Narrow Results

The thermal mass spectra of scalar and pseudo-scalar mesons are studied in an IR-improved soft-wall AdS/QCD model. The Reissner–Nordstrom AdS black hole metric is introduced to describe both the finite temperature and density effects. The thermal spectral function is computed, which shows that the position of peak moves toward a smaller value and the width of peak also broadens to a larger value as the temperature increases. The critical temperature at which the peaks are completely dissolved has been found to be around T_c≃160~200 MeV. The effect of chemical potential is shown to be the same as the one caused by the temperature. It is found that when the temperature approaches to zero T≈0, the melting down of mesons occurs at the scaled critical chemical potential μ₀/κ≈2.2~2.4 GeV.

The Regge spectrum of soft-wall holographic model is generalized to the case of arbitrary twist. Combining this result with the light-front holographic QCD, one arrives at a broad hydrogen-like degeneracy of the same sort as the degeneracy observed experimentally in the spectrum of light mesons.

To realize massive pions, we study variations of the holographic model of massless QCD using the brane configuration proposed by Sakai and Sugimoto. We propose deformations which break the chiral symmetry explicitly and compute the mass of the pions and vector mesons. The observed value of the pion mass can be obtained. We also argue a chiral perturbation corresponding to our deformation.

We investigate AdS/QCD duality for the two-point correlation function of the lowest dimension scalar glueball operator, in the case of the IR soft wall model. We point out the role of the boundary conditions for the bulk-to-boundary propagator in determining the gluon condensates. We show how a low energy QCD theorem can be obtained within the AdS approach, together with a gluon condensate close to the commonly accepted value and robust against perturbation of the background dilaton field.

AdS/QCD is an extra-dimensional approach to modeling hadronic physics, motivated by the AdS/CFT correspondence in string theory. AdS/QCD models are often more accurate than would have been expected at energies below a few GeV. We address the question of why these models are so successful, and respond to some of the criticisms that have been waged against these models.

The combination of Anti-de Sitter space (AdS) methods with light-front (LF) holography provides a remarkably accurate first approximation for the spectra and wavefunctions of meson and baryon light-quark bound states. The resulting bound-state Hamiltonian equation of motion in QCD leads to relativistic light-front wave equations in terms of an invariant impact variable ζ which measures the separation of the quark and gluonic constituents within the hadron at equal light-front time. These equations of motion in physical space-time are equivalent to the equations of motion which describe the propagation of spin-J modes in anti–de Sitter (AdS) space. The eigenvalues give the hadronic spectrum, and the eigenmodes represent the probability distributions of the hadronic constituents at a given scale. A positive-sign confining dilaton background modifying AdS space gives a very good account of meson and baryon spectroscopy and form factors. The light-front holographic mapping of this model also leads to a non-perturbative effective coupling which agrees with the effective charge defined by the Bjorken sum rule and lattice simulations. It displays a transition from perturbative to nonperturbative conformal regimes at a momentum scale ~ 1 GeV. The resulting β-function appears to capture the essential characteristics of the full β-function of QCD, thus giving further support to the application of the gauge/gravity duality to the confining dynamics of strongly coupled QCD.

According to the conjecture of the gauge/gravity duality, a strongly coupled 4D field theory may be described by a dual 5D theory which is a free field theory in the first approximation. The usual description of the axial-vector sector in the bottom–up AdS/QCD models includes, however, the quartic interaction with a bulk scalar field. A question arises, whether it is possible to describe some aspects of the chiral symmetry breaking (CSB) in a way more consistent with the original proposal, i.e. within a free 5D theory? We suggest that a natural candidate for this purpose is the soft wall (SW) model with inverse exponential profile.

One of the most fundamental problems in Quantum Chromodynamics is to understand the origin of the mass scale which controls the range of color confinement and the hadronic spectrum. For example, if one sets the Higgs couplings of quarks to zero, then no mass parameters appear in the QCD Lagrangian, and the theory is conformal at the classical level. Nevertheless, hadrons have a finite mass. We show that a mass gap and a fundamental color confinement scale arise when one extends the formalism of de Alfaro, Fubini and Furlan (dAFF) to frame-independent light-front Hamiltonian theory. Remarkably, the resulting light-front potential has a unique form of a harmonic oscillator in the light-front invariant impact variable if one requires that the action remains conformally invariant. The result is a single-variable relativistic equation of motion for bound states, a "Light-Front Schrödinger Equation," analogous to the nonrelativistic radial Schrödinger equation, which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics, including a massless pion for zero quark mass and linear Regge trajectories with the same slope in the radial quantum number and orbital angular momentum. The same light-front equations with the correct hadron spin dependence arise from the holographic mapping of "soft-wall model" modification of AdS₅ space with a specific dilaton profile. The corresponding light-front Dirac equation provides a dynamical and spectroscopic model of nucleons. A fundamental mass parameter κ appears, determining the hadron masses and the length scale which underlies hadron structure. Quark masses can be introduced to account for the spectrum of strange hadrons. This Light-Front Holographic approach predicts not only hadron spectroscopy successfully, but also hadron dynamics — hadronic form factors, the QCD running coupling at small virtuality, the light-front wavefunctions of hadrons, ρ electroproduction, distribution amplitudes, valence structure functions, etc. Thus the combination of light-front dynamics, its holographic mapping to gravity in a higher-dimensional space and the dAFF procedure provides new insight into the physics underlying color confinement, chiral invariance, and the QCD mass scale, among the most profound questions in physics.

Generalized parton distribution functions (GPDs) describe different features of hadronic physics in a single framework. There are sum rules that relate them to phenomenologically interesting quantities. By means of GPDs calculated in the holographic soft-wall model, perturbative effects are entered into the formalism in an effective way. Using the DGLAP-like equations, GPDs are evolved to the next-to-leading order of approximation and then implications on nucleon form factors and their charge distributions will be discussed at a qualitative level.

By using the AdS/CFT correspondence, we investigate various form factors between nucleons and mesons in a nuclear medium. In order to describe a nuclear medium holographically, we take into account the thermal charged AdS geometry with an appropriate IR cutoff. After introducing an anomalous dimension as a free parameter, we investigate how the nucleon’s mass is affected by the change of the anomalous dimension. Moreover, we study how the form factors of nucleons rely on the properties of the nuclear medium. We show that in a nuclear medium with different numbers of proton and neutron, the degenerated nucleon form factor in the vacuum is split into four different values depending on the isospin charges of nucleon and meson.

AdS/QCD is implemented to calculate the basis light-front quantization (BLFQ) wave function for $J/\psi$ meson. Exclusive $J/\psi$ photoproduction is computed in the dipole picture using this holographic wave function. The bCGC model is employed in the calculation of $J/\psi$ production for dipole amplitude. The differential cross-sections and total cross-sections of $J/\psi$ meson are computed and compared with the experimental data. We find that the AdS/QCD holographic wave function in BLFQ can be employed as a new candidate for the light-front wave function of $J/\psi$ meson in diffractive process.

The axial-vector form factor of the nucleons is studied at a finite temperature using the holographic soft-wall model with the thermal dilaton field. We use the bulk interaction action known from the zero temperature case and apply the profile functions of fields thermalized by the interaction with the thermal dilaton field. The dependence of the axial form factor on the square of the transferred momentum and the temperature is plotted for the ground and excited states of the nucleons.

We consider a five-dimensional fully anisotropic holographic model supported by Einstein-dilaton-three-Maxwell action. This type of model within different warp-factors and the coupling function for the Maxwell field providing the nontrivial chemical potential can describe magnetic/inverse magnetic catalysis phenomena expected from lattice results for heavy and light quarks. To reproduce magnetic catalysis phenomena for heavy quarks, we should modify the warp factor and consider additional polynomial terms in warp factor. It is expected that the first-order phase transition will coincide with chiral phase transition and the phase between first-order phase transition line and confinement/deconfinement line can be interpreted as quarkyonic phase.

We show that a dynamical AdS/QCD model is able to reproduce the linear Regge trajectories for the light-flavor sector of mesons with high spin and also for the scalar and pseudo-scalar ones. In addition the model has confinement by the Wilson loop criteria and a mass gap. We also calculate the decay amplitude of scalars into two pion with a good agreement with the available experimental data.

One of the most important nonperturbative methods for solving QCD is quantization at fixed light-front time τ = t + z/c – Dirac's "Front Form". The eigenvalues of the light-front QCD Hamiltonian predict the hadron spectrum and the eigensolutions provide the light-front wavefunctions which describe hadron structure. More generally, we show that the valence Fock-state wavefunctions of the light-front QCD Hamiltonian satisfy a single-variable relativistic equation of motion, analogous to the nonrelativistic radial Schrödinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. We outline a method for computing the required potential from first principles in QCD. The holographic mapping of gravity in AdS space to QCD, quantized at fixed light-front time, yields the same light front Schrödinger equation; in fact, the soft-wall AdS/QCD approach provides a model for the light-front potential which is color-confining and reproduces well the light-hadron spectrum. One also derives via light-front holography a precise relation between the bound-state amplitudes in the fifth dimension of AdS space and the boost-invariant light-front wavefunctions describing the internal structure of hadrons in physical space-time. The elastic and transition form factors of the pion and the nucleons are found to be well described in this framework. The light-front AdS/QCD holographic approach thus gives a frame-independent first approximation of the color-confining dynamics, spectroscopy, and excitation spectra of relativistic light-quark bound states in QCD.

We discuss an holographic soft wall model to describe nucleon properties. We pay special attention to nucleon spectrum, GPDs in the skewness case for nucleons and electroproduction of the N (1440) Roper resonance in soft-wall AdS/QCD.

In this talk, the references on the mass determination of the 0⁺⁺ scalar glueball is briefly reviewed. Some discussions on the exploration of glueball in theory and experiment are made.

We briefly review motivation and results of the approach to QCD vacuum as a medium describable in terms of statistical ensemble of almost everywhere constant Abelian (anti-)self-dual gluon fields. An overview of the hadronization formalism based on this ensemble is presented. New results for radial excitations of light, heavy-light mesons and heavy quarkonia are presented. A possible interrelation between the present approach and holographic QCD with harmonic confinement is outlined.

In this work we discuss how to construct the Regge trajectories for light mesons and baryons in the context of a bottom-up holographic model, consisting on a 5-dimensional AdS background, deformed with a quadratic function in the holographic coordinate. We fit the scalar meson f₀, vector meson ρ, baryons with spin 1/2⁺ and 3/2^* trajectories. These results are in fine agreement with those exposed in PDG. We also found in this model a universal Regge slope near to 1.1 GeV².

In AdS/QCD models medium effects are usually introduced in the metric, but recently some authors have started to consider the possibility to catch media properties in the dilaton field. In this talk we present a couple of examples about this topic.