient flow scheme is used. The calculations are performed at several lattice spacings allowing for a controlled continuum extrapolation. The results for the discrete beta-function show that it is monotonic without any sign of a fixed point in the range of couplings we cover. As a cross check the continuum results are compared with the well-known perturbative continuum beta-function for small values of the renormalized coupling and perfect agreement is found.

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arXiv2015

Toward the minimal realization of a light composite Higgs

Work in progress is reported on a particularly interesting gauge theory with a fermion doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group. Extending previous studies we outline our strategy as we investigate Goldstone dynamics and Electroweak scale setting from chiral symmetry breaking ($χ$ SB), test the GMOR relation from the spectrum of the Dirac operator and the related chiral condensate, begin to develop and test mixed action based improved analysis of $χ$ SB with new run plans at fixed topology to cross over from the p-regime to the epsilon-regime of $χ$ SB, continue to pursue the light $0^{++}$ scalar and its relation to the dilaton, and probe the scale-dependent running coupling from the perturbative UV scale to the scale of chiral symmetry breaking. Our observations suggest that the model is very close to the conformal window and a light composite scalar, perhaps the Higgs impostor with or without dilaton-like interpretation, appears to emerge with $0^{++}$ quantum numbers. The lightest baryon of the model on the 3 TeV scale has intriguing implications.

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arXiv2015

Baryon spectrum in the composite sextet model

The strongly coupled near-conformal gauge theory with two fermion flavors in the two-index symmetric (sextet) representation of SU(3) is potentially a minimal realization of the composite Higgs mechanism. We discuss the staggered fermion construction of baryonic states, present our first numerical results and comment on implications for dark matter.

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arXiv2014

The lattice gradient flow at tree level

The cut-off effects of the lattice gradient flow -- often called Wilson flow -- are calculated on a periodic 4-torus at leading order in the gauge coupling. A large class of discretizations is considered which includes all frequently used cases in practice. It is shown how the results lead to a smoother continuum extrapolation for the beta-function of SU(3) gauge theory with N_f = 4 flavors of fermions.

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arXiv2014

The lattice gradient flow at tree-level and its improvement

The Yang-Mills gradient flow and the observable E(t), defined by the square of the field strength tensor at t>0, are calculated at finite lattice spacing and tree-level in the gauge coupling. Improvement of the flow, the gauge action and the observable are all considered. The results are relevant for two purposes. First, the discretization of the flow, gauge action and observable can be chosen in such a way that $O(a^2)$, $O(a^4)$ or even $O(a^6)$ improvement is achieved. Second, simulation results using arbitrary discretizations can be tree-level improved by the perturbatively calculated correction factor normalized to one in the continuum limit.

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arXiv2014

Can a light Higgs impostor hide in composite gauge models?

The frequently discussed strongly interacting gauge theory with a fermion flavor doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group is investigated. In previous studies the chiral condensate and the mass spectrum were shown to be consistent with chiral symmetry breaking ($χ$SB) at vanishing fermion mass. The recently reported $β$-function is not inconsistent with this observation, suggesting that the model is very close to the conformal window and a light "Higgs impostor" could emerge as a composite state. In this work we describe the methodology and preliminary results of studying the emergence of the light composite scalar with $0^{++}$ quantum numbers.

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arXiv2014

The chiral condensate from the Dirac spectrum in BSM gauge theories

The eigenvalues of the Dirac operator at finite volume encode whether or not chiral symmetry is spontaneously broken in a massless theory. We apply this framework in a particular BSM context, namely SU(3) gauge theory with N_f=2 massless flavors in the 2-index symmetric (sextet) representation. Our first results are at a single lattice spacing. We find that both the density of near-zero eigenvalues and the renormalization group invariant mode number indicate spontaneous symmetry breaking. Quantitatively, there is a discrepancy between the determination of the fermion condensate in the chiral limit via the eigenvalue spectrum and the determinations from direct measurements of the chiral condensate and the GMOR relation. We comment on possible explanations of this discrepancy and further refinements of this study.

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arXiv2013

Lattice Gauge Theories at the Energy Frontier

This White Paper has been prepared as a planning document for the Division of High Energy Physics of the U. S. Department of Energy. Recent progress in lattice-based studies of physics beyond the standard model is summarized, and major current goals of USQCD research in this area are presented. Challenges and opportunities associated with the recently discovered 126 GeV Higgs-like particle are highlighted. Computational resources needed for reaching important goals are described. The document was finalized on February 11, 2013 with references that are not aimed to be complete, or account for an accurate historical record of the field.

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arXiv2012

The sextet gauge model, light Higgs, and the dilaton

The frequently discussed strongly interacting gauge theory with a fermion flavor doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group is investigated \cite{Fodor:2012ty}. The chiral condensate and the mass spectrum are consistent with chiral symmetry breaking at vanishing fermion mass. In contrast, sextet fermion mass deformations of spectral properties are not consistent with leading conformal scaling behavior near the critical surface of a conformal theory. A recent paper \cite{DeGrand:2012yq} which could not resolve the conformal fixed point of the gauge coupling from the slowly walking scenario of a very small nearly vanishing β-function is not in conflict with chiral symmetry breaking reported here. A light Higgs impostor could emerge as the dilaton from spontaneous symmetry breaking of scale invariance or, without the dilaton mechanism, as a composite state.

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arXiv2012

Conformal finite size scaling of twelve fermion flavors

Extended simulation results and their analysis are reported in a strongly coupled gauge theory with twelve fermion flavors in the fundamental SU(3) color representation. The conformality of the model is probed using mass deformed conformal finite size scaling (FSS) theory driven by the fermion mass anomalous dimension. Two independent conformal FSS fitting procedures are used in the analysis. The first one deploys physics motivated scaling functions, complemented by a second fitting procedure with spline based general B-forms for the scaling functions. The results at fixed gauge coupling show unresolved problems with the conformal hypothesis.

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arXiv2012

Confining force and running coupling with twelve fundamental and two sextet fermions

We investigate two models of much recent interest in lattice Beyond Standard Model studies: N_f=2 fermions in the 2-index symmetric (sextet) representation, and N_f=12 fermions in the fundamental representation, both with SU(3) gauge symmetry. We present results at fixed lattice spacing for the static fermion potential and force as measured via lattice simulations. We show indications that both models are confining in the chiral limit and that neither theory is conformal. This is consistent with our findings for the mass spectrum, which indicate that chiral symmetry is spontaneously broken in both theories.

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arXiv2012

The gradient flow running coupling scheme

The Yang-Mills gradient flow in finite volume is used to define a running coupling scheme. As our main result the discrete beta-function, or step scaling function, is calculated for scale change s=3/2 at several lattice spacings for SU(3) gauge theory coupled to N_f = 4 fundamental massless fermions. The continuum extrapolation is performed and agreement is found with the continuum perturbative results for small renormalized coupling. The case of SU(2) gauge group is briefly commented on.

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arXiv2012

Can the nearly conformal sextet gauge model hide the Higgs impostor?

New results are reported from large scale lattice simulations of a frequently discussed strongly interacting gauge theory with a fermion flavor doublet in the two-index symmetric (sextet) representation of the SU(3) color gauge group. We find that the chiral condensate and the mass spectrum of the sextet model are consistent with chiral symmetry breaking in the limit of vanishing fermion mass. In contrast, sextet fermion mass deformations of spectral properties are not consistent with leading conformal scaling behavior near the critical surface of a conformal theory. A recent paper could not resolve the conformal fixed point of the gauge coupling from the slowly walking scenario of a very small nearly vanishing β-function (DeGrand:2012yq). It is argued that overall consistency with our new results is resolved if the sextet model is close to the conformal window, staying outside with a very small non-vanishing β-function. The model would exhibit then the simplest composite Higgs mechanism leaving open the possibility of a light scalar state with quantum numbers of the Higgs impostor. It would emerge as the pseudo-Goldstone dilaton state from spontaneous symmetry breaking of scale invariance. We will argue that even without association with the dilaton, the scalar Higgs-like state can be light very close to the conformal window. A new Higgs project of sextet lattice simulations is outlined to resolve these important questions.

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arXiv2012

The Yang-Mills gradient flow in finite volume

The Yang-Mills gradient flow is considered on the four dimensional torus T^4 for SU(N) gauge theory coupled to N_f flavors of massless fermions in arbitrary representations. The small volume dynamics is dominated by the constant gauge fields. The expectation value of the field strength tensor squared is calculated for positive flow time t by treating the non-zero gauge modes perturbatively and the zero modes exactly. The finite volume correction to the infinite volume result is found to contain both algebraic and exponential terms. The leading order result is then used to define a one parameter family of running coupling schemes in which the coupling runs with the linear size of the box. The new scheme is tested numerically in SU(3) gauge theory coupled to N_f = 4 flavors of massless fundamental fermions. The calculations are performed at several lattice spacings with a controlled continuum extrapolation. The continuum result agrees with the perturbative 2-loop prediction for small renormalized coupling as expected.

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arXiv2012

Twelve fundamental and two sextet fermion flavors

We report extended simulation results and their new analysis in two important gauge theories with twelve fermion flavors in the fundamental SU(3) color representation and two fermions in the sextet representation. We probe the $N_f=12$ model with respect to the conformal window using mass deformed finite size scaling (FSS) theory driven by the fermion mass anomalous dimension. Our results at fixed gauge coupling show problems with the conformal scenario of the $N_f=12$ model. In the sextet model with two flavors, under the conformal hypothesis, we determine large values for the anomalous fermion mass dimension with $γ\geq 1$. Since our sextet analysis favors the chiral symmetry breaking hypothesis without conformality, the large exponent $γ$ could play an important role in understanding the composite Higgs mechanism. The new results discussed here include our extended data sets and exceed what was presented at the conference.

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arXiv2011

Twelve massless flavors and three colors below the conformal window

We report new results for a frequently discussed gauge theory with twelve fermion flavors in the fundamental representation of the SU(3) color gauge group. The model, controversial with respect to its conformality, is important in non-perturbative studies searching for a viable composite Higgs mechanism Beyond the Standard Model (BSM). To resolve the controversy, we subject the model to opposite hypotheses inside and outside of the conformal window. In the first hypothesis we test chiral symmetry breaking ($χ{\rm SB}$) with its Goldstone spectrum, $F_π$, the $χ{\rm SB}$ condensate, and several composite hadron states as the fermion mass is varied in a limited range with our best effort to control finite volume effects and extrapolation to the massless chiral limit. Supporting results for $χ{\rm SB}$ from the running coupling based on the force between static sources and some preliminary evidence for the finite temperature transition are also presented. In the second test for the alternate hypothesis we probe conformal behavior driven by a single anomalous mass dimension under the assumption of unbroken chiral symmetry. Our results show a very low level of confidence in the conformal scenario. Staggered lattice fermions with stout-suppressed taste breaking are used throughout the simulations.

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arXiv2011

Chiral symmetry breaking in fundamental and sextet fermion representations of SU(3) color

We report new results for lattice gauge theories with twelve fermion flavors in the fundamental representation and two fermion flavors in the two-index symmetric (sextet) representation of the SU(3) color gauge group. Both models are important in searching for a viable composite Higgs mechanism in the Beyond the Standard Model (BSM) paradigm. We subject both models to opposite hypotheses inside and outside of the conformal window. In the first hypothesis we test chiral symmetry breaking ($χ{\rm SB}$) with its Goldstone spectrum, $F_π$, the $χ{\rm SB}$ condensate, and several composite hadron states as the fermion mass is varied in a limited range with our best effort to control finite volume effects. Supporting results for $χ{\rm SB}$ from the running coupling based on the force between static sources is also presented. In the second test for the alternate hypothesis we probe conformal behavior driven by a single anomalous mass dimension under the assumption of unbroken chiral symmetry. Our results show very low level of confidence in the conformal scenario.

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