We consider the bottom-up holographic models for QCD which contain the ultraviolet (UV) cutoff. Such models are supposed to describe exclusively the low-energy sector of QCD. The introduction of UV cutoff in the soft wall model is shown to result in a model with qualitatively different predictions. The ensuing model seems to be able to incorporate the constituent quark mass. It is also demonstrated that in order to reproduce the results of the usual soft wall model for the vector and higher spin mesons in the presence of the UV cutoff one can consider the flat bulk space with a modified dilaton background.
Thermodynamics of strongly interacting matter near critical end point are investigated in a holographic QCD model, which could describe the QCD phase diagram in $T-μ$ plane qualitatively. Critical exponents along different axis ($α,β,γ,δ$) are extracted numerically. It is given that $α\approx 0, β\approx 0.54, γ\approx 1.04, δ\approx 3.0$, the same as 3D Ising mean-field approximation and previous holographic QCD model calculations. We also discuss the possibilities to go beyond mean field approximation by including the full back-reaction of chiral dynamics in the holographic framework.
In this paper we describe qualitatively some aspects of the holographic QCD. Inspired by a successfull 4D description, we try to separate the Confinement and the Chiral Symmetry Breaking dynamics. We also discuss the realization of the baryons as skyrmions in Soft Wall Holographic QCD, and the issue of the Vector Meson Dominance.
The AdS/QCD holographic wave function of basis light-front quantization (BLFQ) for vector meson $J/ψ$ is applied in this manuscript. The exclusive production of $J/ψ$ in diffractive process is computed in dipole model with AdS/QCD holographic wave function. We use IP-Sat and IIM model in the calculation of the differential cross section of the dipole scattering off the proton. The prediction of AdS/QCD holographic wave function in BLFQ gives a good agreement to the experimental data.
We investigate the quark-gluon mixed condensate based on an AdS/QCD model. Introducing a holographic field dual to the operator for the quark-gluon mixed condensate, we obtain the corresponding classical equation of motion. Taking the mixed condensate as an additional free parameter, we show that the present scheme reproduces very well experimental data. A fixed value of the mixed condensate is in good agreement with that of the QCD sum rules.
The holographic QCD prediction for the pion distribution amplitude (DA) $ϕ_{hol}(u)$ is used to compute the pion spacelike electromagnetic form factor $F_π(Q^2)$ within the QCD light-cone sum rule method. In calculations the pion's renormalon-based model twist-4 DA, as well as the asymptotic twist-4 DA are employed. Obtained theoretical predictions are compared with experimental data and with results of the holographic QCD.
We report that the pion radius, decay constant, EM and transition form factors can simultaneously and accurately be predicted in light-front holographic QCD with a universal AdS/QCD mass scale, provided dynamical spin effects are taken into account.
This paper, written in memory of Manoj Banerjee, takes a critical look at holographic models of QCD focusing on ``practical'' models in which the five dimensional theory is treated classically. A number of theoretical and phenomenological challenges to the approach are discussed.
We discuss diffractive photon-production of vector mesons in holographic QCD. At large $\sqrt{s}$, the QCD scattering amplitudes are reduced to the scattering of pair of dipoles exchanging a closed string or a pomeron. We use the holographic construction in AdS$_5$ to describe both the intrinsic dipole distribution in each hadron, and the pomeron exchange. Our results for the heavy meson photon-production are made explicit and compared to some existing experiments.
We discuss the vertex function of two vector and one axial-vector operators in the soft-wall holographic model of QCD. When one of the two vector currents represents an on-shell soft photon, such a vertex is described by two structure functions $w_L$ and $w_T$, which are usually calculated through triangular loop diagrams. We evaluate these functions in the soft-wall model of holographic QCD (HQCD) and compare the outcome to the QCD findings.
We discuss the recent progress made in using bottom-up holographic QCD models in calculating hadronic contributions to the anomalous magnetic moment of the muon, in particular the hadronic light-by-light scattering contribution, where holographic QCD naturally satisfies the Melnikov-Vainshtein constraint by an infinite series of axial vector meson contributions.
A new approach to nonperturbative QCD, holographic light-front QCD, provides a comprehensive model for hadron dynamics and spectroscopy, incorporating color confinement, a universal hadron mass scale, the prediction of a massless pion in the chiral limit, and connections between the spectroscopy of mesons, baryons and tetraquarks across the full hadron spectrum. In this article we present predictions for the Sivers asymmetry and related transverse momentum distributions for the proton based on the light-front wavefunctions of the baryon eigenstate predicted by holographic QCD.
We investigate the transverse charge density in the longitudinally as well as transversely polarized deuteron using the recent empirical description of the deuteron electromagnetic form factors in the framework of holographic QCD. The predictions of the holographic QCD are compared with the results of a standard phenomenological parameterization. In addition, we evaluate GPDs and the gravitational form factors for the deuteron. The longitudinal momentum densities are also investigated in the transverse plane.
We demonstrate another success of the AdS/QCD correspondence by showing that an AdS/QCD holographic light-front wavefunction for the $ρ$ meson generates predictions for the cross-sections of diffractive $ρ$ production that are in agreement with data collected at the HERA electron-proton collider.
We use an AdS/QCD holographic wavefunction to generate predictions for the rate of diffractive ρ-meson electroproduction that are in reasonable agreement with data collected at the HERA electron-proton collider.
In this paper we present a short overview on the dynamical holographic QCD method for hadron physics and QCD matter. The 5-dimensional dynamical holographic QCD model is constructed in the graviton-dilaton-scalar framework with the dilaton background field $Φ$ and the scalar field $X$ dual to the gluon condensate and the chiral condensate operator thus can represent the gluodynamics (linear confinement) and chiral dynamics (chiral symmetry breaking), respectively. The dilaton background field and the scalar field are a function of the 5th dimension, which plays the role of the energy scale, in…
The AdS/QCD dictionary is considered, checking the large-N behavior of 5d dual models of QCD as a guideline. Especially, a consistent chiral symmetry breaking function in the Hard Wall model is derived and the different forms of the field/operator correspondence are emphasized.
Holographic QCD is an extra-dimensional approach to modeling hadrons, the bound states of the strong interactions. In holographic models, the extra spatial dimension creates a waveguide for fields, and the discrete towers of modes propagating in that waveguide are interpreted as hadronic resonances. These models are motivated by the AdS/CFT correspondence, which is a duality that relates theories in different numbers of spatial dimensions. Holographic models have the potential to provide a better understanding of strongly interacting systems of quarks and gluons, as well as unconventional supe…
We focus on the chiral and heavy quark mass expansion of mesons masses and decay constants. We propose a light-front QCD formalism for the evaluation of these quantities, consistent with chiral perturbation theory and heavy quark effective theory.
We investigate the AdS/QCD duality for the two-point correlation functions of the lowest dimension scalar meson and scalar glueball operators, in the case of the Soft Wall holographic model of QCD. Masses and decay constants as well as gluon condensates are compared to their QCD estimates. In particular, the role of the boundary conditions for the bulk-to-boundary propagators is emphasized.
We present a study of flavor structures of transverse charge and anomalous magnetization densities for both unpolarized and transversely polarized nucleons. We consider two different models for the electromagnetic form factors in holographic QCD. The flavor form factors are obtained by decomposing the Dirac and Pauli form factors for nucleons using the charge and isospin symmetry. The results are compared with two standard phenomenological parametrizations.
We predict observables in radiative B decays to vector mesons using holographic AdS/QCD Distribution Amplitudes for the vector mesons. We find that end-point divergences can be avoided when computing power-suppressed contributions in the heavy quark limit. The results reported here can be found in references [1] and [2].
We show that AdS/QCD generates predictions for the rate of diffractive rho meson electroproduction that are in agreement with data collected at the HERA electron-proton collider.
Using light-front holography, we predict simultaneously the pion decay constant and the pion charge radius by taking into account (higher twist) dynamical spin effects whose relative importance is constrained by QCD.
Central heavy-ion collisions may induce sizeable fluctuations of the topological charge. This effect is expected to distort the dispersion relation for the hadron masses. We construct a general setup for a compact description of this phenomenon in the framework of bottom-up holographic approach to QCD. A couple of soft wall holographic models are proposed for the vector mesons. The states having different circular polarizations are shown to have different effective mass. The requirement of stability imposes strong constraints on the possible choice of models.
The light-front wavefunction for mesons predicted by holographic light-front QCD is used to obtain the light-cone distribution amplitudes for the light vector mesons and the transition form factors for B and $B_s$ decays to $ρ$, $K^*$ and $ϕ$ mesons. Here, I present some of our predictions as compared to lattice QCD, OCD sum rules and the available experimental data.
A holographic description of chiral symmetry breaking in the pattern of QCD is reviewed. D7 brane probes are used to include quark fields in a simple non-supersymmetric deformation of the AdS/CFT Correspondence. The axial symmetry breaking is realized geometrically and the quark condensate and meson masses are computable. Surprisingly, treating the model as a description of QCD works quantitatively at the 15% level. Models of this AdS/QCD type typically have a strongly coupled, conformal UV regime that is far from QCD. To systematically move closer to QCD, we propose cutting out the large radi…
The properties of scalar mesons and glueballs at finite temperature are analyzed through a {\it bottom-up} holographic approach. We focus on the spectral functions and mass spectra. A discussion on hadron dissociation and deconfinement phase transition is also put forward.
We consider the light-front holographic QCD with the light-front wave functions for mesons, modified for massive quarks. We evaluate the wave functions, distribution amplitudes, and form factors for $π$, $ρ$, $K$, and $J/ψ$ mesons and photon-to-meson transition form factors for $π$, $η$, and $η^\prime$ mesons. The results are compared with the experimental data, wherever available.
We investigate the spectra of light scalar and vector glueballs in a holografic description of QCD with a dilaton background bulk field. In particular, we study how the glueball masses depend on the conditions on the dilaton background and on the geometry of the bulk.
We consider the electromagnetic form factors of strange vector, axial vector and pseudoscalar mesons in a holographic QCD model. We find the charge radius of charged kaon agrees with the experiment, while the charge radius of charged pion is a little bit smaller than the experimental value, as obtained in other calculations in the hard-wall holographic QCD models. The charge radii of charged rho and $K^\ast$ quantitatively agree with a recent Dyson-Schwinger equation calculation. We also present the electric form factors of vector, axial vector and pseudoscalar mesons in both space-like and ti…
Light-front wavefunctions obtained from holographic light-front QCD are used to obtain the distributions amplitudes for $K^*$ vector meson. Consequently, alternate predictions for rare B transitions to $K^*$ form factors are presented. In this talk, I compare our results for some rare B decay channels to those obtained from QCD sum rules and available experimental data.
The lowest dimensional gluon condensate $G_2$ is analysed at finite temperature and chemical potential using a bottom/up holographic model of QCD. Starting from the free energy of the model, pressure, entropy and quark density are obtained. Moreover, at zero chemical potential, 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.
A dilaton potential is adjusted to recently confirmed lattice QCD thermodynamics data in the temperature range $(0.7 \ldots 3.5) T_c$ where $T_c = 155 \text{MeV}$ is the pseudo-critical temperature. The employed holographic model is based on a gravity--single-field dilaton dual. We discuss conditions for enforcing (for the pure gluon plasma) or avoiding (for the QCD quark-gluon plasma) a first-order phase transition, but still keeping a softest point (minimum of sound velocity).
We describe a new exact relation for large $N_c$ QCD for the long-distance behavior of baryon form factors in the chiral limit, satisfied by all 4D semi-classical chiral soliton models. We use this relation to test the consistency of the structure of two different holographic models of baryons.
In this paper we review the hadroproduction mechanisms of double-charm baryons for the different experimental environments and reinterpret the SELEX and LHCb results.
We reassess a work done by Migdal on the spectrum of low-energy vector mesons in QCD in the light of the AdS-QCD correspondence. Recently, a tantalizing parallelism was suggested between Migdal's work and a family of holographic duals of QCD. Despite the intriguing similarities, both approaches face a major drawback: the spectrum is in conflict with well-tested Regge scaling. However, it has recently been shown that holographic duals can be modified to accomodate Regge behavior. Therefore, it is interesting to understand whether Regge behavior can also be achieved in Migdal's approach. In this…
We study the vertex function of two vector and one axial-vector operators in the soft-wall holographic model of QCD. In particular, we discuss the possible relation, introduced by Son and Yamamoto, between the structure function $w_T$ describing such a vertex when one of the two vector currents represents an on-shell soft photon and the the two-point $Π_\perp^{VV}-Π_\perp^{AA}$ correlation function.
The recent determination of the $β$--function of the QCD running coupling $α_{\overline{MS}}(Q^2)$ to 5-loops{\color{blue},} provides a verification of the convergence of a novel method for determining the fundamental QCD parameter $Λ_s$ based on the Light-Front Holographic approach to nonperturbative QCD. The new 5-loop analysis, together with improvements in determining the holographic QCD nonperturbative scale parameter $κ$ from hadronic spectroscopy, leads to an improved precision of the value of $Λ_s$ in the ${\overline{MS}}$ scheme close to a factor of two; we find $Λ^{(3)}_{\overline{MS…
We employ both top-down and bottom-up holographic dual models of QCD to calculate vertex functions and couplings that are induced by the five dimensional Chern-Simons term. We use these couplings to study the photoproduction of $f_1$ mesons. The Chern-Simons-term-induced interaction leads to a simple relation between the polarization of the incoming photon and the final state $f_1$ meson which should allow a clear separation of this interaction from competing processes.
In this work we calculate the four-point correlation function of vector quark currents of QCD via holographic QCD model. Computing the correlator we take into account the exchange of vector and axial vector bosons and dilaton in the bulk. The result is used for calculation of the two-point correlator of electromagnetic currents in external magnetic field at zero momentum, related to RHIC experiments, chiral magnetic effect and lattice study. At zero temperature we find this quantity to be loosely connected with chiral symmetry breaking and strongly dependent on the confinement properties. Some…
We revisit confinement/deconfinement transition in holographic QCD to consider the back-reaction of a bulk scalar field. The bulk scalar field is dual to a quark bi-linear operator $\bar qq$, and it encodes explicit and spontaneous chiral symmetry breaking of QCD. To perform the Hawking-Page transition analysis with the back-reaction, we first obtain a deformed AdS black hole solution due to a finite quark mass. Through the Hawking-Page analysis with the back-reacted geometry, we study the flavor number $N_f$ and finite quark mass dependence of the critical temperature of the QCD confinement/d…
We construct a Schrodinger-like equation for the longitudinal wave function of a meson in the valence qq-bar sector, based on the 't Hooft model for large-N two-dimensional QCD, and combine this with the usual transverse equation from light-front holographic QCD, to obtain a model for mesons with massive quarks. The computed wave functions are compared with the wave function ansatz of Brodsky and De Teramond and used to compute decay constants and parton distribution functions. The basis functions used to solve the longitudinal equation may be useful for more general calculations of meson stat…
We investigate inhomogeneous chiral condensation under rotation considering finite size effects and boundary conditions in the holographic QCD model. The rotational suppression effect determined by $Ωr$ is confirmed in the holographic model which is not influenced by the boundary conditions. For chiral condensation at the center, it is found that under Neumann boundary condition the finite size exhibits two opposite effects, i.e., catalysis at high temperatures and inverse catalysis at low temperatures. In contrast, under Dirichlet boundary condition, the effect of finite size on condensation …
A holographic dual of hadrons at finite density is considered. We use the zero black-hole mass limit of Reisner-Nordstrom (RN) AdS background with hard wall to describe a confining background with finite quark density. We calculate density-dependence of meson masses and decay constants. In our model, pion decay constant and its velocity go down but all the meson masses go up as density grows.
We establish a holographic QCD model for four flavors, where a light scalar field $X$ and a heavy scalar field $H$ are introduced, separately. The $H$ field is responsible for the breaking of $SU(N_f=4)$ to $SU(N_f=3)$. The ground state and its Regge excitation of meson spectra in the light flavor sector and heavy flavor sector as well as the ligh-heavy mesons are well in agreement with the Particle data group (PDG). Due to the additional introduction of the $H$ field in the model, different Regge slopes for light and heavy mesons can be achieved.
I consider the three-point function of two vector and one axial-vector currents. In the kinematic configuration in which one of the two vector currents corresponds to an on-shell soft photon, such a vertex is described in QCD by two functions, $w_L$ and $w_T$. After reviewing the properties of these functions in QCD, I describe the result obtained using the soft-wall holographic model of QCD with the addition of the Chern-Simons term and I compare it with the QCD outcome. I also discuss a relation, proposed by Son and Yamamoto, that connects $w_L$ and $w_T$ to the two-point functions $Π_{VV}$ …
We study the phenomenology of the recently proposed modified version of the Soft-Wall model of holographic QCD with a negative dilaton profile. We investigate vector and scalar mesons, evaluating mass spectra, decay constants and two-point correlation functions. We then study chiral symmetry breaking and compute the coupling of a scalar meson with two pseudoscalars. Finally, we compare the results with the ones found in the literature in the positive-dilaton case.
The Roper resonance, the first excited state of the nucleon, Is one of the best-established baryon resonances. Yet, its properties have not been consistently explained by effective models of QCD, such as the non-relativistic quark model. In this letter, we propose an alternative approach in the Sakai-Sugimoto model that is one of the holographic models of QCD. In particular, we analyze the helicity amplitude of the electromagnetic transitions at the leading of 't~Hooft coupling $1/λ$. The model incorporates baryon structure at short distance by non-linear mesons surrounded by meson clouds at l…
We calculate the contribution of the higher twist Feynman diagrams to the large-$p_T$ inclusive pion production cross section in proton-proton collisions in the case of the running and frozen coupling approaches within holographic QCD. The structure of infrared renormalon singularities of the higher twist subprocess cross section and it's resummed expression are found. We compare the resummed higher twist cross sections with the ones obtained in the framework of the frozen coupling approximation and leading twist cross section. We discuss the phenomenological consequences of possible higher-tw…
We study the drag force of a relativistic heavy quark using a holographic QCD model with conformal invariance broken by a background dilaton. We analyze the effects of chemical potential and confining scale on this quantity, respectively. It is shown that the drag force in this model is larger than that of $\mathcal{N}=4$ supersymmetric Yang-Mills (SYM) plasma. In particular, the inclusion of the chemical potential and confining scale both enhance the drag force, in agreement with earlier findings. Also, we discuss how chemical potential and confining scale influence the diffusion coefficient.
We calculate the Standard Model (SM) predictions for the differential branching ratio of the rare $B_s \to ϕμ^+ μ^-$ decays using $B_s \to ϕ$ transition form factors (TFFs) obtained using holographic light-front QCD (hQCD) instead of the traditional QCD sum rules (QCDSR) . Our predictions for the differential branching ratio is in better agreement with the LHCb data. Also, we find that the hQCD prediction for $R_{K^*ϕ}$, the ratio of the branching fraction of $B \to K^* μ^+ μ^-$ to that of $B_s \to ϕμ^+ μ^-$ , is in excellent agreement with both the LHCb and CDF results in low $q^2$ range.
We predict the isospin asymmetry as well as the branching ratio for the decay $B -> K* γ$ within QCD factorization using new anti-de Sitter/Quantum Chromodynamics (AdS/QCD) holographic Distribution Amplitudes (DAs) for the K* meson. Our prediction for the branching ratio agrees with that obtained using standard QCD Sum Rules (SR) DAs and with experiment. More interestingly, our prediction for the isospin asymmetry using the AdS/QCD DA does not suffer from the end-point divergence encountered when using the corresponding SR DA. We predict an isospin asymmetry of 3.3 % in agreement with the most…
We investigate the importance of dynamical spin effects in the holographic light-front wavefunctions of the pseudoscalar mesons. We find that these effects are crucial to describe the pion data while they are not necessary to describe the available kaon data. For $η-η^\prime$ system, we find that dynamical spin effects are required to describe their transition form factors data.
An SU(Nf)xSU(Nf) Yang-Mills theory on an extra-dimensional interval is considered, with appropriate symmetry-breaking boundary conditions on the IR brane. UV-brane to UV-brane correlators at high energies are compared with the OPE of two-point functions of QCD quark currents. Condensates correspond to departure from AdS of the (different) metrics felt by vector and axial combinations, away from the UV brane. Their effect on hadronic observables is studied: the extracted condensates agree with the signs and orders of magnitude expected from QCD.
A Double-Dilaton Holographic Model of QCD: Running Coupling and Meson Phenomenology
Universitat Autònoma de Barcelona（巴塞罗那自治大学）；Institut de Física d’Altes Energies (IFAE)（高能物理研究所（IFAE））；The Barcelona Institute of Science and Technology (BIST)（巴塞罗那科学技术学院（BIST））
The aim of these lectures is to convey a working knowledge of Light Front Holographic QCD and Supersymmetric Light Front Holographic QCD. We first give an overview of holographic QCD in general and then concentrate on the application of the holographic methods on QCD quantized in the light front form. We show how the implementation of the supersymmetric algebra fixes the interaction and how one can obtain hadron mass spectra with the minimal number of parameters. We also treat propagators and compare the holographic approach with other non-perturbative methods. In the last chapter we describe …
