arXiv subjects

Ismail Zahed

Publications and source records attributed to Ismail Zahed.

At least 181 records · Page 10Linked to original sources

arXiv2008

Dense and Hot Holographic QCD: Finite Baryonic E Field

We investigate the response of dense and hot holographic QCD (hQCD) to a static and baryonic electric field E using the chiral model of Sakai and Sugimoto. Strong fields with E>(\sqrtλM_{KK})^2 free quark pairs, causing the confined vacuum and matter state to decay. We generalize Schwinger's QED persistence function to dense hQCD. At high temperature and density, Ohm's law is derived generalizing a recent result by Karch and O'Bannon to the chiral case.

hep-th↗

arXiv2008

Electromagnetic Baryon Form Factors from Holographic QCD

In the holographic model of QCD suggested by Sakai and Sugimoto, baryons are chiral solitons sourced by D4 instantons in bulk of size 1/\sqrtλ with λ=g^2N_c. We quantize the D4 instanton semiclassically using \hbar=1/(N_cλ) and non-rigid constraints on the vector mesons. The holographic baryon is a small chiral bag in the holographic direction with a Cheshire cat smile. The vector-baryon interactions occur at the core boundary of the instanton in D4. They are strong and of order 1/\sqrt{\hbar}. To order \hbar^0 the electromagnetic current is entirely encoded on the core boundary and vector-meson dominated. To this order, the electromagnetic charge radius is of order λ^0. The meson contribution to the baryon magnetic moments sums identically to the core contribution. The proton and neutron magnetic moment are tied by a model independent relation similar to the one observed in the Skyrme model.

hep-th↗

arXiv2008

Diffusion in an Expanding Plasma using AdS/CFT

We consider the diffusion of a non-relativistic heavy quark of fixed mass M, in a one-dimensionally expanding and strongly coupled plasma using the AdS/CFT duality. The Green's function constructed around a static string embedded in a background with a moving horizon, is identified with the noise correlation function in a Langevin approach. The (electric) noise decorrelation is of order 1/T(τ) while the velocity de-correlation is of order MD(τ)/T(τ). For MD>1, the diffusion regime is segregated and the energy loss is Langevin-like. The time dependent diffusion constant D(τ) asymptotes its adiabatic limit 2/π\sqrtλ T(τ) when τ/τ_0=(1/3η_0τ_0)^3 where η_0 is the drag coefficient at the initial proper time τ_0.

hep-th↗

arXiv2008

The Chiral Model of Sakai-Sugimoto at Finite Baryon Density

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 point-like nature of the sources and large N_c cause the matter to be repulsive as all baryon interactions are omega mediated. Through the induced DBI action on D8-\bar{D8}, we study the effects of the fixed baryon charge density n_B on the pion and vector meson masses and couplings. Issues related to vector dominance in matter in the context of holographic QCD are also discussed.

hep-th↗

arXiv2008

Dense Holographic QCD in the Wigner-Seitz Approximation

We investigate cold dense matter in the context of Sakai and Sugimoto's holographic model of QCD in the Wigner-Seitz approximation. In bulk, baryons are treated as instantons on S^3\times R^1 in each Wigner-Seitz cell. In holographic QCD, Skyrmions are instanton holonomies along the conformal direction. The high density phase is identified with a crystal of holographic Skyrmions with restored chiral symmetry at about 4 Mkk^3/pi^5. As the average density goes up, it approaches to uniform distribution while the chiral condensate approaches to p-wave over a cell. The chiral symmetry is effectively restored in long wavelength limit since the chiral order parameter is averaged to be zero over a cell. The energy density in dense medium varies as n_B^{5/3}, which is the expected power for non-relativistic fermion. This shows that the Pauli exclusion effect in boundary is encoded in the Coulomb repulsion in the bulk.

hep-th↗

arXiv2007

Transverse momentum spectra of dileptons at NA60

Recently the NA60 collaboration has reported the transverse mass spectra of dimuons coming from In-In collisions at 158 GeV/A. The measured yields display a strong invariant mass dependence not typical of radial flow, suggesting that different sources contribute in different mass regions. We interpret the dimuon transverse mass spectra from an early thermalized partonic phase and hadronic phase constrained by the strictures of broken chiral symmetry. Each phase develops a specific transverse momentum dependence by hydrodynamical expansion. We show that a measurement of the momentum anisotropy at NA60 could provide information on the dominant emission source (hadronic or partonic) in the intermediate mass region 1.5 <= M <= 3.0 GeV.

hep-ph↗

arXiv2007

Random Levy Matrices Revisited

We compare eigenvalue densities of Wigner random matrices whose elements are independent identically distributed (iid) random numbers with a Levy distribution and maximally random matrices with a rotationally invariant measure exhibiting a power law spectrum given by stable laws of free random variables. We compute the eigenvalue density of Wigner-Levy (WL) matrices using (and correcting) the method by Bouchaud and Cizeau (BC), and of free random Levy (FRL) rotationally invariant matrices by adapting results of free probability calculus. We compare the two types of eigenvalue spectra. Both ensembles are spectrally stable with respect to the matrix addition. The discussed ensemble of FRL matrices is maximally random in the sense that it maximizes Shannon's entropy. We find a perfect agreement between the numerically sampled spectra and the analytical results already for matrices of dimension N=100. The numerical spectra show very weak dependence on the matrix size N as can be noticed by comparing spectra for N=400. After a pertinent rescaling spectra of Wigner-Levy matrices and of symmetric FRL matrices have the same tail behavior. As we discuss towards the end of the paper the correlations of large eigenvalues in the two ensembles are however different. We illustrate the relation between the two types of stability and show that the addition of many randomly rotated Wigner-Levy matrices leads by a matrix central limit theorem to FRL spectra, providing an explicit realization of the maximal randomness principle.

cond-mat.stat-mech↗

arXiv2006

Dense Hadronic Matter in Holographic QCD

We provide a method to study hadronic matter at finite density in the context of the Sakai-Sugimoto model. We introduce the baryon chemical potential through the external $U(1)_v$ in the induced (DBI plus CS) action on the D8-probe-brane, where baryons are skyrmions. Vector dominance is manifest at finite density. We derive the baryon density effect on the energy density, the dispersion relations of pion and vector mesons at large $N_c$. The energy density asymptotes to a constant at large density suggesting that dense matter at large $N_c$ freezes, with the pion velocity dropping to zero. Holographic dense matter enforces exactly the tenets of vector dominance, and screens efficiently vector mesons. At the freezing point the $ρ-ππ$ coupling vanishes with a finite rho mass of about 20% its vacuum value.

hep-th↗

arXiv2006

A Gravity Dual of RHIC Collisions

In the context of the AdS/CFT correspondence we discuss the gravity dual of a heavy-ion-like collision in a variant of ${\cal N}=4$ SYM. We provide a gravity dual picture of the entire process using a model where the scattering process creates initially a holographic shower in bulk AdS. The subsequent gravitational fall leads to a moving black hole that is gravity dual to the expanding and cooling heavy-ion fireball. The front of the fireball cools at the rate of $1/τ$, while the core cools as $1/\sqrtτ$ from a cosmological-like argument. The cooling is faster than Bjorken cooling. The fireball freezes when the dual black hole background is replaced by a confining background through the Hawking-Page transition.

hep-th↗

arXiv2006

Ampere's Law and Energy Loss in AdS/CFT Duality

We note that the energy loss in ${\cal N}=4$ SYM measures directly the spatial string tension $σ_S=π\sqrtλT^2/2$ which is at the origin of the area law for large spatial Wilson loops. We show that the latter reflects on the nonperturbative nature of Ampere's law in ${\cal N}=4$ SYM both in vacuum and at finite temperature.

hep-ph↗

arXiv2006

Classical Strongly Coupled QGP II: Screening and Equation of State

We analyze the screening and bulk energy of a classical and strongly interacting plasma of color charges, a model we recently introduced for the description of a quark-gluon plasma at T=(1-3)Tc. The partition function is organized around the Debye-Huckel limit. The linear Debye-Huckel limit is corrected by a virial expansion. For the pressure, the expansion is badly convergent even in the dilute limit. The non-linear Debye-Huckel theory is studied numerically as an alternative for moderately strong plasmas. We use Debye theory of solid to extend the analysis to the crystal phase at very strong coupling. The analytical results for the bulk energy per particle compare well with the numerical results from molecular dynamics simulation for all couplings.

nucl-th↗

arXiv2006

Classical Strongly Coupled QGP I: The Model and Molecular Dynamics Simulations

We propose a model for the description of strongly interacting quarks and gluon quasiparticles at $T=(1-3)T_c$, as a classical and nonrelativistic colored Coulomb gas. The sign and strength of the inter-particle interactions are fixed by the scalar product of their classical {\it color vectors} subject to Wong's equations. The model displays a number of phases as the Coulomb coupling is increased ranging from a gas, to a liquid, to a crystal with antiferromagnetic-like color ordering. We analyze the model using Molecular Dynamics (MD) simulations and discuss the density-density correlator in real time. We extract pertinent decorrelation times, diffusion and viscosity constants for all phases. The classical results when extrapolated to the sQGP suggest that the phase is liquid-like, with a diffusion constant $D\approx 0.1/T$ and a bulk viscosity to entropy density ratio $η/s\approx 1/3$.

nucl-th↗

arXiv2005

Cold Strongly Coupled Atoms Make a Near-perfect Liquid

Feshbach resonances of trapped ultracold alkali atoms allow to vary the atomic scattering length a. At very large values of a the system enters an universal strongly coupled regime in which its properties--the ground state energy, pressure {\it etc.}--become independent of a. We discuss transport properties of such systems. In particular, the universality arguments imply that the shear viscosity of ultracold Fermi atoms at the Feschbach resonance is proportional to the particle number density n, and the Plank constant \hbar η=\hbar n α_η, where α_ηis a universal constant. Using Heisenberg uncertainty principle and Einstein's relation between diffusion and viscosity we argue that the viscosity has the lower bound given by α_η \leq (6π)^{-1}. We relate the damping of low-frequency density oscillations of ultracold optically trapped ^{6}Li atoms to viscosity and find that the value of the coefficient α_ηis about 0.3. We also show that such a small viscosity can not be explained by kinetic theory based on binary scattering. We conclude that the system of ultracold atoms near the Feshbach resonance is a near-ideal liquid.

nucl-th↗

arXiv2004

Holography of Radiation and Jet Quenching

We study the non-linear propagation of radiation in {\cal N=4} SYM at zero and finite temperature using the refined radius/scale duality in AdS/CFT. We argue that a pulse radiation by a quark at the boundary should be described holographically by a "point like object" passing through the center of the AdS bulk. We find that at finite temperature, the radiation stalls at a distance of $1/πT$ with a natural geometric and holographic interpretation. Indeed, the stalling is the holographic analogue of the gravitational in-fall of light towards the black hole in the bulk. We suggest that these results are relevant for jet quenching by a strongly coupled quark-gluon liquid as currently probed in heavy ion colliders at RHIC. In particular, colored jets cannot make it beyond 1/3 fm at RHIC whatever their energy.

hep-th↗

arXiv2004

Ionization of Binary Bound States in a Strongly Coupled Quark-Gluon Plasma

Although at temperatures $T\gg Λ_{QCD}$ the quark-gluon plasma (QGP) is a gas of weakly interacting quasiparticles (modulo long-range magnetism), it is strongly interacting (sQGP) in the temperature range $(1-3) T_c$. One aspect of these interactions is the existence of many binary bound states of quasiparticles. Only $\bar q q$ ones have been so far directly seen on the lattice, for charmed and light quarks, but other attractive channels in $ qg, gg$ are likely to have them as well. It was argued in our previous paper that such bound states account for a significant part of the bulk properties such as density and pressure. Using the same model, we evaluate the energy loss $dE/dx$ due to the ionization of these states. We found that it is substantial, but only in the narrow interval of temperatures $T=(1.4-1.7)T_c$. In contrast to that, we show that radiative and elastic losses are not likely to be modified much by binding, as the total density of color charges is close to what it is for weakly coupled quasiparticles. These distinctions would be important for understanding the energy dependence of jet quenching.

hep-ph↗

arXiv2004

A Diquark Chiral Effective Theory and Exotic Baryons

A chiral effective theory of quarks and diquarks is formulated and applied to exotic tetraquarks and pentaquarks. The effective theory is similar to the chiral quark effective theory with the addition of diquark degrees of freedom and couplings to quarks. Chiral symmetry through generalized Goldberger-Treiman relations, fixes the mass splitting between the even and odd parity diquarks to be about 600 MeV. We provide an estimate of the parameters of the effective theory from both the random instanton model and known data on the low lying scalar nonet. We assess the static properties of the exotic baryons, such as their masses, magnetic moments and decay widths. We show that the small decay widths of the newly reported exotics are largely due to a large tunnelling suppression of a quark between a pair of diquarks. We find $Γ(Θ^+\to K^+n)\simeq 2.5\sim7.0 MeV$ and $Γ(Ξ^{--}\toΞ^- π^-)\simeq 1.7\sim4.8 MeV$.

hep-ph↗

arXiv2004

Towards a Theory of Binary Bound States in the Quark-Gluon Plasma

Although at temperatures $T\gg Λ_{QCD}$ the quark-gluon plasma (QGP) is a gas of weakly interacting quasiparticles (modulo long-range magnetism), it is strongly interacting in the regime $T=(1-3) T_c$. As both heavy ion experiments and lattice simulations are now showing, in this region the QGP displays rather strong interactions between the constituents. In this paper we investigate the relationship between four (previously disconnected) lattice results: {\bf i.} spectral densities from MEM analysis of correlators; {\bf ii.} static quark free energies $F(R)$; {\bf iii.} quasiparticle masses; {\bf iv.} bulk thermodynamics $p(T)$. We show a high degree of consistency among them not known before. The potentials $V(R)$ derived from $F(R)$ lead to large number of binary bound states, mostly colored, in $gq, qq, gg$, on top of the usual $\bar q q$ mesons. Using the Klein-Gordon equation and ({\bf ii-iii}) we evaluate their binding energies and locate the zero binding endpoints on the phase diagram, which happen to agree with ({\bf i}). We then estimate the contribution of all states to the bulk thermodynamics in agreement with ({\bf iv}). We also address a number of theoreticall issues related with to the role of the quark/gluon spin in binding at large $α_s$, although we do not yet include those in our estimates. Also the issue of the transport properties (viscosity, color conductivity) in this novel description of the QGP will be addressed elsewhere.

hep-ph↗

arXiv2004

A schematic model for pentaquarks based on diquarks

QCD instantons are known to produce deeply bound diquarks which may be used as building blocks in the formation of multiquark states, in particular pentaquarks and dibaryons. We suggest a simple model in which the lowest scalar diquark (and possibly the tensor one) can be treated as an independent ``body'', with the same color and (approximately) the mass as a constituent (anti)quark. In this model a new symmetry between states with the same number of ``bodies'' but different number of quarks appear, in particular the 3-``body'' pentaquarks can be naturally related to decuplet baryons. We estimate both the masses and widths of such states, and then discuss the limitations of this model.

hep-ph↗