Holographic QCD Research Articles

Scalar mesons within a dynamical holographic QCD model

We show that the infrared dynamics of string modes dual to q q ¯ states within a Dynamical AdS/QCD model of coupled dilaton-gravity background gives the Regge-like spectrum of f 0 's scalars and higher spin mesons consistent with experimental data. The pion mass and its trajectory were also described with a scale deformation of the metric and a rescaled string mass. The available experimental decay widths of the S → P P decays provided a complementary check of the proposed classification scheme for f 0 ( 980 ) , f 0 ( 1370 ) , f 0 ( 1500 ) and f 0 ( 1710 ) as radial excitations of f 0 ( 600 ) . For f 0 ( 980 ) we estimated a mixing angle of ± 20 o with other structures.

Nucleon-nucleon potential in holographic QCD

We derive the nucleon-nucleon potential in the D4-D8 model of holographic QCD. The nucleon-nucleon potential emerges from one-boson exchange picture with massless pseudo-scalars and an infinite tower of spin one mesons. We observe in particular that the tensor coupling of the ρ meson with nucleon comes out naturally larger than the vector coupling, whereas for ω mesons and axial mesons, such tensor couplings are completely absent. The potential is universally repulsive ∼ 1 / r 2 at short distance, and has the usual long-distance attractive behavior ∼ − 1 / r 3 along a isospin singlet and spin triplet channel. Both the large N c form and the finite N c form are given. In the former, a shallow classical minimum of depth ∼ 0.1 M K K N c / λ forms at about r M K K ≃ 5.5 . We discuss shallow bound states of nucleons with the holographic potential.

On the evaluation of gluon condensate effects in the holographic approach to QCD

In holographic QCD the effects of gluonic condensate can be encoded in a suitable deformation of the 5D metric. We develop two different methods for the evaluation of first order perturbative corrections to masses and decay constants of vector resonances in 5D Hard-Wall models of QCD due to small deformations of the metric. They are extracted either from a novel compact form for the first order correction to the vector two-point function, or from perturbation theory for vector bound-state eigenfunctions: the equivalence of the two methods is shown. Our procedures are then applied to flat and to AdS 5D Hard-Wall models; we complement results of existing literature evaluating the corrections to vector decay constant and to two-pion–one-vector couplings: this is particularly relevant to satisfy the sum rules. We concentrate our attention on the effects for the Gasser–Leutwyler coefficients; we show that as in the Chiral Quark model, the addition of the gluonic condensate improves the consistency, the understanding and the agreement with phenomenology of the holographic model.

Four-point correlator of vector currents and electric current susceptibility in holographic QCD

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 features of the AdS/QCD models are also discussed.

Intersecting D4-branes model of holographic QCD and tachyon condensation

We consider the intersecting D4-brane and anti-D4-brane model of holographic QCD, motivated by the model that has recently been suggested by Van Raamsdonk and Whyte. We analyze such D4-branes by the use of the action with a bi-fundamental ``tachyon'' field, so that we find the classical solutions describing the intersecting D4-branes and the U-shaped D4-branes. We show that the ``tachyon'' field in the bulk theory provides a current quark mass and a quark condensate to the dual gauge theory and that the lowest modes of mesons obtain mass via tachyon condensation. Then evaluating the properties of a pion, one can reproduce Gell-Mann-Oakes-Renner relation.

Three-flavor quark mass dependence of baryon spectra in holographic QCD

We introduce the strange quark mass to the Sakai–Sugimoto model of holographic QCD. We compute mass shifts in the spectra of three-flavor baryons at the leading order in perturbation in quark masses. Comparison with experimental data shows an agreement only qualitatively.

Tachyonic crystals and the laminar instability of the perturbative vacuum in asymptotically free gauge theories

Lattice Monte Carlo studies in SU(3) gauge theory have shown that the topological charge distribution in the vacuum is dominated by thin coherent membranes of codimension one arranged in a layered, alternating-sign sandwich. A similar lamination of topological charge occurs in the 2D $CP^{N-1}$ model. In holographic QCD, the observed topological charge sheets are naturally interpreted as $D6$ branes wrapped around an $S_4$.. With this interpretation, the laminated array of topological charge membranes observed on the lattice can be identified as a "tachyonic crystal", a regular, alternating-sign array of $D6$ and $\bar{D6}$ branes that arises as the final state of the decay of a non-BPS $D7$ brane via the tachyonic mode of the attached string. In the gauge theory, the homogeneous, space-filling $D7$ brane represents the perturbative gauge vacuum, which is unstable toward lamination associated with a marginal tachyonic boundary perturbation $\propto \cos(X/\sqrt{2\alpha'})$. For the $CP^{N-1}$ model, the cutoff field theory can be cast as the low energy limit of an open string theory in background gauge and tachyon fields $A_{\mu}(x)$ and $\lambda(x)$. This allows a detailed comparison with large $N$ field theory results and provides strong support for the tachyonic crystal interpretation of the gauge theory vacuum.

Holographic QCD in medium: a bottom up approach

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.

N-body nuclear forces at short distances in holographic QCD

We provide a calculation of N-body (N>2) nucleon interactions at short distances in holographic QCD. In the Sakai-Sugimoto model of large N_c massless QCD, N baryons are described by N Yang-Mills instantons in 5 spacetime dimensions. We compute a classical short distance interaction hamiltonian for N 'tHooft instantons. This corresponds to N baryons sharing identical classical spins and isospins. We find that genuine N-body nuclear forces turn out to vanish for N>2, at the leading order. This suggests that classical N-body forces are always suppressed compared with 2-body forces.

Quark number susceptibility with finite chemical potential in holographic QCD

We study the quark number susceptibility in holographic QCD with a finite chemical potential or under an external magnetic field at finite temperature. We first consider the quark number susceptibility with the chemical potential. We observe that approaching the critical temperature from high temperature regime, the quark number susceptibility divided by temperature square develops a peak as we increase the chemical potential, which confirms recent lattice QCD results. We discuss this behavior in connection with the existence of the critical end point in the QCD phase diagram. We also consider the quark number susceptibility under the external magnetic field. We predict that the quark number susceptibility exhibits a blow-up behavior at low temperature as we raise the value of the magnetic field. We finally spell out some limitations of our study.

Melting spectral functions of the scalar and vector mesons in a holographic QCD model

We investigate the finite-temperature spectral functions of heavy quarkonia by using the soft-wall AdS/QCD model. We discuss the scalar, the pseudo-scalar, the vector, and the axial-vector mesons and compare their qualitative features of the melting temperature and growing width. We find that the axial-vector meson melts earlier than the vector meson, while there appears only a slight difference between the scalar and pseudo-scalar mesons which also melt earlier than the vector meson.

Nuclear Force from String Theory

Recent “technology” called holography, or gauge/string duality (AdS/CFT correspondence) found in string theory, makes it possible to compute various quantities of strongly coupled gauge theories. This technology was applied to QCD, and it was found that it describes surprisingly well important properties of low energy QCD, the hadron physics. We apply it further to nuclear physics. In this talk, I review a part of the developments of the holographic QCD, and show a computation of nuclear force at short distance, derived using the holographic QCD, which was done in collaboration with T. Sakai and S. Sugimoto [K. Hashimoto, T. Sakai, and S. Sugimoto, “Holographic Baryons: Static Properties and Form Factors from Gauge/String Duality,” Prog. Theor. Phys. 120 (2008) 1093–1137, arXiv:0806.3122 [hep-th]; K. Hashimoto, T. Sakai, and S. Sugimoto, “Nuclear Force from String Theory,” arXiv:0901.4449 [hep-th]]. (This talk was presented at Hyp-X conference at J-PARC, on 25th September, 2009.)

Nuclear matter to strange matter transition in holographic QCD

We construct a simple holographic QCD model to study nuclear matter to strange matter transition. The interaction of dense medium and hadrons is taken care of by imposing the force balancing condition for stable D4/D6/D6 configuration. By considering the intermediate and light flavor branes interacting with baryon vertex homogeneously distributed along R^3 space and requesting the energy minimization, we find that there is a well defined transition density as a function of current quark mass. We also find that as density goes up very high, intermediate (or heavy) and light quarks populate equally as expected from the Pauli principle. In this sense, the effect of the Pauli principle is realized as dynamics of D-branes.

Scalar mesons and glueballs in Dp-Dq hard-wall models

We investigate the light scalar mesons and glueballs in the Dp-Dq hard-wall models, including D3-Dq, D4-Dq, and D6-Dq systems. It is found that only in the D4-D6 and D4-D8 hard-wall models are the predicted masses of the q̄q scalar meson f0 scalar glueball consistent with their experimental or lattice results. This indicates that D4-D6 and D4-D8 hard-wall models are the favorite candidates of the realistic holographic QCD model.

Quark mass dependence of the hadron spectrum in holographic QCD

We compute a shift of baryon mass spectra due to quark masses in perturbation, in Sakai-Sugimoto model of holographic QCD. We find the shift for the ground state nucleons to be \delta M = 4.1 m_{\pi}^2 [GeV^{-1}], which is consistent with the current lattice QCD result. We predict the same value of the shift for N(1535) and Delta, while a larger value 7.7 m_{\pi}^2 [GeV^{-1}] for Roper N(1440). We also present some evidences that the shifts of the vector meson masses are suppressed in the large 't Hooft coupling limit.

Tests of Universality of Baryon Form Factors in Holographic QCD

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.

Confront holographic QCD with Regge trajectories of vectors and axial-vectors

We derive the general 5-dimension metric structure of the $Dp-Dq$ system in type II superstring theory, and demonstrate the physical meaning of the parameters characterizing the 5-dimension metric structure of the \textit{holographic} QCD model by relating them to the parameters describing Regge trajectories. By matching the spectra of vector mesons $\rho_1$ with deformed $Dp-Dq$ soft-wall model, we find that the spectra of vector mesons $\rho_1$ can be described very well in the soft-wall $D3-Dq$ model, i.e, $AdS_5$ soft-wall model. We then investigate how well the $AdS_5$ soft-wall model can describe the Regge trajectory of axial-vector mesons $a_1$. We find that the constant component of the 5-dimension mass square of axial-vector mesons plays an efficient role to realize the chiral symmetry breaking in the vacuum, and a small negative $z^4$ correction in the 5-dimension mass square is helpful to realize the chiral symmetry restoration in high excitation states.

Baryon with Massive Strangeness in Holographic QCD

Baryon with Massive Strangeness in Holographic QCD

Transverse Structure of Hadrons in Holographic QCD

Transverse Structure of Hadrons in Holographic QCD

Form factors of vector and axial-vector mesons in holographic D4-D8 model

We calculate elastic and non-elastic electromagnetic form factors for vector and axial-vector mesons in the holographic D4-D8 brane model. We obtain the mass spectrum and Regge trajectories for these particles. From the elastic form factors we extract the electric radius, the magnetic and quadrupole moments. Form factors for transverse and longitudinal polarizations are also obtained. We find superconvergence sum rules for the vector and axial-vector meson couplings that determine the asymptotic behavior of the form factors at large momentum transfer. Our results show good agreement with other holographic models and QCD.