Chiral and deconfinement phase transitions at finite temperature $T$ and quark number chemical potential $μ$ are simultaneously studied in the quenched dynamical holographic QCD model within the Einstein-Dilaton-Maxwell framework. By calculating the corresponding order parameters, i.e., the chiral condensate and Polyakov loop, it is shown that the transition lines of these two phase transitions are separated in the $T-μ$ plane. The deconfinement phase transition is shown to be always of crossover type and the transition line depends weakly on the baryon number density. Differently, the chiral transition is of crossover at small baryon number density and it turns to be of first order at sufficient large baryon number density. A critical endpoint (CEP), at which the transition becomes second order type, appears in the chiral transition line. This is the first time to realize the CEP of chiral phase transition in the $(T, μ)$ plane using the holographic EMD(Einstein-Maxwell-Dilaton) model for two flavour case. It is observed that between these two phase transition lines, there is a region with chiral symmetry restored and color degrees still confined, which could be considered as the quarkyonic phase. Qualitatively, this behavior is in consistent with the result in the Polyakov-loop improved Nambu-Jona-Lasinio (PNJL) model.
We point out a new configuration in the Witten-Sakai-Sugimoto model, allowing baryons in the pointlike approximation to coexist with fundamental quarks. The resulting phase is a holographic realization of quarkyonic matter, which is predicted to occur in QCD at a large number of colors, and possibly plays a role in real-world QCD as well. We find that holographic quarkyonic matter is chirally symmetric and that, for large baryon chemical potentials, it is energetically preferred over pure nuclear matter and over pure quark matter. The zero-temperature transition from nuclear matter to the quar…
Large-N_c QCD matter at intermediate baryon density and low temperatures has been conjectured to be in the so-called quarkyonic phase, i.e., to have a quark Fermi surface and on top of it a confined spectrum of excitations. It has been suggested that the presence of the quark Fermi surface leads to a homogeneous phase with restored chiral symmetry, which is unstable towards creating condensates breaking both the chiral and translational symmetry. Motivated by these exotic features, we investigate properties of cold baryonic matter in the single flavor Sakai-Sugimoto model searching for a holog…
We explore QCD phase diagram by constructing a holographic QCD model using the Einstein-Maxwell-Scalar system. The chiral transition is investigated by adding a probe scalar and confinement transition is studied by adding a probe string into the system. By interpreting the black hole phase transition in the bulk spacetime as the quarkyonic transition in the dual QCD theory and introducing the bypass mechanism for deconfinement transition, we give an explanation why chiral symmetry breaking and deconfinement transition lines coincide with each other despite their different physical origins.
Holographic QCD at finite baryon number density and zero temperature is studied within the five-dimensional Sakai-Sugimoto model. We introduce a new approximation that models a smeared crystal of solitonic baryons by assuming spatial homogeneity to obtain an effective kink theory in the holographic direction. The kink theory correctly reproduces a first order phase transition to lightly bound nuclear matter. As the density is further increased the kink splits into a pair of half-kink constituents, providing a concrete realization of the previously suggested dyonic salt phase, where the bulk so…
We study the phase diagram of (large-$N_c$) QCD using a simplistic holographic hard-wall model with a dynamical scalar field and a homogeneous Ansatz representing a smeared instanton/baryon density. The resulting phase diagram is qualitatively consistent with expectations, including a mesonic, baryonic, quarkyonic, and quark-gluon plasma phase. As in other holographic models, we also find a baryonic popcorn transition, which appears at large chemical potential as a crossover. We then evaluate the nuclear matter equation of state, which turns out to be rather stiff with a large peaked sound vel…
In this work, we offer a dynamical soft-wall model to describe the gluodynamics and chiral dynamics in one systematical framework. We firstly construct a quenched dynamical holographic QCD (hQCD) model in the graviton-dilaton framework for the pure gluon system, then develop a dynamical hQCD model for the two flavor system in the graviton-dilaton-scalar framework by adding light flavors on the gluodynamical background. For two forms of dilaton background field $Φ=μ_G^2z^2$ and $Φ=μ_G^2z^2\tanh(μ_{G^2}^4z^2/μ_G^2)$, the quadratic correction to dilaton background field at infrared encodes import…
In the large N limit cold dense nuclear matter must be in a lattice phase. This applies also to holographic models of hadron physics. In a class of such models, like the generalized Sakai-Sugimoto model, baryons take the form of instantons of the effective flavor gauge theory that resides on probe flavor branes. In this paper we study the phase structure of baryonic crystals by analyzing discrete periodic configurations of such instantons. We find that instanton configurations exhibit a series of "popcorn" transitions upon increasing the density. Through these transitions normal (3D) lattices …
We analyze the equal-time Bethe-Salpeter quark wave functions of the pion in various models. We discuss how the quenched lattice QCD results with delocalized pion interpolators can be identified with the coarse grained wave functions, typical of low-energy effective models. Actually, we find that one-loop chiral quark models predict that pseudoscalar and tensor wave functions have the same shape, while the axial component is more extended. These facts are accurately confirmed by the lattice. We also show how the transversity information, relevant for the light-cone physics, can be straightforw…
The plasma phase at high temperatures of a strongly coupled gauge theory can be holographically modelled by an AdS black hole. Matter in the fundamental representation and in the quenched approximation is introduced through embedding D7-branes in the AdS-Schwarzschild background. Low spin mesons correspond to the fluctuations of the D7-brane world volume. As is well known by now, there are two different kinds of embeddings, either reaching down to the black hole horizon or staying outside of it. In the latter case the fluctuations of the D7-brane world volume represent stable low spin mesons. …
The vector dominance of the electromagnetic form factors both for mesons and baryons arises naturally in holographic QCD, where both the number of colors and the 't Hooft coupling are taken to be very large, offering a bona-fide derivation of the notion of vector dominance. The crucial ingredient for this is the infinite tower of vector mesons in the approximations made which share features that are characteristic of the quenched approximation in lattice QCD. We approximate the infinite sum by contributions from the lowest four vector mesons of the tower which turn out to saturate the charge a…
We study the rho meson in a uniform magnetic field eB using a holographic QCD-model, more specifically a D4/D8/Dbar8 brane setup in the confinement phase at zero temperature with two quenched flavours. The parameters of the model are fixed by matching to corresponding dual field theory parameters at zero magnetic field. We show that the up- and down-flavour branes respond differently to the presence of the magnetic field in the dual QCD-like theory, as expected because of the different electromagnetic charge carried by up- and down-quark. We discuss how to recover the Landau levels, indicating…
During heavy ion collisions, high temperatures and strong magnetic fields are generated. We employ the gauge-gravity duality to study the N_f=2 QCD phase diagram under these extreme conditions in the quenched approximation, in particular we use the non-antipodal Sakai-Sugimoto model (SSM). We take the different coupling of up and down flavours to the magnetic field into account geometrically, resulting in a split of the chiral phase transition according to flavour. We discuss the influence of the magnetic field on the chiral temperatures -in physical GeV units- in terms of the choice of the co…
A simple holographic model is presented and analyzed that describes chiral symmetry breaking and the physics of the meson sector in QCD. This is a bottom-up model that incorporates string theory ingredients like tachyon condensation which is expected to be the main manifestation of chiral symmetry breaking in the holographic context. As a model for glue the Kuperstein-Sonnenschein background is used. The structure of the flavor vacuum is analyzed in the quenched approximation. Chiral symmetry breaking is shown at zero temperature. Above the deconfinement transition chiral symmetry is restored.…
We first study Z(N) walls in a deconfined phase of Witten's D4-brane background of pure SU(N) Yang-Mills theory, motivated by a recent work in the case of N=4 SYM. Similarly to it, we propose that for a large wall charge k ~ N, it is described by k D2-branes blown up into a NS5-brane wrapping S^3 inside S^4 via Myers effect, and we calculate the tension by suitable U-duality. We find a precise Casimir scaling for the tension formula. We then study the fate of Z(N)-vacua in a presence of fundamental flavors in quenched approximation via gauge/gravity correspondence. In the case of D3/D7 system …
The large N_c N=4 gauge theory with quenched N=2 quark matter in the presence of a magnetic field displays chiral symmetry breaking. We study the temperature and chemical potential dependence of this theory using its gravity dual (based on the D3/D7 brane system). With massless quarks, at zero chemical potential, the theory displays a first order thermal transition where chiral symmetry is restored and simultaneously the mesons of the theory melt. At zero temperature, these transitions with chemical potential are second order and occur at different chemical potential values. Between the three …
Using holography, we study the low-lying mesonic spectrum of a range of asymptotically free gauge theories. First we revisit a simple top-down holographic model of QCD-like dynamics with predictions in the M_rho-M_pi plane. The meson masses in this model are in very good agreement with lattice gauge theory calculations in the quenched approximation. We show that the key ingredient for the meson mass predictions is the running of the anomalous dimension of the quark condensate, gamma. This provides an explanation for the agreement of holographic and quenched lattice gauge theory calculations. W…
With the explicit breaking of scale invariance by the trace anomaly of QCD rephrased in terms of spontaneous breaking, low-energy strong interaction dynamics of dense (and also hot) matter can be effectively captured by -- in addition to the Nambu-Goldstone bosons and the vector mesons -- two dilaton fields, the "soft" ($χ_s$) field that is locked to chiral symmetry and the "hard" ($χ_h$) field which remains unaffected by chiral symmetry. The interplay of the soft and hard dilatons plays a subtle role in how chiral symmetry is manifested in hot and/or dense matter. The scale anomaly in which t…
Finite-density QCD is difficult to study numerically because of the sign problem. We prove that, in a certain region of the phase diagram, the phase quenched approximation is exact to O(Nf/Nc). It is true for any physical observables. We also consider the implications for the lattice simulations and find a quantitative evidence for the validity of the phase quenching from existing lattice QCD results at Nc=3. Our results show that the phase-quench approximation is rather good already at Nc=3, and the 1/Nc correction can be incorporated by the phase reweighting method without suffering from the…
We present the results of a systematic, first-principles study of the spectrum and decay constants of mesons for different numbers of color charges N, via lattice computations. We restrict our attention to states in the non-zero isospin sector, evaluating the masses associated with the ground-state and first excitation in the pseudoscalar, vector, scalar, and axial vector channels. Our results are based on a new set of simulations of four dimensional SU(N) Yang-Mills theories with the number of colors ranging from N=2 to N=17; the spectra and the decay constants are computed in the quenched ap…
Here we analyse the low energy confining phase of the $2+1$ dimensional quenched QCD-like theory in anisotropic non-supersymmetric D$2$ brane background with the holographic approach. Two key features of QCD -- flux-tube tension and mass spectrum of pseudoscalar glueball are studied. The non-supersymmetric D$2$ branes with anisotropy in time direction are considered as the gravity theory. Tuning the anisotropic parameter, we get the Hawking-Page transition in the gravity background. On the dual theory, this refers the confinement-deconfinement phase transition. Using the Wilson loop, the linea…
