arXiv subjects

Fen Zuo

Publications and source records attributed to Fen Zuo.

36 records · Page 2Linked to original sources

arXiv2014

Thermal power terms in the Einstein-dilaton system

We employ the gauge/string duality to study the thermal power terms of various thermodynamic quantities in gauge theories and the renormalized Polyakov loop above the deconfinement phase transition. We restrict ourselves to the five-dimensional Einstein gravity coupled to a single scalar, the dilaton. The asymptotic solutions of the system for a general dilaton potential are employed to study the power contributions of various quantities. If the dilaton is dual to the dimension-4 operator ${\rm Tr} F_{μν}^2$, no power corrections would be generated. Then the thermal quantities approach their asymptotic values much more quickly than those observed in lattice simulation. When the dimension of the dual operator is different from 4, various power terms are generated. The lowest power contributions to the thermal quantities are always quadratic in the dilaton, while that of the Polyakov loop is linear. As a result, the quadratic terms in inverse temperature for both the trace anomaly and the Polyakov loop, observed in lattice simulation, cannot be implemented consistently in the system. This is in accordance with the field theory expectation, where no gauge-invariant operator can accommodate such contributions. Two simple models, where the dilaton is dual to operators with different dimensions, are studied in detail to clarify the conclusion.

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arXiv2014

Quadratic thermal terms in the deconfined phase from holography

Recent lattice simulation has uncovered many interesting properties of SU(N) gauge theory at finite temperature. Especially, above the deconfinement phase transition all the thermodynamics quantities acquire significant quadratic contributions in inverse temperature. Such a term is also found to dominate the logarithmic of the renormalized Polyakov loop. Using the Hawking-Page transition in Anti-de Sitter space as an example, we show how such contributions can be naturally generated in the holographic approach.

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arXiv2014

Temperature and chemical potential dependence of the gluon condensate: a holographic study

The lowest dimensional gluon condensate $G_2$ is analyzed at finite temperature and chemical potential using a holographic model of QCD with conformal invariance broken by a background dilaton. Starting from the free energy of the model, the thermodynamical quantities needed to determine the $T$ and $μ$ dependence of the gluon condensate are evaluated. At high temperature the gluon condensate is independent of chemical potential. Moreover, at $μ=0$ and in the string frame, the temporal and spatial Wilson loops at low temperature are computed; they are related to the (chromo) electric and magnetic components of $G_2$, respectively. The $T$-dependence of the two components is separately determined.

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arXiv2013

Large-distance properties of holographic baryons

Employing the asymptotic instanton solution in an arbitrary background of a set of holographic QCD models, we show that baryon form factors have a precise large-distance behaviour regardless of the background. The dependence coincides with that obtained from general chiral soliton models and large-$N_C$ chiral perturbation theory. The nonlinear terms in the equations of motion are necessary to recover the correct results. We also show that the holographic currents have the right structure at low energy if the solutions of the full equation of motion, instead of the linearized ones, are used. The indication is that in this holographic approach, the linearized approximation used in the meson sector is not appropriate for the solitonic description of the baryons.

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arXiv2013

Holography, chiral Lagrangian and form factor relations

We perform a detailed study of mesonic properties in a class of holographic models of QCD, which is described by the Yang-Mills plus Chern-Simons action. By decomposing the 5 dimensional gauge field into resonances and integrating out the massive ones, we reproduce the Chiral Perturbative Theory Lagrangian up to ${\cal O}(p^6)$ and obtain all the relevant low energy constants (LECs). The numerical predictions of the LECs show minor model dependence, and agree reasonably with the determinations from other approaches. Interestingly, various model-independent relations appear among them. Some of these relations are found to be the large-distance limits of universal relations between form factors of the anomalous and even-parity sectors of QCD.

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arXiv2012

Holography and chiral Lagrangian

In a wide class of holographic models described by the Yang-Mills and Chern-Simons terms, we derive all the ${\cal O}(p^6)$ Chiral Perturbation Theory low-energy constants. Various model-independent relations exist among the constants up to this order, generalizing the previously found relations at $p^4$ order. Some of them can be extended to relations between different amplitudes.

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arXiv2012

Photon-to-pion transition form factor and pion distribution amplitude from holographic QCD

We try to understand the recently observed anomalous behavior of the photon-to-pion transition form factor in the holographic QCD approach. First the holographic description of the anomalous γ^*γ^*π^0 form factor is reviewed and applied to various models. It is illustrated that in describing the anomalous form factor, the holographic approach is asymptotically dual to the perturbative QCD (pQCD) framework, with the pion mode π(z)\sim z corresponding to the asymptotic pion distribution amplitude. This indicates some inconsistency in light-front holography, since π(z)\sim z would be dual to φ(x)\sim \sqrt{x(1-x)} there. After clarifying these subtleties, we employ the relation between the holographic and the perturbative expressions to study possible asymptotic violation of the transition form factor. It is found that if one require that the asymptotic form factor possess a pQCD-like expression, the pion mode can only be ultraviolet-enhanced by logarithmic factors. The minimally deformed pion mode will then be of the form π(z)\sim z\ln (zΛ)^{-1}. We suppose that this deformation may be due to the coupling of the pion with a nontrivial open string tachyon field, and then the parameter $Λ$ will be related to the quark condensate. Interestingly, this pion mode leads immediately to Radyushkin's logarithmic model, which fitted very well the experimental data in the large-Q^2 region. On the other side, the pQCD interpretation with a flat-like pion distribution amplitude, proposed by Radyushkin and Polyakov, fails to possess a holographic expression.

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arXiv2011

Skyrmion properties from holographic instantons of large size

Based on the intanton approximation of Skyrmion and recent progresses on the holographic approach, we study baryon properties using Skyrmions generated from holographic instantons. First we employ Atiyah and Manton's early observation to show that the instanton approximation gives the correct infrared behavior for the Skyrmion solution, and thus those of the electromagnetic form factors of the nucleon. We then use Skyrmions generated from flat-space instanton solutions to study various baryon properties, treating the instanton size as an arbitrary variable. It is found that, a large instanton size is required to reproduce the physical axial coupling. From such an instanton configuration, the predictions for other baryon quantities in the Skyrme model improve the original results quite a lot. Some of the results reproduce those by quantizing the full instanton solution, while no fine-tuning of the mesonic parameters are needed. From these we conclude that the baryons in the low energy region can be effectively described by large instantons.

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arXiv2011

The cross section of the process $e^{+}+e^{-}\to J/ψ+η_c$ within the QCD light-cone sum rules

We calculate the cross section of the exclusive process $e^{+} +e^{-}\to J/ψ+η_c$ at the leading order approximation within the QCD light-cone sum rules (LCSR) approach. It is found that the form factor $F_{VP}(V=J/ψ,P=η_c)$ depends mainly on the behavior of the twist-2 distribution amplitude of the $η_c$-meson at the scale of this process. Thus in order to obtain a reliable estimation of the cross section, it is important to have a realistic distribution amplitude of the $η_c$ meson, and to deal with the evolution of the distribution amplitude to the effective energy scale of the process. Our results show that one can obtain a compatible prediction with the Belle and BarBar experimental data.

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arXiv2010

Improved soft-wall model with a negative dilaton

I propose to change the sign of the dilaton in the infrared~(IR) soft-wall AdS/QCD model, in order to implement confinement. The deformed model exhibits interesting properties, especially in describing chiral symmetry breaking. The expectation value of the scalar field $X$, which determines the quark mass and condensate, approaches a constant in the IR limit, rather than blows up in the original model. In contrast to the estimate in Ref. \cite{Shifman:2007xn}, this kind of solution will not lead to chiral symmetry restoration for highly-excited states, due to the property of the harmonic-oscillator equation. Instead, it will guarantee the Regge behavior of the axial meson spectrum, and also the pseudoscalar mesons. The value of the condensate can be fixed by requiring that the pion be massless in the chiral limit, but only under some approximation in the present model. We also find that, by relaxing the IR boundary conditions, the unphysical massless state in the vector channel can be eliminated.

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arXiv2010

$γ^{*}ρ^0\toπ^0$ Transition Form Factor in Extended AdS/QCD Models

The $γ^{*}ρ^0\toπ^0$ transition form factor is extracted from recent result for the $γ^* γ^* π^0$ form factor obtained in the extended hard-wall AdS/QCD model with a Chern-Simons term. In the large momentum region, the form factor exhibits a $1/Q^4$ behavior, in accordance with the perturbative QCD analysis, and also with the Light-Cone Sum Rule (LCSR) result if the pion wave function exhibits the same endpoint behavior as the asymptotic one. The appearance of this power behavior from the AdS side and the LCSR approach seem to be rather similar: both of them come from the {"}soft" contributions. Comparing the expressions for the form factor in both sides, one can obtain the duality relation $z\propto \sqrt{u(1-u)}$, which is compatible with one of the most important relations of the Light-Front holography advocated by Brodsky and de Teramond. In the moderate $Q^2$ region, the comparison of the numerical results from both approaches also supports a asymptotic-like pion wave function, in accordance with previous studies for the $γ^* γ^* π^0$ form factor. The form factor at zero momentum transfer gives the $γ^{*}ρ^0π^0$ coupling constant, from which one can determine the partial width for the $ρ^0(ω)\to π^0 γ$ decay. We also calculate the form factor in the time-like region, and study the corresponding Dalitz decays $ρ^0(ω)\to π^0 e^+e^-, π^0μ^+μ^-$. Although all these results are obtained in the chiral limit, numerical calculations with finite quark masses show that the corrections are extremely small. Some of these calculations are repeated in the Hirn-Sanz model and similar results are obtained.

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arXiv2009

Remarks on the two-dimensional power correction in the soft wall model

We present a direct derivation of the two-point correlation function of the vector current in the soft wall model by using the AdS/CFT dictionary. The resulting correlator is exactly the same as the one previously obtained from dispersion relation with the same spectral function as in this model. The coefficient $C_2$ of the two-dimensional power correction is found to be $C_2=-c/2$ with $c$ the slope of the Regge trajectory, rather than $C_2=-c/3$ derived from the strategy of first quantized string theory. Taking the slope of the $ρ$ trajectory $c\approx0.9{GeV}^2$ as input, we then get $C_2\approx-0.45{GeV}^2$. The gluon condensate is found to be $<α_sG^2>\approx0.064{GeV}^4$, which is almost identical to the QCD sum rule estimation. By comparing these two equivalent derivation of the correlator of scalar glueball operator, we demonstrate that the two-dimensional correction can't be eliminated by including the non-leading solution in the bulk-to-boundary propagator, as was done in \cite{Colangelo2}. In other words, the two-dimensional correction does exist in the scalar glueball case. Also it is manifest by using the dispersion relation that the minus sign of gluon condensate and violation of the low energy theorem are related to the subtraction scheme.

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arXiv2008

Heavy-to-light transition form factors and their relations in light-cone QCD sum rules

The improved light-cone QCD sum rules by using chiral current correlator is systematically reviewed and applied to the calculation of all the heavy-to-light form factors, including all the semileptonic and penguin ones. By choosing suitable chiral currents, the light-cone sum rules for all the form factors are greatly simplified and depend mainly on one leading twist distribution amplitude of the light meson. As a result, relations between these form factors arise naturally. At the considered accuracy these relations reproduce the results obtained in the literature. Moreover, since the explicit dependence on the leading twist distribution amplitudes is preserved, these relations may be more useful to simulate the experimental data and extract the information on the distribution amplitude.

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arXiv2008

Semileptonic $B(B_s, B_c)$ decays in the light-cone QCD sum rules

Semileptonic $B$($B_s, B_c$) decays are investigated systematically in the light-cone QCD sum rules. Special emphasis is put on the LCSR calculation on weak form factors with an adequate chiral current correlator, which turns out to be particularly effective to control the pollution by higher twist components of spectator mesons. The result for each channel depends on the distribution amplitude of the the producing meson. The leading twist distribution amplitudes of the related heavy mesons and charmonium are worked out by a model approach in the reasonable way. A practical scenario is suggested to understand the behavior of weak form factors in the whole kinematically accessible ranges. The decay widths and branching ratios are estimated for several $B$($B_c$) decay modes of current interest.

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arXiv2007

Couplings of the Rho Meson in a Holographic dual of QCD with Regge Trajectories

The couplings $g_{ρHH}$ of the $ρ$ meson with any hadron H are calculated in a holographic dual of QCD where the Regge trajectories for mesons are manifest. The resulting couplings grow linearly with the exciting number of H, thus are far from universal. A simple argument has been given for this behavior based on quasi-classical picture of excited hadrons. It seems that in holographic duals with exact Regge trajectories the $g_{ρHH}$ universality should be violated. The $ρ$-dominance for the electromagnetic form factors of H are also strongly violated, except for the lowest state, the pion. Quite unexpected, the form factor of the pion is completely saturated by the contribution of the $ρ$. The asymptotic behavior of the form factors are also calculated, and are found to be perfectly accordant with the prediction of conformal symmetry and pertubative QCD.

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arXiv2007

Semileptonic $B_c$ decays and Charmonium distribution amplitude

In this paper we study the semileptonic decays of the $B_c$ meson in the Light-Cone Sum Rule (LCSR) approach. The result for each channel depends on the corresponding distribution amplitude of the final meson. For the case of $B_c$ decaying into a pseudoscalar meson, to twist-3 accuracy only the leading twist distribution amplitude (DA) is involved if we start from a chiral current. If we choose a suitable chiral current in the vector meson case, the main twist-3 contributions are also eliminated and we can consider the leading twist contribution only. The leading twist distribution amplitudes of the charmonium and other heavy mesons are given by a model approach in the reasonable way. Employing this charmonium distribution amplitude we find the cross section $σ(e^+e^-\to J/ψ+η_c)\simeq22.8 {fb}$ which is consistent with Belle and BaBar's data. Based on this model, we calculate the form factors for various $B_c$ decay modes in the corresponding regions. Extrapolating the form factors to the whole kinetic regions, we get the decay widths and branching ratios for various $B_c$ decay modes including their $τ$ modes when they are kinematically accessible.

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arXiv2006

$B_c(B){\to}D l\tildeν$ form factors in Light-Cone Sum Rules and the $D$-meson distribution amplitude

In this paper we calculate the weak form factors of the decays $B_c(B)\to Dl\tildeν$ by using the chiral current correlator within the framework of the QCD light-cone sum rules (LCSR). The expressions of the form factors only depend on the leading twist distribution amplitude (DA) of the $D$ meson. Three models of the $D$-meson distribution amplitude are employed and the calculated form factor $F_{B_c\to D}(0)$ is given. Our prediction, by using the $D$-meson distribution amplitude with the exponential suppression at the end points, is compatible with other approaches, and favors the three-points sum rules (3PSR) approach with the Coulumb corrections included.

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arXiv2006

Form Factor for $B{\to}D l\tildeν$ in Light-Cone Sum Rules With Chiral Current Correlator

Within the framework of QCD light-cone sum rules (LCSR), we calculate the form factor for $B{\to}D l\tildeν$ transitions with chiral current correlator. The resulting form factor depends on the distribution amplitude(DA) of the $D$-meson. We try to use three kinds of DA models of the $D$-meson. In the velocity transfer region $1.14 < y < 1.59$, which renders the Operator Product Expansion (OPE) near light-cone $x^2=0$ go effectively, the yielding behavior of form factor is in agreement with that extracted from the data on $ B\to D l\tildeν$, within the error. In the larger recoil region $1.35 < y < 1.59$, the results are observed consistent with those of perturbative QCD (pQCD). The presented calculation can play a bridge role connecting those from the lattice QCD with the heavy quark symmetry and perturbative QCD(pQCD) to have an all-around understanding of $B{\to}D l\tildeν$ transitions.

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