Meson supercurrents and the Meissner effect in the Sakai-Sugimoto model
Anton Rebhan, Andreas Schmitt, Stefan A. Stricker
备注：
46 pages, 5 figures, v2: small modifications and added footnotes in discussion of axial current and number density (p19, p24); typos corrected; references added; version to appear in JHEP
The Sakai-Sugimoto model provides a holographic description for chiral symmetry breaking. We use this model to investigate chirally broken phases in an external magnetic field at finite isospin and baryon chemical potentials. The equations of motion for the bulk gauge fields are solved analytically and the free energy is computed from the Yang-Mills and Chern-Simons contributions to the D8 brane action. In the case of a neutral pion condensate, a magnetic field is found to induce nonzero gradients of the Goldstone boson fields corresponding to meson supercurrents. A charged pion condensate, on the other hand, expels the magnetic field due to the Meissner effect. Upon comparing the Gibbs free energies of these two phases we find that the rotation of the chiral condensate into a charged pion condensate for finite isospin chemical potentials is partially undone by switching on a magnetic field, and we determine the critical magnetic field which removes the charged pion condensate in a first-order phase transition.
We estimate the consequences of finite masses of pseudoscalar mesons on the decay rates of scalar glueballs in the Witten-Sakai-Sugimoto model, a top-down holographic model of low-energy QCD, by extrapolating from the calculable vertex of glueball fields and the eta' meson which follows from the Witten-Veneziano mechanism for giving mass to the latter. Evaluating the effect on the recently calculated decay rates of glueballs in the Witten-Sakai-Sugimoto model, we find a strong enhancement of the decay of scalar glueballs into kaons and eta mesons, in fairly close agreement with experimental da…
We study the instability of the low-temperature QCD vacuum at large-N_c due to an axial chemical potential, using the holographic Sakai-Sugimoto model. We explicitly construct the ground state of the theory, which corresponds to a translationally non-invariant configuration.
We reconsider the realization of baryons in the Sakai-Sugimoto model. In this theory, which is the gravity dual of a QCD-like theory, baryons appear as soliton solutions. These solitons were approximated as flat-space instantons in previous studies. However, with this approximation, it has been shown that one does not reproduce some model-independent predictions for the behavior of baryon electromagnetic form factors which are connected with long-range pion physics. This made it appear that the long-range pion physics of baryons may be hidden in (intractable) α' corrections in the gravity dual…
A brief review of the Witten-Sakai-Sugimoto model is given, which is a top-down holographic model of low-energy QCD with chiral quarks derived from type-IIA superstring theory. The main predictions of the model, in particular concerning meson spectra, the gluon condensate, the QCD string tension, the mass of the $η'$ and of baryons are discussed and compared quantitatively with available experimental and/or lattice results. Then some recent results of potential interest to the physics program at the future FAIR facility are presented: The spectrum of glueballs and their decay rates into pions,…
We add smeared D0 charges to the D4 background and discuss Sakai-Sugimoto model under this background. The corresponding gauge theory develops a glue condensate $< tr (F_{μν}\tilde F^{μν})> $. The D8 branes go less deep than in the original S-S model and massless Goldstones are still found in the spectrum. The effects of the condensate on the meson spectra, pion decay constant, and couplings of the vector mesons and Goldstones are then investigated.
In this proceeding we review some of our recent results for the vector meson electromagnetic form factors and structure functions in the Sakai-Sugimoto model. The latter is a string model that describes many features of the non-perturbative regime of Quantum Chromodynamics in the large $N_c$ limit.
We discuss recent results on the calculation of glueball decay rates in the Witten-Sakai-Sugimoto model, which favor the $f_0(1710)$ meson as a glueball candidate. The flavor asymmetric decay of $f_0(1710)$ is frequently attributed to a putative chiral suppression in glueball decays, which is however questionable in view of the large constituent quark masses induced by chiral symmetry breaking. We find that this can be explained by what we call nonchiral enhancement when finite quark masses are included in the holographic model, with good quantitative agreement with experimental data for $f_0(…
We extend our investigation of resonance production in the Sakai-Sugimoto model to the case of negative parity baryon resonances. Using holographic techniques we extract the generalized Dirac and Pauli baryon form factors as well as the helicity amplitudes for these baryonic states. Identifying the first negative parity resonance with the experimentally observed S_{11}(1535), we find reasonable agreement with experimental data from the JLab-CLAS collaboration. We also estimate the contribution of negative parity baryons to the proton structure functions.
The magnetized pure pion gradient ($\mathcal{5}ϕ$) phase in the deconfined Sakai-Sugimoto model is explored at zero and finite temperature. We found that the temperature has very small effects on the phase. The thermodynamical properties of the phase shows that the excitations behave like a scalar solitonic free particles. By comparing the free energy of the pion gradient phase to the competing multiquark-pion gradient (MQ-$\mathcal{5}ϕ$) phase, it becomes apparent that the pure pion gradient is less thermodynamically preferred than the MQ-$\mathcal{5}ϕ$ phase. However, in the parameter space …
We continue our study of the dynamics of the flavour sector of the Sakai-Sugimoto model in the presence of an external magnetic field, uncovering several features of the meson spectrum at high and low temperatures. We employ both analytical and numerical methods to study the coupled non-linear equations that result from the gravity dual.
Till date, the only consistent description of the deconfinement phase of the Sakai-Sugimoto model appears to be provided by the analysis of [1] (arXiv:1107.4048). The current version of the analysis, however, has a subtlety regarding the monodromy of quarks around the Euclidean time circle. In this note, we revisit and resolve this issue by considering the effect of an imaginary baryon chemical potential on quark monodromies. With this ingredient, the proposal of [1] for investigating finite temperature QCD using holography is firmly established. Additionally, our technique allows a holographi…
We investigate the effect of a magnetic field on the vacuum of low-temperature QCD at large-N_c in the presence of a chiral chemical potential, using the holographic Sakai-Sugimoto model. Above some critical chemical potential we find an instability, which triggers a decay of the homogeneous vacuum to a non-homogeneous configuration with a spiral form, which we construct explicitly. We find that this decay is suppressed for sufficiently large magnetic field, and determine the critical strength. We find that the new vacuum does not exhibit the chiral magnetic effect. This is related to the way …
We investigate the production of positive parity baryon resonances in proton electromagnetic scattering within the Sakai-Sugimoto model. The latter is a string model for the non-perturbative regime of large $N_c$ QCD. Using holographic techniques we calculate the generalized Dirac and Pauli form factors that describe resonance production. We use these results to estimate the contribution of resonance production to the proton structure functions. Interestingly, we find an approximate Callan-Gross relation for the structure functions in a regime of intermediate values of the Bjorken variable.
We revisit and extend previous calculations of glueball decay rates in the Sakai-Sugimoto model, a holographic top-down approach for QCD with chiral quarks based on D8 probe branes in Witten's holographic model of nonsupersymmetric Yang-Mills theory. The rates for decays into two pions, two vector mesons, four pions, and the strongly suppressed decay into four pi0 are worked out quantitatively, using a range of the 't Hooft coupling which closely reproduces the decay rate of rho and omega mesons and also leads to a gluon condensate consistent with QCD sum rule calculations. The lowest holograp…
In this follow-up paper of 1105.2217 we further discuss the occurrence of a magnetically induced tachyonic instability of the rho meson in the two-flavour Sakai-Sugimoto model, uplifting two remaining approximations in the previous paper. That is, firstly, the magnetically induced splitting of the branes is now taken into account, evaluating without approximations the symmetrized trace which enters in the non-Abelian Dirac-Born-Infeld (DBI) action. This leads to an extra mass generating effect for the charged heavy-light rho meson through a holographic Higgs mechanism. Secondly, we compare the…
In this paper, we study the fate of the holographic zero sound mode at finite temperature and non-zero baryon density in the deconfined phase of the Sakai-Sugimoto model of holographic QCD. We establish the existence of such a mode for a wide range of temperatures and investigate the dispersion relation, quasi-normal modes, and spectral functions of the collective excitations in four different regimes, namely, the collisionless quantum, collisionless thermal, and two distinct hydrodynamic regimes. For sufficiently high temperatures, the zero sound completely disappears, and the low energy phys…
We examine the mesonic thermal spectrum of the Sakai-Sugimoto model of holographic QCD by finding the quasinormal frequencies of the supergravity dual. If flavour is added using D8 and anti-D8 branes there exist embeddings where the D-brane worldvolume contains a black hole. For these embeddings (the high-temperature phase of the Sakai-Sugimoto model) we determine the quasinormal spectra of scalar and vector mesons arising from the worldvolume DBI action of the D-brane. We stress the importance of a coordinate change that makes the in-falling quasinormal modes regular at the horizon allowing a…
Motivated by recent studies of deep inelastic scattering (DIS) off the $\mathcal{N}=4$ super-Yang-Mills (SYM) plasma, holographically dual to $AdS_5\times S^5$ black hole, we in this note use the spacelike flavor current to probe the internal structure of one holographic quark-gluon plasma, which is described by the Sakai-Sugimoto model at high temperature phase (i.e., the chiral symmetric phase). The plasma structure function is extracted from the retarded flavor current-current correlator. Our main aim here is to explore the effect of non-conformality on these physical quantities. As usual, …
We derive the generalized Skyrme model as a low-energy effective model of the Sakai-Sugimoto model. The novelty with the past is the presence of the sextic term equal to the topological charge squared. This term appears when the $ω$ meson, and the tower of states on top of it, are integrated out. We claim that, in the small 't Hooft coupling limit, the instanton is well described by a Skyrmion arising from the low energy effective Lagrangian of the Sakai-Sugimoto model. The sextic term plays a dominant role in this limit. Moreover, when a pion mass term is added we recover the BPS Skyrme model…
The Nambu-Jona-Lasinio and Sakai-Sugimoto models are juxtaposed, focusing on the models' dynamically generated masses and the phase diagrams. The models are studied in the parameter space of temperature, constant electromagnetic fields and the quark chemical potential. When the chemical potential is zero or small, the models show good agreement. However, the responses of the models under the large chemical potential are very different. In the presence of the background magnetic field, while the large chemical potential triggers the de Haas-van Alphen oscillation and the boundstate destabilizat…
In the chiral magnetic effect an imbalance in the number of left- and right-handed quarks gives rise to an electromagnetic current parallel to the magnetic field produced in noncentral heavy-ion collisions. The chiral imbalance may be induced by topologically nontrivial gluon configurations via the QCD axial anomaly, while the resulting electromagnetic current itself is a consequence of the QED anomaly. In the Sakai-Sugimoto model, which in a certain limit is dual to large-N_c QCD, we discuss the proper implementation of the QED axial anomaly, the (ambiguous) definition of chiral currents, and…
We use holographic QCD to study Pomeron- and Reggeon-mediated central production in Regge regime hadronic scattering. We focus specifically on $η$ production in proton-proton collisions. While previous work studied the Pomeron-mediated process, the mesonic Regge trajectories ("Reggeons") we now incorporate contribute significantly at experimentally probed energies. We use the five-point open string amplitude (in flat space) to construct approximate propagators for the Reggeon states, and the five-point closed string amplitude for the Pomeron. Using these "holographic" Reggeons and Pomerons, an…
The D4-D8 brane system of Sakai-Sugimoto model at high quark density is studied in the weak coupling regime. We show that the color superconducting phase (for Nc~3) or the chiral density wave (for Nc->infinity) disappears at very large chemical potential, or equivalently at very large compactified dimension that the model possesses. We also comment on the prospects in the strong coupling regime along with the QCD phase diagram.
The holographic Witten-Sakai-Sugimoto model is an almost parameter-free approximation to large-N QCD with chiral quarks that in a number of cases gives surprisingly good quantitative results for light meson physics. It also allows for predictions of the decay rates of glueballs into pseudoscalar and vector mesons through the coupling of modes on the flavor branes to the massive gravitational modes in the bulk. Including effects from finite quark masses leads to an enhancement of the decay of scalar glueballs into heavier pseudoscalars in good agreement with the decay pattern observed for the s…
The Null Melvin Twist to D4 brane background has been studied and putting in a bunch of coincident D8 and anti-D8 branes {\it a la} Sakai-Sugimoto model results in a breakdown of a global symmetry to its diagonal subgroup. The same is studied at finite temperature which results in the unchanged form of the chiral symmetry breaking and restoration curve. The effect of turning on a finite chemical potential do not changes the structure of the phase diagram with respect to Null Melvin Twist. Moreover, the confining and de-confining phase transition has the same structure as in the absence of Null…
Using gauge theory/string duality, we calculated the jet quenching parameter $\hat{q}$ of the Sakai-Sugimoto model in various phases. Being different from the $\mathcal{N}=4$ SYM theory where $\hat{q}\propto T^3$, we find that $\hat{q}\propto T^4/T_d$, where $T_d$ is the critical temperature of the confined/deconfined phase transition. By analyzing the $\hat{q}$ in different phases of this theory, we get better understanding about some statements in previous works, such as the non-universality and the explanation of discrepancies between the theory predictions and experiments.
In this paper, we calculate the transverse momentum diffusion coefficient kappa_T of heavy quark and gluon penetration length in the deconfinement phase of Sakai-Sugimoto model, which is known as a holographic dual of large N_c QCD. We find that for the heavy quark moving through the thermal plasma with a constant velocity v<1, the transverse momentum diffusion coefficient kappa_T is proportional to lamda*gamma^{1/3}*T^4/T_d, and the gluon penetration length delta x is proportional to E^{2/5}. These results are different from those calculated in N=4 super-Yang-Mills theory, where kappa_T is pr…
We study transport properties of the finite temperature Sakai-Sugimoto model. The model represents a holographic dual to 4+1 dimensional supersymmetric SU(N_c) gauge theory compactified on a circle with anti-periodic boundary conditions for fermions, coupled to N_f left-handed quarks and N_f right-handed quarks localized at different points on the compact circle. We analytically compute the speed of sound and the sound wave attenuation in the quenched approximation. Since confinement/deconfinement (and the chiral symmetry restoration) phase transitions are first order in this model, we do not …
Using the Sakai-Sugimoto model we study the effect of an external magnetic field on the dynamics of fundamental flavours in both the confined and deconfined phases of a large N_c gauge theory. We find that an external magnetic field promotes chiral symmetry breaking, consistent with the "magnetic catalysis" observed in the field theory literature, and seen in other studies using holographic duals. The external field increases the separation between the deconfinement temperature and the chiral symmetry restoring temperature. In the deconfined phase we investigate the temperature-magnetic field …
We modify the Sakai-Sugimoto model of chiral symmetry breaking to take into account the open string tachyon which stretches between the flavour D8-branes and anti D8-branes. There are several reasons of consistency for doing this: (i) Even if it might be reasonable to ignore the tachyon in the ultraviolet where the flavour branes and antibranes are well separated and the tachyon is small, it is likely to condense and acquire large values in the infrared where the branes meet. This takes the system far away from the perturbatively stable minimum of the Sakai-Sugimoto model; (ii) The bifundament…
In the context of holographic QCD we analyze Sakai-Sugimoto's chiral model at finite baryon density and zero temperature. The baryon number density is introduced through compact D4 wrapping S^4 at the tip of D8-\bar{D8}. Each baryon acts as a chiral point-like source distributed uniformly over R^3, and leads a non-vanishing U(1)_V potential on the brane. For fixed baryon charge density n_B we analyze the bulk energy density and pressure using the canonical formalism. The baryonic matter with point like sources is always in the spontaneously broken phase of chiral symmetry, whatever the density…
The Sakai-Sugimoto model is the preeminent example of a string theory description of holographic QCD, in which baryons correspond to topological solitons in the bulk. Here we investigate the validity of various approximations of the Sakai-Sugimoto soliton that are used widely to study the properties of holographic baryons. These approximations include the flat space self-dual instanton, a linear expansion in terms of eigenfunctions in the holographic direction and an asymptotic power series at large radius. These different approaches have produced contradictory results in the literature regard…
The baryon spectrum in the Sakai-Sugimoto model in the D4 background with smeared D0 charges is studied. We follow the instanton description of baryons by Hata et al.[Prog. Theor. Phys. 117, 1157]. The background corresponds to an excited state with nonzero glue condensate $\langle {\rm tr} (F_{μν}\tilde F^{μν})\rangle$ which is proportional to the D0 charge density. The baryon size shrinks when we turn on small D0 charge density. But for larger D0 charge density where massive modes in the gauge theory may also take effect, the size of baryons will grow. The difference between baryon masses wi…
We have considered the holographic large $N_c$ QCD model proposed by Sakai and Sugimoto and evaluated the non-Abelian DBI-action on the D8-brane upto $(α')^4$ terms. Restricting to the pion sector, these corrections give rise to four derivative contact terms for the pion field. We derive the Weinberg's phenemenological lagrangian. The coefficients of the four derivative terms are determined in terms of $g_{YM}^2$. The low energy pion-pion scattering amplitudes are evaluated. Numerical results are presented with the choice of $M_{KK}=0.94 GeV$ and $N_c=11$. The results are compared with the amp…
We extend the work of Aarts et al., including an imaginary chemical potential for quark number into the Sakai-Sugimoto model and codimension k defect theories. The phase diagram of these models are a function of three parameters, the temperature, chemical potential and the asymptotic separation of the flavour branes, related to a mass for the quarks in the boundary theories. We compute the phase diagrams and the pressure due to the flavours of the theories as a function of these parameters and show that there are Roberge-Weiss transitions in the high temperature phases, chiral symmetry restore…
Using the Witten-Sakai-Sugimoto model in the D0-D4 background, we holographically compute the vacuum decay rate of the Schwinger effect in this model. Our calculation contains the influence of the D0-brane density which could be identified as the $θ$ angle or chiral potential in QCD. Under the strong electromagnetic fields, the instability appears due to the creation of quark-antiquark pairs and the associated decay rate can be obtained by evaluating the imaginary part of the effective Euler-Heisenberg action which is identified as the action of the probe brane with a constant electromagnetic …
We re-analyze D8 brane embeddings in the geometry of a D4 brane wrapped on a circle that describe chiral symmetry breaking in a strongly coupled non-supersymmetric gauge theory. We argue that if the holographic fields are correctly interpreted, the original embeddings describe a complex quark mass and condensate in the theory. We show that in this interpretation when a quark mass is present there is a massive pseudo Goldstone boson (pion). A previously identified massless fluctuation is, we argue, not a physical state in the field theory. We also determine the behaviour of the quark condensate…
We construct a string-inspired model for the central production of $η$ and $η'$ mesons in proton-proton collisions, via double Pomeron exchange. Using general symmetry considerations, we construct a low-energy differential cross section for double glueball exchange in terms of some undetermined coupling constants and form factors. We extend this model to the Regge regime, replacing the glueball propagators with Pomeron trajectories, and modifying the interaction term by a factor derived from the 5-string scattering amplitude in flat space. We then fix the couplings which remain undetermined, u…
We extend the holographic analysis of light-baryon spectrum in \cite{key-50} to the case involving the heavy flavors. With the construction of the Witten-Sakai-Sugimoto model in the D0-D4 background, we use the mechanism proposed in \cite{key-59,key-60,key-61} by including two light and one heavy flavor branes, to describe the heavy-light baryons as heavy mesons bound to a flavor instanton. The background geometry of this model corresponds to an excited state in the dual field theory with nonzero glue condensate $\left\langle \mathrm{Tr}\mathcal{F}\wedge\mathcal{F}\right\rangle =8π^{2}N_{c}\ti…
With the construction of the Witten-Sakai-Sugimoto model in the D0-D4 background, we systematically investigate the holographic baryon spectrum in the case of three flavors. The background geometry in this model is holographically dual to $U\left(N_{c}\right)$ Yang-Mills theory in large $N_{c}$ limit involving an excited state with a nonzero $θ$ angle or glue condensate $\left\langle \mathrm{Tr}\mathcal{F}\wedge\mathcal{F}\right\rangle =8π^{2}N_{c}\tildeκ$, which is proportional to the charge density of the smeared D0-branes through a parameter $b$ or $\tildeκ$. The classical solution of baryo…
The binding energy of baryon has been studied in the dual $AdS_5\times S^5$ string theory with a black hole interior. In this picture baryon is constructed of a $D_5$ brane vertex wrapping on $S^5$ and $N_c$ fundamental strings connected to it. Here, we calculate the baryon binding energy in Sakai-Sugimoto model with a $D_4/D_8/\bar{D_8}$ in which the supersymmetry is completely broken. Also we check the $T$ dependence of the baryon binding energy. We believe that this model represents an accurate description of baryons due to the existence of Chern-Simones coupling with the gauge field on the…
Using the AdS/CFT correspondence, we calculate the vacuum decay rate for the Schwinger effect in confining large $N_{c}$ gauge theories. The instability is induced by the quark antiquark pair creation triggered by strong electromagnetic fields. The decay rate is obtained as the imaginary part of the Euler-Heisenberg effective Lagrangian evaluated from the D-brane action with a constant electromagnetic field in holographic QCD models such as the Sakai-Sugimoto model and the deformed Sakai-Sugimoto model. The decay rate is found to increase with the magnetic field parallel to the electric field,…
In the presence of a quark chemical potential, a magnetic field induces an axial current in the direction of the magnetic field. We compute this current in the Sakai-Sugimoto model, a holographic model which, in a certain limit, is dual to large-N_c QCD. We also compute the analogous vector current, for which an axial chemical potential is formally introduced. This vector current can potentially be observed via charge separation in heavy-ion collisions. After implementing the correct axial anomaly in the Sakai-Sugimoto model we find an axial current in accordance with previous studies and a va…
This paper continues the investigation of the modified Sakai-Sugimoto model proposed in arXiv:0708.3233. Here we discuss in detail numerical solutions to the classical equations for the brane profile and the tachyon condensate. An ultraviolet cut-off turns out to be essential because the numerical solutions tend to rapidly diverge from the desired asymptotic solutions, beyond a sufficiently large value of the holographic coordinate. The required cut-off is determined by the non-normalizable part of the tachyon and is parametrically far smaller than that dictated by consistency of a description…
Recently, a homogeneous ansatz has been used to study cold dense nuclear matter in the Sakai-Sugimoto model of holographic QCD. To justify this homogeneous approximation we here investigate a homogeneous ansatz within a low-dimensional toy version of Sakai-Sugimoto to study finite baryon density configurations and compare it to full numerical solutions. We find the ansatz corresponds to enforcing a dyon salt arrangement in which the soliton solutions are split into half-soliton layers. Within this ansatz we find analogues of the proposed baryonic popcorn transitions, in which solutions split i…
The Sakai-Sugimoto model is the most pre-eminent model of holographic QCD, in which baryons correspond to topological solitons in a five-dimensional bulk spacetime. Recently it has been shown that a single soliton in this model can be well approximated by a flat-space self-dual Yang-Mills instanton with a small size, although studies of multi-solitons and solitons at finite density are currently beyond numerical computations. A lower-dimensional analogue of the model has also been studied in which the Sakai-Sugimoto soliton is replaced by a baby Skyrmion in three spacetime dimensions with a wa…
We investigate properties of baryons in a family of holographic field theories related to the Sakai-Sugimoto model of holographic QCD. Starting with the $N_f=2$ Sakai-Sugimoto model, we truncate to a 5D Yang-Mills action for the gauge fields associated with the noncompact directions of the flavour D8-branes. We define a free parameter $γ$ that controls the strength of this Yang-Mills term relative to the Chern-Simons term that couples the abelian gauge field to the SU(2) instanton density. Moving away from $γ= 0$ should incorporate some of the effects of taking the Sakai-Sugimoto model away fr…
We propose several quark mass deformations of the holographic model of massless QCD using the D4/D8/D8bar brane configuration proposed by Sakai and Sugimoto. The deformations are based on introducing additional D4 or D6 branes away from the QCD D4 branes. The idea is similar to extended technicolor theories, where the chiral symmetry breaking by additional D-branes are mediated to QCD to induce non-zero quark masses. In the D-brane picture as well as the holographic dual gravity description, the quark and the pion masses are generated by novel worldsheet instantons with finite area. We also de…
We investigate behavior of a noncritical model in external electric field and explore its phase structure in the quenched approximation N_f>> N_c. We compute the conductivity of QCD plasma in this model and compare it with the predictions of Sakai-Sugimoto model, D3-D7 system and the lattice simulation. We find that, while the behavior of conductivity in noncritical model as a function of temperature and baryon density is similar to those of D3-D7 system, the phase diagram of noncritical model resembles the phase diagram of Sakai-Sugimoto model.
We investigate the surface of the chiral phase transition in the three-dimensional parameter space of temperature, baryon chemical potential and magnetic field in two different approaches, the field-theoretical Nambu-Jona-Lasinio (NJL) model and the holographic Sakai-Sugimoto model. The latter is a top-down approach to a gravity dual of QCD with an asymptotically large number of colors and becomes, in a certain limit, dual to an NJL-like model. Our main observation is that, at nonzero chemical potential, a magnetic field can restore chiral symmetry, in apparent contrast to the phenomenon of ma…
We revisit the $U(1)_A$ anomaly in the holographic model of low-energy QCD by Witten, Sakai, and Sugimoto, presenting a new and direct derivation of the Witten-Veneziano mechanism for generating the mass of the $η'$ through an anomalous mixing of the Ramond-Ramond $C_1$ field with the singlet component of the pseudoscalar mesons. The latter turns out to have a kinetic mixing with the normalizable modes of the $C_1$ field representing pseudoscalar glueballs, yielding additional vertices for their production and their decay that dominate over those of the unmixed case considered previously in th…
We calculate the electric conductivity $σ$ in deconfined QCD matter using a holographic QCD model, i.e., the Sakai-Sugimoto Model with varying magnetic field $B$ and chiral anomaly strength. After confirming that our estimated $σ$ for $B=0$ is consistent with the lattice-QCD results, we study the case with $B\neq 0$ in which the coefficient $α$ in the Chern-Simons term controls the chiral anomaly strength. Our results imply that the transverse conductivity, $σ_\perp$, is suppressed to be $\lesssim 70\%$ at $B\sim 1\,\mathrm{GeV}^2$ as compared to the $B=0$ case when the temperature is fixed as…
To realize massive pions, we study variations of the holographic model of massless QCD using the D4/D8/anti-D8 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.
Properties of the tensor glueball are discussed in the Witten-Sakai-Sugimoto Model, a top-down holographic approach to the non-perturbative region of Quantum Chromodynamics (QCD).
We study a brane/anti-brane configuration which is separated along a compact direction by constructing a tachyon effective action which takes into account transverse scalars. Such an action is relevant in the study of HQCD model of Sakai and Sugimoto of chiral symmetry breaking, where the size of the compact circle sets the confinement scale. Our approach is motivated by string theory orbifold constructions and gives a route to model inhomogeneous tachyon decay. We illustrate the techniques involved with a relatively simple example of a harmonic oscillator on a circle. We will then repeat the …
We attempt to give a holographic description of the microscopic theory of a BCS superconductor. Exploiting the analogy with chiral symmetry breaking in QCD we use the Sakai-Sugimoto model of two D8 branes in a D4 brane background with finite baryon number. In this case there is a new tachyonic instability which is plausibly the bulk analog of the Cooper pairing instability. We analyze the Yang-Mills approximation to the non-Abelian Born-Infeld action. We give some exact solutions of the non-linear Yang-Mills equations in flat space and also give a stability analysis, showing that the instabili…
